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1 Saturn after Cassini/Huygens<br />
2<br />
3<br />
Revised Version April 10, 2009<br />
4<br />
Icy Satellites: Geological Evolution and Surface Processes<br />
5<br />
Ralf Jaumann 1,2,* , Roger N. Clark 3 , Francis Nimmo 4 , Amanda R. Hendrix 5 , Bonnie J. Buratti 5 6<br />
,<br />
Tilmann Denk 2 , Jeff M. Moore 6 , Paul M. Schenk 7 , Steve J. Ostro ✝5 and Ralf Srama 8<br />
7<br />
8<br />
9<br />
10<br />
11<br />
12<br />
13<br />
14<br />
15<br />
16<br />
17<br />
1<br />
DLR, Institute <strong>of</strong> <strong>Planetary</strong> Research. Rutherfordstrasse 2, 12489 Berlin, Germany;<br />
2<br />
Freie Universität Berlin, Institute <strong>of</strong> Geological <strong>Sciences</strong>, Malteserstr. 74-100, 12249 Berlin, Germany;<br />
3<br />
U.S. Geological Survey, Denver Federal Center, Denver CO 80225;<br />
4<br />
Dept. <strong>Earth</strong> & <strong>Planetary</strong> <strong>Sciences</strong>, <strong>University</strong> <strong>of</strong> <strong>California</strong> <strong>Santa</strong> Cruz, <strong>Santa</strong> Cruz, CA 95064, USA;<br />
5<br />
Jet Propulsion Laboratory, <strong>California</strong> Institute <strong>of</strong> Technology, Pasadena, CA 91109, USA;<br />
6<br />
NASA Ames Research Center, MS 245-3 M<strong>of</strong>fett Field, CA 94035-1000, USA;<br />
7<br />
Lunar and <strong>Planetary</strong> Institute Houston TX 77058, USA;<br />
8<br />
Max Planck Institut für Kernphysik, 69117, Heidelberg, Germany.<br />
18 *<br />
Corresponding author (Fax :+493067055 402 ; Email address: ralf.jaumann@dlr.de<br />
19<br />
20<br />
✝<br />
deceased<br />
21<br />
22<br />
Content<br />
Abstract<br />
23 21.1 Introduction<br />
24 21.2 Cassini’s Exploration <strong>of</strong> Saturn’s Icy Satellites<br />
25 21.3. Morphology, Geology and Topography<br />
26 21.4 Composition and Alteration <strong>of</strong> Surface Materials<br />
27 21.5 Constraints on top-meter Structure and Composition by Radar<br />
28 21.6 Geologic Evolution<br />
29<br />
30<br />
21.7 Conclusions<br />
31 Abstract<br />
32 The sizes <strong>of</strong> the Saturnian icy satellites range from ~1500 km in diameter (Rhea) to ~20 km<br />
33 (Calypso), and even smaller 'rocks' <strong>of</strong> only a kilometer in diameter are common in the system. All<br />
34 these bodies exhibit remarkable, unique features and unexpected diversity. In this chapter, we will<br />
35 mostly focus on the 'medium-sized icy objects' Mimas, Tethys, Dione, Rhea, Iapetus, Phoebe and<br />
36 Hyperion, and consider small objects only where appropriate, whereas Titan and Enceladus will be<br />
37 described in separate chapters. Mimas and Tethys show impact craters caused by bodies that were<br />
38 almost large enough to break them apart. Iapetus is unique in the Saturnian system because <strong>of</strong> its<br />
39 extreme global brightness dichotomy. Tectonic activity varies widely - from inactive Mimas<br />
40 through extensional terrains on Rhea and Dione to the current cryovolcanic eruptions on Enceladus<br />
41 – and is not necessarily correlated with predicted tidal stresses. Likely sources <strong>of</strong> stress include<br />
42 impacts, despinning, reorientation and volume changes. Accretion <strong>of</strong> dark material originating from<br />
43<br />
44<br />
outside the Saturnian system may explain the surface contamination that prevails in the whole<br />
satellite system, while coating by Saturn’s E-ring particles brightens the inner satellites.<br />
45 So far, among the surprising Cassini discoveries are the volcanic activity on Enceladus, the sponge-<br />
46 like appearance <strong>of</strong> Hyperion and the equatorial ridge on Iapetus - unique features in the solar<br />
47 system. The bright-ray system on Rhea was caused by a relatively recent medium impact which<br />
48 formed a ~40 km crater at 12°S latitude, 112°W longitude, while the wispy streaks on Dione and<br />
49 Rhea are <strong>of</strong> tectonic origin. Compositional mapping shows that the dark material on Iapetus is<br />
50<br />
51<br />
52<br />
composed <strong>of</strong> organics, CO2 mixed with H2O ice, and metallic iron, and also exhibits possible<br />
signatures <strong>of</strong> ammonia, bound water, H2 or OH-bearing minerals, and a number <strong>of</strong> as-yet<br />
unidentified substances. The spatial pattern, Rayleigh scattering effect, and spectral properties<br />
53 argue that the dark material on Iapetus is only a thin coating on its surface. Radar data indicate that<br />
54 the thickness <strong>of</strong> the dark layers can be no more than a few decimeters; this is also consistent with<br />
55 the discovery <strong>of</strong> small bright-ray and bright-floor craters within the dark terrain. Moreover, several<br />
56 spectral features <strong>of</strong> the dark material match those seen on Phoebe, Iapetus, Hyperion, Dione and<br />
57 Epimetheus as well as in the F-ring and the Cassini Division, implying that throughout the<br />
58 Saturnian system. All dark material appears to have a high content <strong>of</strong> metallic iron and a small<br />
59<br />
content <strong>of</strong> nano-phase hematite. However, the complete composition <strong>of</strong> the dark material is still<br />
1
60 unresolved, and additional laboratory work is required. As previously concluded for Phoebe, the dark<br />
61 material appears to have originated external to the Saturnian system.<br />
62 The icy satellites <strong>of</strong> Saturn <strong>of</strong>fer an unrivalled natural laboratory for understanding the geological<br />
63<br />
64<br />
diversity <strong>of</strong> different-sized icy satellites and their interactions within a complex planetary system.<br />
65 21.1 Introduction<br />
66 Covering a wide range <strong>of</strong> diameters, the satellites <strong>of</strong> Saturn can be subdivided into three major<br />
67 classes: ‘icy rocks,’ with radii < 10 km, 'small satellites' with radii
Siarnaq 17531 895.53 0.2960 46.002 20 2.3 0.0026 20.1R 0.06<br />
76<br />
77 Tab. 1 Physical and orbital characteristics <strong>of</strong> the small (
134 suggested endogenic activities. However, the nature <strong>of</strong> these activities was not well understood. For<br />
135 a more detailed discussion <strong>of</strong> our knowledge before Cassini, see Dougherty et al. (2009) and Orton<br />
136 et al. (2009). In terms <strong>of</strong> geology, the Cassini mission was designed to provide significantly better<br />
137 area coverage, spatial resolution, and spectral range, resulting in numerous new discoveries. A<br />
138 detailed description <strong>of</strong> the Cassini mission can be found in Dougherty et al. (2009) and Seal et al.<br />
139<br />
140<br />
(2009). Maps <strong>of</strong> the Saturnian satellites are included in Roatsch et al. (2009).<br />
141 21.2 Cassini’s Exploration <strong>of</strong> Saturn’s Icy Satellites<br />
142 The Cassini spacecraft is equipped with instruments tailored to investigate the surfaces,<br />
143 environments and interiors <strong>of</strong> icy satellites. The optical remote sensing (ORS) instrument suite<br />
144 includes cameras and spectrometers designed for high spatial and spectral resolution covering<br />
145 wavelengths between 0.06µm and 1000µm. With the imaging subsystem (ISS) (Porco et al., 2004),<br />
146 morphologic, stratigraphic and other geological surface properties can be observed at spatial<br />
147 resolutions down to a few meters (locally) or a few hundred meters (globally), depending on flyby<br />
148 distances. Moreover, as many as 33 color and polarization filter combinations permit mapping<br />
149 geologically diverse terrain spatially at wavelengths between ~0.3µm and ~1.1µm. The visible and<br />
150 infrared mapping spectrometer (VIMS; 0.4µm to 5.1µm) provides chemical and compositional<br />
151 spectral information with spatial resolutions from a few kilometers (globally) to better than one<br />
152 hundred meters (locally) (Brown et al., 2004; Jaumann et al., 2006). The composite infrared<br />
153 spectrometer (CIRS) determines global and regional surface temperatures and thermal properties on<br />
154<br />
155<br />
a kilometer scale (Flasar et al., 2004). The ultraviolet imaging spectrograph (UVIS) provides<br />
information about thin atmospheres and volcanic plume structures as well as about water ice and<br />
156 other minor constituents on the surface (Esposito et al., 2004), operating in the 60nm - 190nm<br />
157 wavelength range. A suite <strong>of</strong> magnetosphere and plasma science (MAPS) instruments characterizes<br />
158 the satellites’ environments by in-situ methods. Micron-sized dust grains and neutral molecules<br />
159 released from the surface by active or passive processes (volcanoes/sputtering) carry surface<br />
160 composition information to distances as far as hundreds <strong>of</strong> kilometers above the surface. Released<br />
161 material affects the plasma surrounding the satellite and is registered by variations in the magnetic<br />
162 field, ion density, and neutral gas and dust density as well as gas and dusty composition.<br />
163 The primary task <strong>of</strong> the radio science subsystem (RSS) is to determine the mass <strong>of</strong> the moons (Tab.<br />
164 1) by tracking deviations in Cassini's trajectory. RADAR data can provide unique information about<br />
165 the upper sub-surface (Elachi et al., 2004), though few close-up RADAR SAR observations were<br />
166 performed during satellite flybys. The lack <strong>of</strong> a scan platform for the remote sensing instruments<br />
167 prevents the radio science, RADAR and remote sensing systems from operating simultaneously<br />
168 during a flyby because the antenna and remote sensing instruments are oriented 90° apart on the<br />
169 spacecraft. Even joint surface scans by the ORS instruments require significant compromises<br />
170 between individual observations due to data rate and integration time constraints. While the ISS<br />
171 instrument needs short 'dwell' times for mosaicking, the VIMS depend on long 'dwells' for<br />
172 improved signal-to-noise ratios. On the other hand, the CIRS and UVIS instruments contain line-<br />
173 scanning devices, which depend on either slow or fast slews to scan the surfaces. Thus, reaching<br />
174 satisfactory compromises is a major challenge in the planning process.<br />
175 Since the Cassini spacecraft orbits the planet rather than individual satellites, the moons can only be<br />
176<br />
177<br />
observed at various distances and illumination conditions, ideally during very close-targeted flybys.<br />
Depending on the distance <strong>of</strong> a moon to Saturn, the flybys occur at very different velocities. In<br />
178 March 2008, for instance, Enceladus was passed at ~14 km/s, while the Iapetus flyby in September<br />
179 2007 took place at a leisurely 2.4 km/s. The flyby geometry can also vary significantly. The closest-<br />
180 ever approach was during the Enceladus flyby on October 9, 2008, when the spacecraft skimmed as<br />
181 low as 25 km over Enceladus’ surface. Other, more typical targeted flyby altitudes occur between<br />
182 100 km and 2000 km. A flyby can be polar, equatorial, or in between, and at the closest approach,<br />
183 the sub-spacecraft point over a moon can be located either over its illuminated or over its unlit side.<br />
184 The MAPS instruments measure densities or field gradients over time. Sputtering processes lead to<br />
185 high dust and neutral-gas densities above the surface, so that environmental in-situ instruments rely<br />
186 on measurements as close to the surface as possible. Further plasma density variations caused by<br />
187 satellites moving in the magnetosphere are empty flux tubes (wakes) and the drop <strong>of</strong> plasma along<br />
188 the related L-shell. Crossing those regions is <strong>of</strong> high value for in-situ plasma investigations that<br />
189 provide indirect information about the satellite surface and the density <strong>of</strong> its neutral and plasma<br />
190 environment.<br />
191 During the nominal mission, Cassini performed nine targeted as well as numerous close flybys <strong>of</strong><br />
192<br />
icy satellites (Tab. 2).<br />
4
193<br />
Target Orbit Flyby date Altitude<br />
[km]<br />
Solar Phase Angle 2 hrs before,<br />
at, and 2 hrs after C/A [deg]<br />
Phoebe 0 June 11, 2004 1997 85 / 25 / 90 T<br />
Dione B December 15, 2004 72100 108 / 85 / 62 nT<br />
Iapetus B/C December 31, 2004 123399 87 / 94 / 100 nT<br />
Enceladus 3 February 17, 2005 1176 25 / 114 / 158 nT<br />
Enceladus 4 March 9, 2005 500 47 / 42 / 130 T<br />
Hyperion 9 June 10, 2005 168000 25 / 35 / 47 nT<br />
Enceladus 11 July 14, 2005 170 47 / 63 / 132 T<br />
Mimas 12 August 2, 2005 62700 41 / 57 / 91 nT<br />
Tethys 15 September 24, 2005 1503 21 /68 / 155 T<br />
Hyperion 15 September 26, 2005 505 51 / 50 / 127 T<br />
Dione 16 October 11, 2005 500 23 / 66 / 156 T<br />
Iapetus 17 November 12, 2005 415400 89 / 91 / 93 nT<br />
Rhea 18 November 26, 2005 1264 19 / 87 / 161 T<br />
Rhea 22 March 21, 2006 82200 113 / 137 / 157 nT<br />
Iapetus 23 April 11, 2006 602400 123 / 124 / 125 nT<br />
Enceladus 27 September 9, 2006 39900 105 / 118 / 105 nT<br />
Dione 33 November 21, 2006 72300 125 / 144 / 119 nT<br />
Hyperion 39 February 16, 2007 175000 66 / 64 / 64 nT<br />
Tethys 47 June 27, 2007 18400 148 / 116 / 49 nT<br />
Tethys 49 August 29, 2007 55500 135 / 102 / 72 nT<br />
Rhea 49 August 30, 2007 5800 125 / 46 / 41 nT<br />
Iapetus 49 September 10, 2007 1620 139 / 59 / 27 T<br />
Dione 50 September 30, 2007 43400 89 / 49 / 19 nT<br />
Hyperion 51 October 21, 2007 122000 115 / 116 / 115 nT<br />
Mimas 53 December 3, 2007 84200 161 / 139 / 82 nT<br />
Enceladus 61 March 12, 2008 52 115 / 135 / 65 T<br />
Enceladus 80 August 11, 2008 50 108 / 110 / 73 T<br />
Enceladus 88 October 9, 2008 25 108 / 112 / 73 T<br />
Enceladus 91 October 31, 2008 200 108 / 113 / 73 T<br />
Tethys 93 November 16, 2008 57100 79 / 41 / 58 nT<br />
Tethys 94 November 24, 2008 24200 123 / 159 / 84 nT<br />
Tethys 115 July 26, 2009 68400 105 / 89 / 79 nT<br />
Rhea 119 October 13, 2009 40400 140 / 82 / 25 nT<br />
Mimas 119 October 14, 2009 44200 152 / 102 / 44 nT<br />
Hyperion 119 October 16, 2009 127000 155 / 140 / 126 nT<br />
Enceladus 120 November 2, 2009 100 177 / 90 / 5 T<br />
Enceladus 121 November 21, 2009 1600 146 / 87 / 36 T<br />
Rhea 121 November 21, 2009 24400 127 / 58 / 10 nT<br />
Tethys 123 December 26, 2009 52900 136 / 77 / 17 nT<br />
Dione 125 January 27, 2010 45100 158 / 106 / 53 nT<br />
Mimas 126 February 13, 2010 9500 173 / 99 / 21 nT<br />
Rhea 127 March 2, 2010 100 176 / 87 / 3 T<br />
Dione 129 April 7, 2010 500 167 / 79 / 11 T<br />
Enceladus 130 April 28, 2010 100 171 / 93 / 9 T<br />
Enceladus 131 May 18, 2010 200 153 / 108 / 29 T<br />
5<br />
Target [T]/Non<br />
Targeted [nT]
Tethys 132 June 3, 2010 52600 154 / 99 / 45 nT<br />
194<br />
195<br />
196<br />
Tab. 2 Mission characteristics <strong>of</strong> the Cassini icy satellites flybys; C/A ... closest approach<br />
197 Three <strong>of</strong> the targeted flybys were dedicated to Enceladus, two in 2005 and one in 2008. In addition,<br />
198 a very close but non-targeted Enceladus flyby happened in February 2005. As the data provided by<br />
199 the MAPS instruments in March 2005 had raised suspicions about potential internal activity (Spahn<br />
200 et al., 2006; Waite et al., 2006), the July 2005 flyby was lowered from 1000 km to 173 km to allow<br />
201 for increased sensitivity in MAPS measurements. This flyby supplied overwhelming evidence from<br />
202 multiple instruments <strong>of</strong> current activity on Enceladus, eventually confirmed by direct images <strong>of</strong><br />
203 plumes taken during a distant flyby in November 2005. The locations <strong>of</strong> the plumes were found to<br />
204 coincide with linear tectonic features near the south pole, dubbed 'tiger stripes'. The CIRS<br />
205 instrument also identified these 'tiger stripes' as unusually warm linear features (Spencer et al.,<br />
206 2006); the tiger strips contain relatively large ice particles identified by VIMS (Brown et al., 2006;<br />
207<br />
208<br />
Jaumann et al., 2008).<br />
All other moons had only one targeted flyby during the nominal mission, if any. The Tethys,<br />
209 Hyperion, and Dione flybys in the late summer/early fall <strong>of</strong> 2005 were performed within 3 weeks <strong>of</strong><br />
210 each other during the same orbit <strong>of</strong> Cassini around Saturn. Due to Hyperion's chaotic rotation, it<br />
211 was impossible to predict which part <strong>of</strong> its surface would be visible and how the surface might look.<br />
212 By coincidence, the viewing perspective during the approach was almost identical with the best<br />
213 Voyager 1 views. It showed a giant crater, dark material within numerous smaller craters and a very<br />
low mean density <strong>of</strong> only 0.54g/cm 3 214<br />
. The 'wispy streaks' <strong>of</strong> Dione were observed at very oblique<br />
215 viewing angles during the October 2005 flyby, and were confirmed to be tectonic in origin (Wagner<br />
216 et al., 2006). The images from this targeted Dione flyby are among the most spectacular pictures <strong>of</strong><br />
217 the mission. The Rhea flyby in November 2005 was dedicated to radio science, and the results from<br />
218 that flyby are reported in Matson et al. (2009). However, while the spacecraft itself was inertially<br />
219 pointed with its antenna towards <strong>Earth</strong>, the cameras slewed across the surface, obtaining the<br />
220 highest-resolved images <strong>of</strong> Rhea to date.<br />
221 In 2006, no targeted flybys took place, primarily because the spacecraft spent most <strong>of</strong> its time in<br />
222 highly inclined orbits. The very close but non-targeted Rhea flyby in August 2007 was dedicated to<br />
223 remote sensing and revealed small details <strong>of</strong> the prominent bright-ray crater and its ejecta<br />
224 environment. This might be the youngest large crater in the Saturnian system. The targeted Iapetus<br />
225 flyby in September 2007 over the anti-Saturnian and trailing hemispheres yielded very high-<br />
226 resolution observations <strong>of</strong> the highest parts <strong>of</strong> the equatorial ridge, the transition zone between the<br />
227 dark and the bright terrain, and the bright trailing side. Combined with data from a more distant,<br />
228 non-targeted flyby on December 31, 2004, and other distant observations, these data permitted<br />
229 developing a model <strong>of</strong> the formation <strong>of</strong> the global and unique brightness dichotomy <strong>of</strong> this unusual<br />
230 moon (Denk and Spencer, 2008). For more detailed descriptions <strong>of</strong> the Cassini mission, see<br />
231<br />
232<br />
Dougherty et al. (2009), Orton et al. (2009) and Seal et al. (2009).<br />
233 21.3. Morphology, Geology and Topography<br />
234 Voyager’s partial global mapping at 1 km resolution suggested that the morphology <strong>of</strong> the<br />
235 Saturnian satellites was dominated by impact craters (Smith et al., 1981). Indeed, Voyager’s best<br />
236<br />
237<br />
images, 500 m resolution views <strong>of</strong> Rhea, showed a landscape saturated with craters. Voyager also<br />
found evidence <strong>of</strong> limited volcanic and tectonic activity in the form <strong>of</strong> relatively smooth plains and<br />
238 linear features (Smith et al., 1981, 1982; Plescia and Boyce, 1982, 1983; Moore et al., 1985; Moore<br />
239 and Ahren, 1983), especially on Enceladus. The Enceladus discoveries aside, Cassini’s considerably<br />
240 improved resolution and global mapping coverage confirmed the abundance <strong>of</strong> impact craters on all<br />
241 satellites (Dones et al., 2009) and also revealed surprisingly varied degrees <strong>of</strong> past endogenic<br />
242 activity on Tethys, Rhea and especially Dione, indicating that these satellites were more dynamic<br />
243 than originally thought. Observed deformations range from linear grooves (Mimas) and nearly<br />
244 global-scale grabens (Tethys, Rhea) to repeated episodes <strong>of</strong> tectonics and resurfacing (Dione,<br />
245<br />
246<br />
Enceladus). To date, little or no direct evidence <strong>of</strong> past or present endogenic activity has been<br />
identified on Hyperion, Phoebe or Iapetus (other than its potentially endogenic equatorial ridge).<br />
247 This diversity is in itself a puzzle that has not yet been explained. In the following sections, we<br />
248<br />
249<br />
examine the features observed, their morphology, and implications for internal evolution.<br />
250<br />
Craters<br />
6
251 Comparison with craters on larger icy satellites (e.g. Ganymede) is illuminating because surface<br />
252 gravities differ by a factor <strong>of</strong> 5 to 10. Central-peak craters are common on Saturnian satellites.<br />
253 Central peaks in larger craters can be as high as 10 km, many <strong>of</strong> them rising above the local mean<br />
254 elevation. Cassini has revealed little evidence <strong>of</strong> terrace formation, and floor uplift dominates clearly<br />
255 over rim collapse in the formation <strong>of</strong> complex craters (Schenk and Moore, 2007). Pancake (formerly<br />
256 called pedestal) ejecta have been observed in smooth plains on Dione. This kind <strong>of</strong> ejecta deposit,<br />
257 once only known as a feature <strong>of</strong> the Galilean satellites, now appears to be common on icy satellites<br />
258 in general. The fact that it is not obvious on other satellites is most likely due to the difficulty <strong>of</strong><br />
259 identifying it on these rugged, heavily cratered surfaces.<br />
260 A transition to larger, more complex crater landforms (e.g. central pit craters, multi-ring basins) on<br />
261 these satellites is predicted to occur at much larger diameters (>150 km) than those seen on<br />
262 Ganymede and Callisto (Schenk, 1993), due to their weaker gravity. True central-pit craters like<br />
263 those on Ganymede or Callisto (Schenk, 1993) have not been observed, although the central<br />
264 complex <strong>of</strong> Odysseus features a central depression and closely resembles its counterparts on large<br />
265 icy moons. The dominant morphology at larger diameters (>150 km) is peak-ring or similar, with a<br />
266 prominent central peak surrounded by a ring or an elevated circular plateau. The number <strong>of</strong> such<br />
267 basins discovered by Cassini on Iapetus and Rhea was greater than expected. Indeed, at least 10<br />
268 basins larger than 300 km across have been seen on Iapetus (e.g. Giese et al., 2008).<br />
269 The most ancient large basins on Rhea and Tethys have been modified by relaxation and long-term<br />
270 impact bombardment (e.g. Schenk and Moore, 2007). The younger basins are generally deep and<br />
271<br />
272<br />
unrelaxed. However, the floor <strong>of</strong> Evander, is the largest basin identified on Dione, has been relaxed<br />
and uplifted to the level <strong>of</strong> the original surface, and although its rim and peak remain prominent<br />
273 (e.g. Schenk and Moore, 2007). Relaxation <strong>of</strong> this basin is consistent with elevated heat flows on<br />
274 Dione, as indicated by the relaxation <strong>of</strong> smaller (diameters <strong>of</strong> 25-50 km) craters within the extensive<br />
275 smooth plains to the north <strong>of</strong> Evander. Relaxation in this basin, which has a low superposed crater<br />
276 density, is consistent with elevated heat flows, as indicated by the smaller relaxed craters nearby and<br />
277 the extensive smooth plains to the north. The relaxation state <strong>of</strong> impact basins can be determined by<br />
278 comparing their depth to that <strong>of</strong> unmodified craters and examining their shape. The most ancient<br />
279 large basins on Rhea (e.g. Tirawa) and Tethys are roughly 10 to 20% shallower than predicted and<br />
280 feature prominent domical or conical central uplifts that approach or exceed the local elevation <strong>of</strong><br />
281 the satellite (Schenk and Moore, 2007), which suggests they have been modified to some degree by<br />
282 relaxation. The youngest basins appear to be deeper and possibly unrelaxed. The most prominent<br />
283 examples include Herschel on Mimas (diameter ~120 km, depth ~11 km) and Odysseus (diameter<br />
284 ~450 km, depth >8 km). In neither case do the central uplifts rise above the local topography. A<br />
285 prominent exception is the relatively young 400 km wide Evander basin on Dione, which may be<br />
286 either ~3Gyr (Wagner et al., 2006) or less than 2Gyr old (Kirch<strong>of</strong>f and Schenk, 2009). For a more<br />
287 detailed discussion <strong>of</strong> surface ages, see Dones et al. (2009).<br />
288 Iapetus’ leading side shows more relief than the other medium-sized Saturnian satellites (Thomas et<br />
289 al., 2007b; Giese et al., 2008). The lack <strong>of</strong> basin relaxation on Iapetus is consistent with the<br />
290 presence <strong>of</strong> a thick (50–100 km) lithosphere in Iapetus’ early history. Such a lithosphere might also<br />
291 support Iapetus’ prominent equatorial ridge. The ridge shows a complex morphology, probably the<br />
292 result <strong>of</strong> impact erosion in some places and <strong>of</strong> post-formation tectonic modifications in others.<br />
293<br />
294<br />
The abundance <strong>of</strong> large impact basins, even in Voyager’s limited view, suggested that such basins<br />
could have disruptive effects on these satellites (Smith et al., 1981). Several attempts to discover<br />
295 evidence <strong>of</strong> these effects (e.g. seismic shaking, global fracturing) have met with limited success (e.g.<br />
296<br />
297<br />
Moore et al., 2004), but Cassini-based studies are now in progress.<br />
298 Tectonics<br />
299 The Voyager 1 and 2 spacecraft revealed that the medium-sized icy Saturnian satellites have<br />
300 undergone varying degrees <strong>of</strong> tectonic deformation (Smith et al. 1981; 1982; Plescia and Boyce<br />
301 1982, 1983). Although such deformation can provide constraints on the interior structure and<br />
302 evolution <strong>of</strong> a satellite, the imaging resolution was <strong>of</strong>ten insufficient to permit sustainable<br />
303 conclusions. Post-Voyager advances include a vastly better understanding <strong>of</strong> tectonic deformation<br />
304 and thermal evolution in the Jovian system thanks to Galileo, and greatly improved experimental<br />
305 measurements <strong>of</strong> relevant ice properties (Schmitt et al., 1998). In this section, we combine this<br />
306 improved understanding with the results <strong>of</strong> the ongoing Cassini mission to discuss the current state<br />
307 <strong>of</strong> knowledge <strong>of</strong> icy-satellite tectonics in the Saturnian system. We also examine the extent to which<br />
308 cryovolcanic processes may be relevant. Most <strong>of</strong> the topics covered in this section are treated in<br />
309<br />
greater detail in Collins et al. (2009).<br />
7
310 Tectonics on icy satellites comprises three major aspects: (1) the various mechanisms by which ice<br />
311 deforms when subjected to tectonic stress; (2) the mechanisms by which tectonic stress may arise;<br />
312 and (3) the observational constraints on these deformation mechanisms.<br />
313 From a tectonics point <strong>of</strong> view, ice is rather similar to silicate materials: under surface conditions its<br />
314 behavior is rigid or brittle, while at greater depths it is ductile. The rheology <strong>of</strong> ice is complicated<br />
315 and cannot be discussed in depth here; useful discourses on its elastic, brittle and viscous properties<br />
316 may be found in Petrenko and Whitworth (1999), Beeman et al. (1988) and Durham and Stern<br />
317 (2001), respectively. Although a real ice shell will exhibit all three kinds <strong>of</strong> behavior, it can be<br />
318 modeled as a simple elastic layer (e.g. McNutt, 1984). This effective elastic thickness depends on<br />
319 the temperature gradient within the ice shell during deformation and can be inferred from<br />
320 topographical measurements (e.g. Giese et al., 2007). Thus, measuring the elastic thickness places a<br />
321 useful constraint on satellite thermal evolution (e.g. Nimmo et al., 2002). Pressures within Saturnian<br />
322<br />
323<br />
satellites are generally low (see Tab. 3).<br />
R<br />
(km)<br />
Ms<br />
(10 20 kg)<br />
a<br />
(10 3 km)<br />
e H<br />
(m)<br />
3eH<br />
(m)<br />
8<br />
Tidal<br />
(10 10 W)<br />
Rad.<br />
(10 10 W)<br />
Stress<br />
(kPa)<br />
Mimas 198 0.37 186 .0202 9180 556 78 0.013 8400 7.1 I<br />
Enceladus 252 1.08 238 .0045 3940 53 2 0.038 630 23 III<br />
Tethys 533 6.15 295 0. 7270 0 0 0.22 0 37 II<br />
Dione 562 11 377 .0022 2410 16 0.86 0.39 85 97 II<br />
Rhea 764 23 527 .001 1440 4.3 0.067 0.81 17 124 II<br />
Titan 2575 1346 1222 .0292 254 22 45 47 26 3280 -<br />
Iapetus 736 18 3561 .0283 5 0.4 2.6x10 -5 0.63 1.7 88 I/II<br />
324<br />
325<br />
326<br />
Tab. 3 Data from Yoder (1995) and Thomas et al. (2007). R, Ms, a and e are the radius, mass,<br />
semi-major axis, period and eccentricity, respectively. H is the permanent tidal bulge assuming<br />
327<br />
328<br />
329<br />
330<br />
h2=2.5. 3eH is the approximate magnitude <strong>of</strong> the diurnal tidal bulge – note that this is likely an<br />
overestimate for small satellites because their rigidity will reduce h2. “Tidal” is the tidal heat<br />
production assuming a homogeneous body with k2=1.5 (again, a likely overestimate for small<br />
satellites) and Q=100. “Rad” is the radiogenic heat production assuming a chondritic rate <strong>of</strong><br />
3.5x10 -12 331 W/kg. “Stress” is the approximate diurnal tidal stress given by EeH/R where E is<br />
332<br />
333<br />
Young’s modulus (assumed 9 GPa). Pcen is the central pressure, assuming uniform density. “MG” is<br />
the “metamorphic grade”, modified after Johnson (1998), which is a qualitative assessment <strong>of</strong> the<br />
334<br />
335<br />
degree <strong>of</strong> deformation: I=unmodified, II=intermediate, III=heavily modified<br />
336 Thus, although ice transmutes into high-pressure forms at pressures greater than about 0.2GPa (e.g.<br />
337 Sotin et al., 1998), this effect is relevant mainly on Titan.<br />
338 Tectonic features may be used to infer the direction and (sometimes) the magnitude <strong>of</strong> the driving<br />
339 stresses. Observations can thus place constraints on the mechanisms which gave rise to these<br />
340 tectonic features. We briefly discuss several mechanisms which are potential candidates for<br />
341 producing tectonic features.<br />
342 Satellites experience both rotational and tidal deformation. If the rotation rate or the magnitude <strong>of</strong><br />
343<br />
344<br />
the tide changes, the shape <strong>of</strong> the satellite will change as well, generating global-scale patterns <strong>of</strong><br />
stress. A recent compilation <strong>of</strong> the present-day shapes <strong>of</strong> the Saturnian satellites is given by Thomas<br />
345 et al. (2007b); a good general description <strong>of</strong> tidal and rotational satellite deformation is given in<br />
346 Murray and Dermott (1999).<br />
347 If a synchronously rotating satellite’s orbital eccentricity is zero, the tidal bulge will be at a fixed<br />
348 geographic point, and <strong>of</strong> constant amplitude. The maximum amplitude H <strong>of</strong> this static (or<br />
349 permanent) tidal bulge depends on the mass and radius <strong>of</strong> the satellite, the mass <strong>of</strong> the primary, the<br />
350<br />
351<br />
352<br />
distance between them, and the rigidity <strong>of</strong> the satellite. The latter is described by the h2 Love<br />
number, where h2 has a value <strong>of</strong> 5/2 for a fluid body <strong>of</strong> uniform density which, however, declines as<br />
the rigidity (or shear modulus) µ <strong>of</strong> the satellite increases (e.g. Murray and Dermott, 1999).<br />
353 In a synchronous satellite with non-zero orbital eccentricity, the amplitude and direction <strong>of</strong> the<br />
354 static tidal bulge will change slightly, resulting in a time-varying diurnal tide. Causing diurnal<br />
355 stresses, this tide has an amplitude expressed by 3eH, where e is the eccentricity and H is the<br />
356 equilibrium tidal bulge (e.g. Greenberg and Geissler, 2002). Obliquity librations are potentially an<br />
357 additional source <strong>of</strong> tidal stress (Bills and Ray, 2000); they can be large if their period is a small<br />
358<br />
multiple <strong>of</strong> the orbital period (Wisdom, 2004).<br />
Pcen<br />
(MPa)<br />
MG
359 Table 3 shows the diurnal tidal stresses <strong>of</strong> the icy Saturnian satellites at zero rigidity, demonstrating<br />
360 the general decrease in stress as the semi-major axis lengthens. For comparison, the diurnal stresses<br />
361 on Europa thought to be responsible for cycloidal features are a few tens <strong>of</strong> kPa (Hoppa et al.,<br />
362 1999a). Because the principal stresses rotate during each orbit, contraction, extension and mixed-<br />
363 mode horizontal principal stresses which promote strike-slip motions are all possible as a satellite<br />
364 orbits (Hoppa et al., 1999b). This motion can lead to both shear heating (Nimmo and Gaidos, 2002)<br />
365 and the monotonic accumulation <strong>of</strong> strike-slip <strong>of</strong>fsets (Hoppa et al., 1999b).<br />
366 A satellite surface may move longitudinally with respect to the static tidal bulge due to tidal<br />
367 (Greenberg and Weidenschilling, 1984) or atmospheric torques (Lorenz et al., 2008) or, in the case<br />
368 <strong>of</strong> floating ice shells, if the thermally equilibrated ice shell thickness variations are not compatible<br />
369 with rotational equilibrium (Ojakangas and Stevenson, 1989a). This non-synchronous rotation leads<br />
370 to surface stresses (Helfenstein and Parmentier, 1985). Stresses from non-synchronous rotation<br />
371 increase with the angular speed <strong>of</strong> rotation but will dissipate with time owing to viscous relaxation<br />
372 (Greenberg and Weidenschilling, 1984; Wahr et al., 2008). Maximum tensile stress depends on the<br />
373 amount <strong>of</strong> reorientation and the degree <strong>of</strong> flattening, f, which depends in turn on the satellite<br />
374 rotation rate (Leith and McKinnon, 1996). In non-rigid satellites, the stresses due to non-<br />
375 synchronous rotation will exceed the diurnal tidal stresses for a reorientation typically in excess <strong>of</strong><br />
376 roughly one degree. In certain circumstances, the surface <strong>of</strong> a satellite may reorient relative to its<br />
377 axis <strong>of</strong> rotation (Willemann, 1984; Matsuyama and Nimmo, 2008) and give rise to global fracture<br />
378 patterns (e.g. Schenk et al., 2008). This process <strong>of</strong> true polar wander is conceptually very similar to<br />
379<br />
380<br />
non-synchronous rotation (Melosh, 1980; Leith and McKinnon, 1996), and 'reorientation' will in the<br />
following be understood as implying either non-synchronous rotation or true polar wander. In<br />
381 general, rotation axis motion occurs roughly perpendicular to the tidal axis because this path is<br />
382 energetically favored (Matsuyama and Nimmo, 2007). True polar wander can occur because <strong>of</strong> ice<br />
383 shell thickness variations (Ojakangas and Stevenson, 1989b), long-wavelength density anomalies<br />
384 (Janes and Melosh, 1988; Nimmo and Pappalardo, 2006), volatile redistribution (Rubincam, 2003)<br />
385 or impacts, either directly (Chapman and McKinnon, 1986) or due to the creation <strong>of</strong> a new impact<br />
386 basin (Melosh, 1975; Murchie and Head, 1986; Nimmo and Matsuyama, 2007). Like non-<br />
387 synchronous rotation, the maximum stress developed by true polar wander depends on the amount<br />
388 <strong>of</strong> reorientation and the effective flattening f, with the value <strong>of</strong> f depending on whether any<br />
389 reorientation <strong>of</strong> the tidal axis has occurred (Matsuyama and Nimmo, 2008).<br />
390 Satellites that are initially rotating at a rate faster than synchronous will despin (i.e. reduce their<br />
391 rotation rate) to synchronous rotation in timescales that are generally very short compared to the age<br />
392 <strong>of</strong> the solar system (e.g. Murray and Dermott, 1999). Spin rate reductions result in shape changes<br />
393 and stresses – compressive at the equator as the equatorial bulge collapses, and tensile at the poles<br />
394 as the poles elongate. The maximum differential stress caused by despinning in a hydrostatic body<br />
395 depends mainly on the satellite radius and the difference between the initial and the final angular<br />
396 rotation velocities (Melosh, 1977). Spin-up may occur in some circumstances (e.g. if the satellite<br />
397 undergoes differentiation), and in this case the signs <strong>of</strong> all the stresses are reversed. Stresses along<br />
398 lines <strong>of</strong> longitude are always larger than the stresses along lines <strong>of</strong> latitude. Thus, irrespective <strong>of</strong><br />
399 whether a satellite is spinning down or spinning up, equatorial tectonic features are expected to be<br />
400 oriented in a north-south direction. This result is important when considering the origin <strong>of</strong> the<br />
401<br />
402<br />
global equatorial ridge on Iapetus.<br />
Synchronous satellites, which are evolving outwards due to tidal torques from the primary, will<br />
403 undergo both a reduction in their spin rate and a parallel reduction in the tidal bulge amplitude. This<br />
404 combination <strong>of</strong> despinning and tidal bulge reduction causes a stress pattern in which a region<br />
405 around the sub-planet point experiences compressive stress, middle latitudes experience horizontal<br />
406 shear stress, and the poles undergo tension (Melosh, 1980; Helfenstein and Parmentier, 1983). The<br />
407 maximum principal stress difference is similar to that produced by despinning alone (Melosh,<br />
408 1980).<br />
409 A large variety <strong>of</strong> mechanisms can lead to volume changes within a satellite, and thus to extensional<br />
410 or contractional features on the surface (Squyres and Cr<strong>of</strong>t, 1986; Kirk and Stevenson, 1987;<br />
411 Mueller and McKinnon, 1988). Volume changes generate isotropic stress fields on the surface. A<br />
412 potentially important source <strong>of</strong> expansion or contraction is the large density contrast between ice I<br />
413 and water. As water freezes to ice I, large surface extensional stresses may result (Cassen et al.,<br />
414 1979; Smith et al., 1981; Nimmo, 2004a). The reason for this effect is that, on a spherical body, the<br />
415 increased volume <strong>of</strong> ice relative to water drives the ice shell outwards; and this increase in radius<br />
416<br />
results in extension. A fractional change in radius <strong>of</strong> 0.1% gives rise to stresses <strong>of</strong> about 10MPa.<br />
9
417 This effect is much reduced if there are high-pressure ice phases freezing simultaneously (Squyres,<br />
418 1980; Showman et al., 1997).<br />
419 Similar but smaller effects are caused by thermal expansion or contraction (Ellsworth and Schubert,<br />
420 1983; Hillier and Squyres, 1991; Showman et al., 1997; Nimmo, 2004a). A global temperature<br />
change <strong>of</strong> 100K will cause stresses on the order <strong>of</strong> 100 MPa for a thermal expansivity <strong>of</strong> 10 -4 K -1 421 .<br />
422 Warming may also lead to silicate dehydration (e.g. Squyres and Cr<strong>of</strong>t, 1986), which in turn causes<br />
423 expansion.<br />
424 Satellites are quite likely to have non-axisymmetric structures, in which case some <strong>of</strong> the above-<br />
425 mentioned global mechanisms may lead to local deformation. For instance, local zones <strong>of</strong> weakness<br />
426 or pre-existing structures can result in enhanced tidal dissipation and deformation (e.g. Sotin et al.,<br />
427 2002; Nimmo, 2004b) and/or the alteration <strong>of</strong> local stress trajectories. Convection (thermal or<br />
428 compositional) and buoyancy forces due to lateral shell thickness variations can generate local<br />
429 deformation. They are discussed at greater length in Collins et al. (2009). We will not discuss these<br />
430 processes further here, as they do not tend to produce globally organized tectonic structures.<br />
431 Large impact events can also potentially create global fracture networks. This mechanism is implicit<br />
432 in the model presented by E. Shoemaker in Smith et al. (1981) for the catastrophic breakup and re-<br />
433 accretion <strong>of</strong> the medium-sized icy satellites <strong>of</strong> Saturn. Presumably, impacts not quite large enough<br />
434 to completely disrupt a satellite could do considerable damage in the form <strong>of</strong> global fracturing, and<br />
435 post-Voyager studies <strong>of</strong> these satellites focused in part on attempts to find evidence <strong>of</strong> an incipient<br />
436 break-up <strong>of</strong> this kind. For instance, the surface <strong>of</strong> Mimas is crossed by a number <strong>of</strong> linear-to-<br />
437<br />
438<br />
439<br />
arcuate, sub-parallel troughs (Fig. 1) that are thought to be the consequence <strong>of</strong> global-scale fractures<br />
during the Herschel impact event (McKinnon, 1985; Schenk, 1989a).<br />
440 Fig. 1 Linear troughs on Mimas, extending east to west. These troughs are interpreted as fractures,<br />
441 possibly related to the Herschel crater. Orthographic map projection at 400m/pixel is centered on<br />
442 the antipode to Herschel, the largest impact basin on Mimas.<br />
443<br />
444 Voyager and Cassini mapping indicates that these features, if related to Herschel, do not form a<br />
445 simple radial or concentric pattern. Nor has it been demonstrated that any <strong>of</strong> these features form on<br />
446 a plane intersecting Herschel, or whether they should do so. It thus remains possible that these are<br />
447 random fractures formed during freeze expansion <strong>of</strong> the Mimas interior. Detailed mapping remains<br />
448 to be done to test these hypotheses. Mimas does not otherwise exhibit endogenic landforms and<br />
449 will, therefore, not be discussed further.<br />
450 Enceladus shows the greatest tectonic deformation <strong>of</strong> any <strong>of</strong> the Saturnian satellites, and very<br />
451 significant spatial variations in surface age (Smith et al., 1982; Kargel and Pozio, 1996; Porco et al.,<br />
452 2006; Jaumann et al., 2008). Spencer et al. (2009) discusses the tectonic behavior <strong>of</strong> Enceladus in<br />
453 much greater detail; only a very brief summary is given here.<br />
454 Three different terrain types exist on Enceladus. Ancient cratered terrains cover a broad band from<br />
0 to 180 o longitude. Centred on 90 o and 270 o 455<br />
longitude there are younger, deformed terrains roughly<br />
90 o wide at the equator. The southern polar region south <strong>of</strong> 55 o 456<br />
S is heavily deformed and almost<br />
457 uncratered. The most prominent tectonic features <strong>of</strong> this region are the linear depressions called<br />
458 ‘tiger stripes’ (Porco et al., 2006). These tiger stripes are typically ~500m deep, ~2 km wide, up to<br />
459<br />
460<br />
~130 km long and spaced ~35 km apart. Crosscutting fractures and ridges with almost no<br />
superimposed impact craters characterize the area between the stripes. The tiger stripes are<br />
461 apparently the source <strong>of</strong> the geysers observed to emanate from the south polar region (Spitale and<br />
462 Porco, 2007). The energy source <strong>of</strong> these geysers is unknown, but it may be shear heating generated<br />
463 by strike-slip motion at the tiger stripes (Nimmo et al., 2007). The coherent (but latitudinally<br />
464 asymmetrical) global pattern <strong>of</strong> deformation on Enceladus is currently unexplained, but it is most<br />
465 probably due to shape changes, perhaps related to a hypothetical episode <strong>of</strong> true polar wander<br />
466 (Nimmo and Pappalardo, 2006).<br />
467<br />
468<br />
Nearly encircling the globe, Ithaca Chasma is the most prominent feature on Tethys (Fig. 2).<br />
469 Fig. 2 Tethys and Ithaca Chasma. Topography was derived from Cassini stereo images and has a<br />
470 horizontal resolution <strong>of</strong> 2-5 km and a vertical accuracy <strong>of</strong> 150-800m. Pr<strong>of</strong>iles have superimposed<br />
471<br />
472<br />
model flexural pr<strong>of</strong>iles (in red). Te is elastic thickness. (Reproduced from Giese et al. (2007))<br />
473 It extends approximately 270° around Tethys and is not a complete circle. It is narrowly confined to<br />
474 a zone which lies along a large circle whose pole is only ~20° from the center <strong>of</strong> Odysseus, the<br />
475<br />
relatively fresh largest basin on the satellite which is 450 km wide (Smith et al., 1982; Moore and<br />
10
476 Ahern, 1983). Smith et al. (1981) suggested that Ithaca Chasma was formed by freeze-expansion <strong>of</strong><br />
477 Tethys's interior. They noted that if Tethys was once a sphere <strong>of</strong> liquid water covered by a thin solid<br />
478 ice crust, freezing in the interior would have produced an expansion <strong>of</strong> the surface comparable to<br />
479 the area <strong>of</strong> the chasm (~10% <strong>of</strong> the total satellite surface area). This hypothesis fails to explain why<br />
480 the chasm occurs only within a narrow zone; besides, it is difficult to account for the geophysical<br />
481 energy needed for the presence <strong>of</strong> a molten interior. Expansion <strong>of</strong> the satellite's interior should have<br />
482 caused fracturing over the entire surface in order to effectively relieve stresses in a rigid crust<br />
483 (Moore and Ahern, 1983).<br />
484 The nearly concentric geometrical relationship between Odysseus and Ithaca Chasma prompted<br />
485 Moore and Ahern (1983) to suggest that the trough system was an immediate manifestation <strong>of</strong> the<br />
486 impact event, perhaps caused by a damped, whole-body oscillation <strong>of</strong> the satellite. Based on the<br />
487 deep topography <strong>of</strong> Odysseus, Schenk (1989b) also suggested that it had not undergone substantial<br />
488 relaxation, and that Ithaca Chasma was the equivalent <strong>of</strong> a ring graben formed during the impact<br />
489 event by a prompt collapse <strong>of</strong> the floor involving a large portion <strong>of</strong> the interior. In a Cassini-era<br />
490 study, Giese et al. (2007) doubted the Odysseus-related hypothesis (Moore and Ahern, 1983;<br />
491 Schenk, 1989b) on the basis <strong>of</strong> crater densities within Ithaca Chasma relative to Odysseus, from<br />
492 which they concluded that Ithaca Chasma is older than Odysseus and therefore could not have<br />
493 influenced its formation. They went on to measure photogrammetrically the raised flanks <strong>of</strong> Ithaca<br />
494 Chasma, which they attributed to flexural uplift. Assuming that these raised flanks were indeed due<br />
495 to flexural uplift, they derived a mechanical lithospheric thickness <strong>of</strong> ~20 km and surface heat<br />
fluxes <strong>of</strong> 18-30 mW/m 2 at assumed strain rates <strong>of</strong> 10 -17 to 10s -1 496<br />
497<br />
. They concluded that Ithaca Chasma<br />
is an endogenic tectonic feature but could not explain why it is confined to a narrow zone<br />
498 approximating a large circle.<br />
499 Several Voyager-era studies <strong>of</strong> Dione (Plescia, 1983; Moore, 1984) noted a near-global network <strong>of</strong><br />
500 tectonic troughs and the presence <strong>of</strong> a smooth plain crossed by both ridges and troughs located near<br />
501 the 90°W longitude and extending eastward. Cassini coverage revealed the westward extension <strong>of</strong><br />
502 this plain (Wagner et al., 2006) and demonstrated that the fractured and the cratered dark plains are<br />
503 spectrally distinct (Stephan et al., 2008). Like the eastern portion observed by Voyager, the western<br />
504 plain also exhibits both ridges and troughs (Fig. 3).<br />
505<br />
Fig. 3 (a) Image mosaic <strong>of</strong> Dione, centered on 180 o 506<br />
longitude. Red and white arrows point to Amata<br />
507 and Turnus impact structures, respectively. (b) Topography <strong>of</strong> Dione, from Schenk and Moore<br />
508 (2007). An ancient degraded impact basin centered just west <strong>of</strong> Amata can be identified by the two<br />
509 concentric rings in the DEM (white arrows). DEM has a dynamic range <strong>of</strong> 8 km (red is highest,<br />
510<br />
511<br />
magenta is lowest).<br />
512 However, the ridge system is much more pronounced. The western ridges are planimetrically<br />
513 narrower and morphologically better expressed (e.g. their relative lack <strong>of</strong> degradation). These<br />
514 western ridges also conform much more closely to the boundary between the plain and the older<br />
515 cratered rolling terrain to the west. The linear to arcuate troughs interpreted to have been created by<br />
516 tectonic extension form a complex network that divides Dione’s surface into large polygons (Smith<br />
517 et al., 1981, 1982; Plescia, 1983; Moore, 1984). The troughs <strong>of</strong>ten form parallel to sub-parallel sets,<br />
518<br />
519<br />
somewhat reminiscent <strong>of</strong> the grooved terrain on Ganymede. The global orientation <strong>of</strong> Dione’s<br />
tectonic fabric is non-random (Moore, 1984) and exhibits patterns consistent with a decline in<br />
520 oblateness due to either despinning or orbital recession (Melosh 1977, 1980).<br />
521 The topography derived from Cassini images <strong>of</strong> Dione shows one clear case <strong>of</strong> tectonic orientation<br />
522 regionally influenced by a preexisting but very degraded large impact basin centered on the smaller<br />
523 crater Amata (Schenk and Moore, 2007 (Figs. 3 and 4)).<br />
524 Fig. 4 Fracture zones on Dione. Global base map <strong>of</strong> Cassini imaging has a resolution <strong>of</strong> 400m. The<br />
525 circle represents the outer ridge corresponding to the rim <strong>of</strong> the degraded basin near Amata shown<br />
526<br />
527<br />
in Fig. 3.<br />
528 Moore et al (2004) identified another example <strong>of</strong> tectonic lineament orientation associated with an<br />
529 unambiguously identified impact feature, 90 km Turnus. Such crater-focused tectonics has been<br />
530 identified on Ganymede from Galileo data (e.g. Pappalardo and Collins, 2005). This crater-<br />
531 lineament relationship on Dione is the most pronounced among any <strong>of</strong> the middle-sized icy<br />
532 satellites except Enceladus (Spencer et al., 2009). Moore (1984) proposed a tectonic history <strong>of</strong><br />
533<br />
extension followed by (at least regional) compression, the latter being responsible for the ridges.<br />
11
534 Cassini-era images <strong>of</strong> the tectonic troughs (mostly grabens) show them to be morphologically fresh,<br />
535 implying that extension continued into geologically recent times (Wagner et al., 2006).<br />
536 After Voyager, Rhea was held to be the least endogenically evolved satellite larger than 1000 km in<br />
537 diameter. Moore et al. (1985) pointed out a few parallel lineaments and noted several large ridges or<br />
538 scarps (which they termed ‘megascarps') and speculated that these features were formed by a period<br />
539 <strong>of</strong> extension followed by compression. Cassini images, and digital terrain models derived from<br />
540 them, <strong>of</strong> the trailing hemisphere <strong>of</strong> Rhea seen from both middle northern and far southern latitudes<br />
541 clearly show that the wispy arcuate albedo markings seen in Voyager images are associated with a<br />
542 graben/extensional fault system dominantly trending north-south (Moore and Schenk, 2007;<br />
543<br />
544<br />
Wagner et al. 2007) (Fig. 5a).<br />
545 Fig. 5 (a) Cassini image <strong>of</strong> the northern trailing hemisphere <strong>of</strong> Rhea showing the bright scarps <strong>of</strong><br />
546 the northern extension <strong>of</strong> the graben system and their relationship to 'wispy' terrain. (b) Mosaic <strong>of</strong><br />
547 Cassini images showing curvilinear grabens (black arrows) and an orthogonal set <strong>of</strong> ridges and<br />
548<br />
549<br />
troughs (white arrow). Base mosaic resolution is 1 km/pixel.<br />
550 This is the first identification <strong>of</strong> unambiguous regional endogenic activity on Rhea. The digital<br />
551 terrain models <strong>of</strong> Rhea derived from Cassini images also show a set <strong>of</strong> roughly north-south trending<br />
552<br />
553<br />
ridges (Figs. 5b, and 6) that correspond to the 'megascarps' <strong>of</strong> Moore et al. (1985).<br />
554<br />
555<br />
556<br />
Fig. 6 Preliminary stereo topography map, from Schenk and Moore (2007). White arrows indicate<br />
graben systems. The DEM has a dynamic range <strong>of</strong> 10 km (red is highest, magenta is lowest).<br />
557 If these ridges are in fact due to compressional tectonics, their geometrical relation to the<br />
558 graben/scarp system may also indicate a genetic relationship. However, the poor morphological<br />
559 presentation <strong>of</strong> these ridges implies that they are old, while the 'wispy terrain' grabens are fresh and<br />
560 presumably much more recently formed.<br />
561 While the troughs and ridges suggest mainly extensional and (minor) compressional tectonics<br />
562 (Thomas, 1988), the en-echelon pattern <strong>of</strong> the scarps and troughs on the trailing hemisphere<br />
563 suggests shear stress (Wagner et al., 2007). This pattern may have been influenced by the possible<br />
564 presence <strong>of</strong> a large, degraded impact basin (basin C <strong>of</strong> Moore et al., 1985).<br />
565<br />
566<br />
The equatorial ridge <strong>of</strong> Iapetus (Fig. 7) is the most enigmatic feature on any <strong>of</strong> the medium-sized<br />
567 Fig. 7 Topography <strong>of</strong> Iapetus as derived from stereo Cassini imaging. Note the sharply defined<br />
568 equatorial ridge. The DEM at left is referenced to a sphere, the DEM at right to a biaxial ellipsoid.<br />
569<br />
570<br />
(Adapted from Giese et al., 2008)<br />
571 icy satellites (Porco et al., 2005a; Giese et al., 2008); it extends across more than half <strong>of</strong> Iapetus'<br />
572 circumference (Giese et al., 2008). Different parts show very different morphologies, from<br />
573 trapezoidal to triangular cross-sections with steep walls and a sharp rim to isolated mountains with<br />
574 moderate slopes and rounded tops (Giese et al., 2008). Although the ridge is variable in height, it<br />
575 rises up to 10 km above the local mean but also has a gently sloping flank. The ridge is abundantly<br />
576<br />
577<br />
cratered, indicating that it is comparable in age to other terrains on Iapetus (Schmedemann et al.,<br />
2008). Although the equatorial location <strong>of</strong> the ridge might be due to despinning (Castillo-Rogez et<br />
578 al., 2007), it represents such a large mass that, if it had formed in another location, it would almost<br />
579 certainly have reoriented to the equator anyway. However, as already mentioned above, irrespective<br />
580 <strong>of</strong> whether a satellite is spinning down or spinning up, equatorial tectonic features can be expected<br />
581 to be oriented north-south, making it difficult to explain the ridge with despinning. One possibility<br />
582 is that despinning stresses combined with lithospheric thickness variations due to convection may<br />
583 have resulted in equatorial extension and diking (Roberts and Nimmo, 2009; Melosh and Nimmo,<br />
584 2009). Other proposals suggest that the morphology <strong>of</strong> the ridge indicates that the surface was<br />
585 warped up by tectonic faulting (Giese et al., 2008), that it is a result <strong>of</strong> extensional forces acting<br />
586 above an ascending current <strong>of</strong> solid-state convection from a two-cell convection pattern<br />
587 (Czechowski and Leliwa-Kopystyński, 2008), or that it was formed by the impact <strong>of</strong> an ancient<br />
588 debris disk on Iapetus (Ip, 2006). The great apparent elastic thickness suggested by the survival <strong>of</strong><br />
589 the ridge may be used to place constraints on the thermal evolution <strong>of</strong> Iapetus (Castillo-Rogez et al.,<br />
590 2007).<br />
591 Although the global shape <strong>of</strong> Iapetus does suggest an earlier, more rapid spin rate (Castillo-Rogez et<br />
592<br />
al., 2007), despinning cannot explain the equatorial ridge (see above): stresses in the north-south<br />
12
593 direction are always smaller than those in the east-west direction, and will result in N-S oriented<br />
594 features (Melosh, 1977). Thus, the origin <strong>of</strong> the equatorial ridge is currently a mystery.<br />
595 The deformation <strong>of</strong> Saturnian satellites varies wildly, from currently active and heavily deformed<br />
596 Enceladus to nearby Mimas, which is tectonically uninteresting. Nor does the degree <strong>of</strong> deformation<br />
597 correlate in any straightforward way with predicted tidal stresses. However, there are at least two<br />
598 global yet somewhat contradictory aspects, which may be identified. On the one hand, coherent<br />
599 global tectonic patterns are evident on Dione, Tethys and Enceladus and to a lesser extent on Rhea<br />
600 and Iapetus. On the other, several satellites, especially Tethys and Enceladus, show very<br />
601 pronounced spatial variations in the extent <strong>of</strong> deformation. These two aspects suggest that global<br />
602 sources <strong>of</strong> stress are common, but also that lateral variations in the mechanical properties <strong>of</strong> the<br />
603 satellites’ ice shells play an important role.<br />
604 Regarding this second point, localization <strong>of</strong> deformation (e.g. at the south pole <strong>of</strong> Enceladus or at<br />
605 Ithaca Chasma) may arise naturally in ice shells (e.g. Sotin et al., 2002; Nimmo, 2004b) but is<br />
606 currently poorly understood and makes modeling tectonic deformation much harder. As far as<br />
607 global deformation patterns are concerned, several satellites (Enceladus, Iapetus, perhaps Dione)<br />
608 show patterns with simple symmetries about the rotational and tidal axes. This symmetry may be<br />
609 due either to feature reorientation or the operation <strong>of</strong> mechanisms (tides, despinning etc.), which<br />
610 have a natural symmetry. On Dione, Rhea and Iapetus at least, diurnal tidal stresses are rather small<br />
611 (Tab. 3), and most <strong>of</strong> the tectonic features are apparently ancient, which suggests that despinning,<br />
612 volume changes or reorientation are more likely to be responsible for the observed deformation than<br />
613<br />
614<br />
present-day tidal stresses.<br />
As in the Galilean satellites, extensional features are generally more common than compressional<br />
615 features on the other Saturnian satellites. This observation is readily explained if we assume that the<br />
616 satellites once contained subsurface oceans which progressively froze (e.g. Smith et al., 1981;<br />
617 Nimmo, 2004a); alternative explanations are that compressional features such as folds (Prockter and<br />
618 Pappalardo, 2000) are less easy to recognize than extensional features, or that larger stresses are<br />
619<br />
620<br />
required to form compressional features (see Pappalardo and Davis, 2007).<br />
621 Cryovolcanism<br />
622 Cryovolcanism can be defined as 'the eruption <strong>of</strong> liquid or vapor phases (with or without entrained<br />
623 solids) <strong>of</strong> water or other volatiles that would be frozen solid at the normal temperature <strong>of</strong> the icy<br />
624 satellite’s surface' (Geissler, 2000). Cryovolcanism therefore includes effusive eruptions <strong>of</strong> icy<br />
625 slurries and explosive eruptions consisting primarily <strong>of</strong> vapor (Fagents, 2003). Voyager-era data<br />
626 suggested that such activity might be common in the outer solar system, resulting in a flurry <strong>of</strong><br />
627 theoretical papers (e.g. Stevenson, 1982; Allison and Clifford, 1987; Crawford and Stevenson,<br />
628 1988; Kargel. 1991; Kargel et al., 1991). Higher-resolution images from Galileo generally revealed<br />
629 tectonic rather than cryovolcanic resurfacing processes, although some features may be due to<br />
630 cryovolcanic activity (e.g. Schenk et al., 2001; Fagents et al., 2000; Miyamoto et al., 2005). In the<br />
631 following, we discuss the current state <strong>of</strong> knowledge about cryovolcanism in the Saturnian<br />
632 satellites. Enceladus, which is definitely cryovolcanically active (Porco et al., 2006), and Titan,<br />
633 which may be (Lopes et al., 2007; Nelson et al., 2009a,b), are discussed by Spencer et al. (2009),<br />
634 Jaumann et al. (2009) and Sotin et al. (2009).<br />
635<br />
636<br />
The principal difference between cryovolcanism and silicate volcanism is that in the former, the<br />
melt is denser than the solid (by ~10%). This means that special circumstances must exist for an<br />
637 eruption <strong>of</strong> non-vapor cryovolcanic materials to occur (though <strong>of</strong> course on <strong>Earth</strong> it is not<br />
638 uncommon for denser basaltic lavas to erupt through lighter granitic material). Such circumstances<br />
639 include one or more <strong>of</strong> the following: (1) the melt includes dissolved volatiles, which exsolve and<br />
640 reduce the bulk melt density (Crawford and Stevenson, 1988); (2) the melt is driven upwards by<br />
641 pressure differences caused by tidal effects (Greenberg et al., 1998), surface topography (Showman<br />
642 et al., 2004) or freezing (Fagents, 2003; Manga and Wang, 2007); (3) compositional differences<br />
643 render the ice shell locally dense or the melt less dense (e.g. by the addition <strong>of</strong> ammonia) (Kargel<br />
644 1992, 1995). The latter mechanism can be nothing more than a local effect, since if the bulk ice<br />
645 shell is denser than the underlying material; it will sink if the ice shell is disrupted.<br />
646 Erupted material consisting primarily <strong>of</strong> water plus solid ice will simultaneously freeze and boil, the<br />
647 latter process ceasing once a sufficiently thick ice cover is formed (Allison and Clifford, 1987). The<br />
648 resulting flow velocity will depend on the viscosity <strong>of</strong> the ice-water slurry (Kargel et al., 1991) as<br />
649 well as on flow thickness and local slope (Wilson et al., 1997; Miyamoto et al., 2005). If the erupted<br />
650 material is dominated by the vapor phase, the vapor plume will expand outwards (Tian et al., 2007)<br />
651<br />
and escape entirely if the molecular thermal velocities exceed the escape velocity. Any associated<br />
13
652 solid material will follow ballistic trajectories, ultimately forming deposits centered on the eruption<br />
653 vents (Fagents et al., 2000).<br />
654 The plume properties measured on Enceladus by Cassini imply considerably smaller velocities for<br />
655 icy dust grains than for vapor. On hypothesis is that the dust grains condense and grow in vertical<br />
656 vents <strong>of</strong> variable width. Repeated wall collisions and re-acceleration by the gas in the vents lead to<br />
657 effective friction, which explains the observed plume properties (Schmidt et al., 2008). ). Most dust<br />
658 grains are growing by direct condensation <strong>of</strong> the ascending water vapor within the ice channels and<br />
659 contain almost pure water ice; they are free <strong>of</strong> atomic sodium. In 2008 the CDA instrument while<br />
660 analyzing the composition <strong>of</strong> E-ring particles discovered a second type <strong>of</strong> icy dust particles. These<br />
661 grains are rich in sodium salts (0.5 - 2% in mass) and there origin is explained by the existence <strong>of</strong><br />
662 an ocean <strong>of</strong> liquid water below the surface <strong>of</strong> Enceladus and which is in direct contact with its rock<br />
663 core (Postberg et al., 2009). This shows that cryovolcanism can provide valuable information about<br />
664 geological conditions <strong>of</strong> the interior.<br />
665 The plains <strong>of</strong> Dione are another candidate for cryovolcanic resurfacing (Fig. 8). Several Voyager-<br />
666<br />
667<br />
era studies concluded that these plains were<br />
668 Fig. 8 Cassini view <strong>of</strong> smooth plains on Dione. Older heavily cratered terrains lie to the west. Note<br />
669 the prominent north-south ridge and the subtle linear features to the east. Mosaic resolution is<br />
670<br />
671<br />
400m/pixel.<br />
672<br />
673<br />
formed in this way (e.g. Plescia and Boyce, 1982; Plescia, 1983; Moore, 1984). Moore (1984)<br />
suggested that several branching broad-rimmed troughs north <strong>of</strong> the equator at 60°W may have<br />
674 been the source vents. Surprisingly, the plains unit may be topographically higher than the cratered<br />
675 terrain (Moore and Schenk, 2007). This observation seems a little difficult to reconcile with the<br />
676 plains being emplaced as a low-viscosity cryovolcanic flow. One possibility is that the plains<br />
677 material was emplaced as a deformable plastic unit (like terrestrial glaciers), but this, or any other,<br />
678 explanation needs both further evidence and thought. The ridges discussed above bound (or at least<br />
679 occur near the edges) the plains unit in the west and southeast and may be related to their formation.<br />
680 A plains unit on Tethys is located antipodal to the very large Odysseus impact feature. Moore and<br />
681 Ahern (1983) proposed that the plains were formed by cryovolcanism. Alternatively, Moore et al.<br />
682 (2004) considered the possibility that it was extreme seismic shaking caused by energy focused<br />
683 there by the Odysseus impact that formed the antipodal plains. If seismic shaking was responsible,<br />
684 there would probably be a significant transition zone between that unit and the adjacent hilly and<br />
685 cratered terrain. Digital terrain models derived from Cassini images indicate that the transition<br />
686 between the cratered terrain and the plains is abrupt and the plains themselves are level, supporting<br />
687 the hypothesis that they are endogenic (Moore and Schenk, 2007).<br />
688 Both Tethys and Dione show evidence for weak plasma tori (Burch et al., 2007) somewhat<br />
689 analogous to the much more marked feature at Enceladus, which is caused by geysers (Porco et al.,<br />
690 2005). It is not yet clear whether surface activity or some other process is generating these tori.<br />
691 Similarly, Clark et al. (2008a) report a possible, but currently unconfirmed, detection <strong>of</strong> material<br />
692 around Dione, which may also indicate some surface activity. The Cassini magnetometer has also<br />
693 measured small amounts <strong>of</strong> mass loading near Dione (Burch et al., 2007).<br />
694<br />
695<br />
So far, none <strong>of</strong> the digital terrain models <strong>of</strong> Rhea developed in the Cassini era shows any regions<br />
that would qualify as plains, supporting the perception that Rhea shows no record <strong>of</strong> cryovolcanic<br />
696 resurfacing. However, unlike Dione, the diffuse nature <strong>of</strong> the wisps (the 'wispy terrain') on Rhea has<br />
697 yet to be explained as numerous smaller bright scarps associated with fracture and faulting, and may<br />
698 yet be shown to be an expression <strong>of</strong> cryo-pyroclastic mantling (Stevenson, 1982). Furthermore,<br />
699 Rhea, surprisingly, exhibits evidence for faint particle ring (Jones et al., 2008), the origin <strong>of</strong> which<br />
700 is unclear but may be related to endogenic or impact processes. On the other hand, Cassini VIMS<br />
701 observations found no evidence <strong>of</strong> a water vapor column around Rhea (Pitman et al., 2008). On<br />
702 Iapetus, no evidence <strong>of</strong> cryovolcanism has been detected thus far. If volcanism ever operated on<br />
703<br />
704<br />
these satellites, it occurred in ancient times and its evidence is now unseen.<br />
705 21.4 Composition and Alteration <strong>of</strong> Surface Materials<br />
706 The composition <strong>of</strong> the satellites <strong>of</strong> Saturn is determined by remote spectroscopy, by sampling<br />
707 materials that were sputtered from the satellite surfaces or, in the case <strong>of</strong> Enceladus, injected into space<br />
708 where they can be directly sampled by spacecraft instruments.<br />
709 It has long been known that the surfaces <strong>of</strong> Saturn’s major satellites, Mimas, Enceladus, Tethys,<br />
710<br />
Dione, Rhea, Hyperion, Iapetus and Phoebe, are predominantly icy objects (e.g. Fink and Larsen,<br />
14
711 1975; Clark et al., 1984, 1986; Roush et al., 1995; Cruikshank et al., 1998a; Owen et al., 2001;<br />
712 Cruikshank et al., 2005; Clark et al., 2005, 2008a; Filacchione et al., 2007, 2008). While the<br />
713 reflectance spectra <strong>of</strong> these objects in the visible range indicate that a coloring agent is present on<br />
714 all surfaces except Mimas, Enceladus and Tethys (Buratti 1984). Only Phoebe and the dark<br />
715 hemisphere <strong>of</strong> Iapetus display near-IR spectra markedly different from very pure water ice (e.g.<br />
716 Cruikshank et al., 2005; Clark et al., 2005).<br />
717 The major absorptions in icy satellite spectra shown in Figs 9 a-e are due to water ice and can be<br />
718<br />
719<br />
observed at 1.5µm., 2.0µm., 3.0µm., and 4.5µm.<br />
720 Fig. 9 VIMS spectra <strong>of</strong> the Saturnian icy satellites: (a) The major absorptions in icy satellite spectra<br />
721 are due to water ice and appear at 1.5, 2.0, 3.0, and 4.5µm; (b) Representative (A) bright and (B)<br />
722 dark region Dione spectra (the gaps in the bright region spectra (A) near 1µm are due to sensor<br />
723<br />
724<br />
725<br />
saturation); the inset shows an area <strong>of</strong> high concentration <strong>of</strong> CO2 (after Clark et al., 2008a); (c)<br />
Spectra <strong>of</strong> Hyperion showing an increase in blue reflectivity and absorptions <strong>of</strong> water ice and CO2<br />
(after Clark et al., 2008a); (d) Spectra <strong>of</strong> Iapetus showing a range <strong>of</strong> blue peak intensities,<br />
726<br />
727<br />
absorptions <strong>of</strong> different intensities by water ice and CO2 absorption bands (after Clark et al.,<br />
2008a); (e) Spectra <strong>of</strong> Iapetus showing a range <strong>of</strong> blue peak intensities and absorptions <strong>of</strong> different<br />
728<br />
729<br />
intensities by water ice and CO2 absorption bands (after Clark et al., 2008a). However, no CO2 was<br />
found on the small satellites Atlas, Pandora, Janus, Epimetheus, Telesto, and Calypso (Buratti et<br />
730<br />
731<br />
al., 2009a).<br />
732<br />
733 Weaker ice absorptions appear at 1.04µm and 1.25µm in the spectra <strong>of</strong> bright regions where the ice<br />
734<br />
735<br />
is purer. Trapped CO2 was first detected in Galileo near-infrared mapping spectrometer (NIMS)<br />
spectra <strong>of</strong> the Galilean satellites (Carlson et al., 1996) and in the Saturnian system on Iapetus<br />
736 (Buratti et al, 2005), Phoebe (Clark et al., 2005), Hyperion (Cruikshank et al., 2007), and Enceladus<br />
737<br />
738<br />
(Brown et al., 2006). Weak CO2 absorptions were also detected in VIMS data on Dione, Tethys,<br />
Mimas and Rhea (Clark et al., 2008a).<br />
739 Organic compounds were detected directly in the Enceladus plume by the INMS (Waite et al.,<br />
740 2006) and in trace amounts by the VIMS on Phoebe (Clark et al., 2005), Hyperion (Cruikshank et<br />
741 al., 2007) and Iapetus (Cruikshank et al., 2008), and possibly Enceladus (Brown et al., 2006). While<br />
742 the spectral signatures indicate aromatic hydrocarbon molecules, exact identifications have remained<br />
743 elusive, partly because the detections have low signal-to-noise ratios. Another spectral feature<br />
744 observed in VIMS data <strong>of</strong> dark material on Phoebe, Iapetus and Dione appears to be an absorption<br />
745 centered near 2.97µm (Fig. 9 d). The detection <strong>of</strong> this feature is complicated by the fact that VIMS<br />
746 has an order-sorting filter change at this wavelength. Clark et al. (2005) presented evidence that this<br />
747 feature is real and maps to geological patterns on Dione and Iapetus (Clark et al., 2008a, 2009).<br />
748 However, a better understanding <strong>of</strong> the instrument response is needed to increase confidence in the<br />
749 identification, including confirmation by a different instrument. If correct, the feature strength<br />
750 indicates approximately 1% ammonia in the dark material.<br />
751 The position shift <strong>of</strong> amorphous ice absorptions from their crystalline counterparts (Mastrapa et<br />
752 al., 2008 and references therein) can be used to determine whether amorphous or crystalline ice is<br />
753<br />
754<br />
present on a surface. Traditionally, amorphous ice displays almost no 1.65µm feature and the<br />
3.1µm Fresnel peak shifts, broadens and weakens. Newman et al. (2008) used variations in the<br />
755 3.1µm peak and the 1.65µm absorption to argue for amorphous ice around the 'tiger stripes' on<br />
756 Enceladus. However, Clark et al. (2009) showed that scattering and sub-micron grain sizes also<br />
757 influence the intensities <strong>of</strong> the 3.1-µm peak and the 1.65-µm absorption, and can be confused with<br />
758 temperature and crystallinity effects. Further examination <strong>of</strong> the ice feature positions in spectra <strong>of</strong><br />
759 Enceladus and other satellites in the Saturnian system showed that positions and strengths are<br />
760 consistent with those <strong>of</strong> fine-grained crystalline ice (Clark et al., 2009). Newman et al. (2009)<br />
761 showed that water ice on Dione exists exclusively in crystalline form.<br />
762 The spectra <strong>of</strong> the satellites in the visible wavelength range are smooth with no sharp spectral features,<br />
763 but all show an ultraviolet absorber affecting the blue end <strong>of</strong> the visible-wavelength spectral slope.<br />
764 Visible spectra can be classified by their spectral slope: from bluish Enceladus and Phoebe to redder<br />
765 Iapetus, Hyperion and Epimetheus. In the 1µm to1.3µm range, the spectra <strong>of</strong> Enceladus, Tethys,<br />
766 Mimas and Rhea are characterized by negative slope, consistent with a surface mainly dominated by<br />
767 water ice, while the spectra <strong>of</strong> Iapetus, Hyperion and Phoebe show considerable reddening, pointing<br />
768 out the relevant role played by the darkening materials present on the surface (Filacchione et al., 2007,<br />
769<br />
2008). In between these two classes are Dione and Epimetheus, which have a relatively flat global<br />
15
770 spectrum in this range. At wavelengths less than about 0.5µm, all satellite spectra display a UV<br />
771 absorber, which might be due to charge transfer in oxides such as nano-phase hematite (Clark et al.,<br />
772 2008a), with the apparent strength <strong>of</strong> the absorber generally indicating the amount <strong>of</strong> contaminant.<br />
773 Trends in the UV absorber are complicated by ice dust contributions to satellite surfaces transmitted<br />
774 from Enceladus via the E-ring. Satellites close to the rings, such as Janus, Atlas and Prometheus, show<br />
775 an intermediate red tint between that <strong>of</strong> the rings and Enceladus. Mimas and Tethys show a red tint<br />
776 between that <strong>of</strong> Janus, Atlas and Prometheus and that <strong>of</strong> Enceladus on the other (e.g. Cuzzi et al.,<br />
777 2009; Buratti et al., 2009a).<br />
778 Disk-integrated ultraviolet observations by the Cassini UVIS (Esposito et al., 2004; Hendrix and<br />
779<br />
780<br />
Burrati, 2009) (Fig. 10) show Tethys and Dione exhibit marked UV<br />
781 Fig. 10 Cassini UVIS disk-integrated spectra <strong>of</strong> the icy moons, all at phase angles between 10° and<br />
782<br />
783<br />
13.6°.<br />
784 leading-trailing differences whereas Mimas, Enceladus and Rhea do not, suggesting compositional<br />
785 variations among the satellites as well as spectrally active components in the ultraviolet. UVIS<br />
786 spectra <strong>of</strong> all the icy moons <strong>of</strong> Saturn are dominated by a pronounced water-ice absorption edge at<br />
787 ~0.165µm. Its strength varies with the abundance <strong>of</strong> water ice, and its wavelength depends on grain<br />
788 size. Nearly all UVIS spectra display a small absorption feature at ~0.185µm (Hendrix and Hansen,<br />
789 2008b) whose origin is not clear at present. It is possible that this feature is due to water ice as it is<br />
790<br />
791<br />
seen in published reflectance spectra <strong>of</strong> water ice (Pipes et al., 1974; Hapke et al., 1981). Published<br />
optical constants (Warren, 1982) do not exhibit the feature, but the constants have not been well<br />
792 measured in the ~0.17µm to 0.4µm range.<br />
793 Cassini spectral coverage <strong>of</strong> the icy moons is complete in the 0.4µm to 5.2µm and in the 0.1µm to<br />
794 0.19µm region, but there is a gap in the 0.19µm to 0.4µm region (except for the broad band spectral<br />
795 measurements <strong>of</strong> ISS). This is an important wavelength region for non-water ice absorptions, and the<br />
796 Cassini UVIS data in the far UV provide a critical tiepoint between the far UV and the visible band that<br />
797 is helpful in understanding the nature <strong>of</strong> the non-ice species present in the surfaces <strong>of</strong> the moons. The<br />
798 icy moons are dark in the far UV compared to their spectra in the Vis-NIR. For instance, the disk-<br />
799 integrated albedo <strong>of</strong> Enceladus at 0.19µm is ~0.4, while at 0.439µm (Verbiscer et al., 2005) it is ~1.2.<br />
800 Water ice is thought to have a relatively flat spectral signature in this wavelength range, which suggests<br />
801 that some other species darkens Enceladus in the FUV. Ammonia, which exhibits an abrupt absorption<br />
802 edge at 0.2µm (Pipes et al., 1974), is a candidate (and ammonia hydrate has been tentatively detected at<br />
803 Enceladus and Tethys in <strong>Earth</strong>-based spectra (Verbiscer et al., 2005; Verbiscer et al., 2008)), though at<br />
804<br />
805<br />
this time other species cannot be ruled out other materials.<br />
806 Dark Material<br />
807 Cassini (1672) was the first to infer the presence <strong>of</strong> dark material in the Saturnian system, which<br />
808 was verified by Murphy et al. (1972) and Zellner (1972). Dark material appears throughout the<br />
809 system, most predominantly on Dione, Rhea, Hyperion, Iapetus and Phoebe. The nature <strong>of</strong> the dark<br />
810 material especially on Iapetus has been studied by numerous authors, <strong>of</strong>ten with conflicting<br />
811 conclusions, including Cruikshank et al. (1983), Wilson and Sagan (1995, 1996) Vilas et al. (1996),<br />
812<br />
813<br />
Jarvis et al. (2000), Owen et al. (2001), Buratti et al. (2002) and Vilas et al. (2004).<br />
The dark material tends to darken and redden the satellites’ spectra, particularly in the visible. In<br />
814 dark-region spectra, additional absorptions are seen at 2.42µm and 4.26µm, with the latter due to<br />
815<br />
816<br />
CO2, as well as stronger water ice absorption near 3µm (Fig. 21.5.1d). The 2.42µm feature was first<br />
tentatively identified as a CN overtone band by Clark et al. (2005) in spectra <strong>of</strong> Phoebe, but<br />
817 evidence <strong>of</strong> CN fundamentals in the 4µm to 5µm region has not been confirmed. At present, the<br />
818 2.42µm feature is believed to be due to OH or hydrogen (Clark et al., 2008a, b, 2009), but<br />
819 complications with the VIMS calibration have limited any detailed definition <strong>of</strong> the absorption.<br />
820 Diverse spectral features <strong>of</strong> the dark material on Dione match those seen on Phoebe, Iapetus, Hyperion<br />
821 and Epimetheus as well as in the F-ring and the Cassini Division, implying that its composition is the<br />
822 same throughout the Saturnian system (Clark et al., 2008a, 2009). Models <strong>of</strong> the dark material on<br />
823 Iapetus have used tholins to explain the reddish visible spectral slope (e.g. Cruikshank et al., 2005).<br />
824 However, VIMS data, which better isolate the spectral signature <strong>of</strong> the dark material from that <strong>of</strong> icy<br />
825 regions thanks to the high spatial resolution afforded by close fly-bys, show that the dark material has<br />
826 a remarkably linear spectrum (Clark et al., 2008a, 2009) (Fig. 12) not explained by tholins which have<br />
827 curved spectral response. Tholins also have strong CH absorbers in the 3-4-μm region as well as other<br />
828<br />
marked absorptions at wavelengths longer than 2 μm that are not seen in, for example, spectra <strong>of</strong> the<br />
16
829 dark regions <strong>of</strong> Iapetus. The problem <strong>of</strong> matching one spectrum might be accomplished with a<br />
830 specific mixture <strong>of</strong> many compounds, but with spatially resolved spectra on the satellites we now<br />
831 see a large range <strong>of</strong> mixtures, so models must show consistency across that range <strong>of</strong> observed<br />
832 mixtures, and many models do not.<br />
833 The source <strong>of</strong> the dark material in the Saturnian system is unclear. Several researchers (e.g. Clark et<br />
834 al., 2005) suggest a source outside the system, perhaps the Kuiper belt or comets. Owen et al.<br />
835 (2001) suggested that an impact on Titan created a spray <strong>of</strong> dark, organic-rich material, part <strong>of</strong><br />
836 which was swept up by Iapetus’ leading hemisphere, another part being transported to the inner<br />
837 system to accrete onto the surfaces <strong>of</strong> the inner icy moons. The Dione VIMS spectra provide a key to<br />
838 solving some <strong>of</strong> the mysteries <strong>of</strong> the dark material. Clark et al. (2008a) showed that a pattern <strong>of</strong><br />
839 bombardment by fine particles below 0.5 μm impacted Dione from the trailing-side direction. Several<br />
840 lines <strong>of</strong> evidence point to an external origin <strong>of</strong> the dark material on Dione, including its global spatial<br />
841 pattern, local patterns including crater and cliff walls shielding implantations on slopes facing away<br />
842 from the trailing side, exposing clean ice, and slopes facing the trailing direction that show higher<br />
843 abundances <strong>of</strong> dark material.<br />
844 A blue scattering peaks is seen in the spectrum <strong>of</strong> the dark material and was initially observed in the<br />
845 spectra <strong>of</strong> Tethys, Dione and Rhea by Noland et al. (1974). Clark et al. (2008a) suggest that the<br />
846 blue peak is caused by particles less than 0.5μm in diameter embedded in the ice. The VIMS resolved<br />
847 areas on Phoebe, Iapetus and Hyperion also show blue peaks (Fig. 9 c, d, e), indicating that this is a<br />
848 common property <strong>of</strong> satellites in the Saturnian system (Clark et al., 2008a, 2009). The blue scattering<br />
849<br />
850<br />
851<br />
peak with a strong UV/visible absorption was modelled with ice plus 0.2 μm diameter carbon grains<br />
(to provide the Rayleigh scattering effect) plus nano-phase hematite (Fe2O3) to produce the UV<br />
absorber (Fig. 11). But carbon is inconsistent with the linear-red slope seen in the purest dark material<br />
852 spectra from 0.4µm to 2.5µm. Metallic iron is the only known compound to display this spectral<br />
853 property and nano-phase metallic iron plus nano-phase hemtatite mixed with ice show consistency<br />
854<br />
855<br />
with the mixture spectra observed on the icy satellites and in Saturn’s rings (Clark et al., 2009).<br />
856 Fig. 11 Spectrum <strong>of</strong> Dione compared to a spectrum <strong>of</strong> silicon carbide. (b) Spectrum <strong>of</strong> Dione<br />
857 compared to the spectrum <strong>of</strong> a laboratory mixture <strong>of</strong> 99.25wt% H2O ice, 0.5wt% carbon black, and<br />
858 0.25wt% nano-crystalline hematite (reagent by Sigma-Aldrich). The carbon black grains are 0.2µm<br />
859 in diameter. Both the carbon black and hematite grains create Rayleigh scattering in the sample, but<br />
860 at wavelengths shorter than about 0.5µm, absorption by hematite causes a downturn similar to that<br />
861<br />
862<br />
seen in the Dione spectra (after Clark et al., 2008a).<br />
863 Previous models <strong>of</strong> the dark material on Iapetus typically used tholins (e.g. Cruikshank et al., 2005).<br />
864 However, VIMS data, which better isolate the spectral signature <strong>of</strong> the dark material from that <strong>of</strong> icy<br />
865 regions thanks to the high spatial resolution afforded by close fly-bys, show that the dark material has<br />
866<br />
867<br />
a remarkably linear spectrum (Clark et al., 2008a, 2009) (Fig. 12).<br />
868 Fig. 12 Spectrum <strong>of</strong> the dark material <strong>of</strong> Iapetus compared to laboratory spectra <strong>of</strong> mixtures <strong>of</strong> iron<br />
869<br />
870<br />
powder, H2O, CO2 and NH3. The small Rayleigh scattering peak and UV absorber can be explained<br />
by traces <strong>of</strong> iron oxide nano-powder. The 3-micron absorption is best explained by a combination <strong>of</strong><br />
871<br />
872<br />
873<br />
bound water, ice and trace ammonia. Finer-grained iron would require higher abundances <strong>of</strong> water,<br />
ammonia and carbon dioxide to match the spectrum <strong>of</strong> Iapetus.<br />
874 Tholins have strong CH absorbers in the 3-4-μm region as well as other marked absorptions at<br />
875 wavelengths longer than 2μm that are not seen in, for example, spectra <strong>of</strong> the dark regions <strong>of</strong> Iapetus.<br />
876 Using VIMS data, Clark et al. (2008a) found the general composition <strong>of</strong> the dark material in the<br />
877<br />
878<br />
Saturnian system likely contains bound water, CO2 and, tentatively, ammonia. More recently, Clark et<br />
al. (2008b, 2009) argued that the signature <strong>of</strong> iron identifies the major coloring agent on the icy<br />
879 surfaces in the Saturnian system. Metallic iron does not exhibit sharp diagnostic spectral features,<br />
880 but it does have unique spectral properties in that its absorption increases linearly with decreasing<br />
881 wavelength (Clark et al., 2009).<br />
882 The theory <strong>of</strong> Clark et al. (2008b, 2009) envisages small-particle meteoroid dust contaminating the<br />
883 system as well as meteoroids with a high metallic-iron content. As one moves closer to the rings,<br />
884 the dark material diminishes (e.g. Mimas and Enceladus have little dark material). However, the<br />
885 competing process <strong>of</strong> accretion by Saturn’s E-ring, which originates on Enceladus, also contributes<br />
886 to the greater abundance <strong>of</strong> high-albedo material on Mimas and Enceladus (Buratti et al. 1990,<br />
887<br />
Vebiscer et al., 2007), The main rings do not show any strong signature <strong>of</strong> metallic iron particles,<br />
17
888 but the Cassini Division displays spectra that closely match those <strong>of</strong> Iapetus in Iapetus’ transition<br />
889 zone from dark to light material, implying mixtures <strong>of</strong> the same materials. The ring spectra are<br />
890 much redder in the 0.3µm to 0.6µm region than the spectra <strong>of</strong> dark material on Phoebe, Iapetus and<br />
891 Hyperion. Cosmic rays impacting the huge surface area <strong>of</strong> the rings break up water molecules, thus<br />
892 creating an atmosphere <strong>of</strong> hydrogen and oxygen. The hydrogen escapes, leaving an atmosphere <strong>of</strong><br />
893 oxygen around the rings (Johnson et al., 2006). Oxygen ions are very reactive, and when they<br />
894<br />
895<br />
encounter iron particles, they oxidize them and turn them into nano-phase hematite (Fe2O3). The<br />
spectra <strong>of</strong> nano-phase hematite mixed with ice and dark particles closely match the UV red slope<br />
896 seen in the spectra <strong>of</strong> Dione (Clark et al., 2008a) (Fig. 11). To explain the dark material in the<br />
897 Saturnian system, Clark et al. (2008b, 2009) suggest that oxidized sub-micron iron particles from<br />
898 meteorites, mixed with ice, might explain all the colors and spectral shapes observed in the system.<br />
899 A small amount <strong>of</strong> carbon might also be identified in the mixture, which would change the slope <strong>of</strong><br />
900 iron and further lower the albedo. Sub-micron particles also explain the origin <strong>of</strong> the observed<br />
901<br />
902<br />
Rayleigh scattering. Trace amounts <strong>of</strong> CO2, ammonia and organics would not significantly<br />
influence the visible and UV spectra down to 0.3µm. Finally, the inner satellites may be exposed to<br />
903 as much iron as the outer satellites, but E-ring ice particles mix with and cover up the iron particles.<br />
904 Species found by INMS, CAPS and MIMI in the magnetosphere are candidates for surface<br />
905 components as the magnetospheric material originates either directly from satellite surfaces or from<br />
906 ring particles. Compositional analyses <strong>of</strong> Enceladus’ plume are described in Spencer et al. (2009).<br />
907 The idea <strong>of</strong> iron in the Saturn system is supported by the Cassini CDA instrument, which has<br />
908<br />
909<br />
910<br />
discovered small (
947 late 2004 suggest external material emplacement (e.g. dark material on ram-facing crater walls at<br />
948 high latitudes) (Porco et al., 2005a), but this was later found to be due to photometric effects (Denk<br />
949 et al., 2008). The initial theory <strong>of</strong> an exogenically-created dark pattern (Cook and Franklin, 1970)<br />
950 suggested that pre-existing dark material was uncovered by meteoritic bombardment; this idea was<br />
951 extrapolated by Wilson and Sagan (1996). Researchers theorized (Soter, 1974) that the dark<br />
952 material is exogenically emplaced on Iapetus’ leading hemisphere as material is lost from the moon<br />
953 Phoebe (Burns, 1979). Retrograde Phoebe dust from a distance <strong>of</strong> 215 Saturn radii would travel<br />
954 inward from the effects <strong>of</strong> Poynting-Robertson drag and impact the leading hemisphere <strong>of</strong> Iapetus<br />
955 orbiting at 59 Saturn radii. However, Phoebe is spectrally neutral at visible wavelengths, while the<br />
956 Iapetus dark material is reddish (Cruikshank et al., 1983; Squyres et al., 1984). If the material does<br />
957 come from Phoebe, then some sort <strong>of</strong> chemistry or impact volatilization must occur to change the<br />
958 color and darken the material (Cruikshank et al., 1983; Buratti and Mosher, 1995). Another<br />
959 possibility is that the exogenic source <strong>of</strong> the dark material is Hyperion (Matthews, 1992; Marchi et<br />
960 al., 2002), Titan (Wilson and Sagan, 1995; Owen, 2001), Iapetus itself (Tabak and Young, 1989) or<br />
961 the debris remnants <strong>of</strong> a collision between a former outer moon and a planetocentric object (Denk<br />
962 and Neukum, 2000). Matthews (1992) suggested that the hypothetical impact that disrupted<br />
963 Hyperion created a debris cloud that subsequently hit Iapetus. Both Hyperion and Titan dark<br />
964 materials are spectrally reddish (Thomas and Veverka, 1985; McDonald et al., 1994; Owen et al.,<br />
965 2001), though not as dark as those <strong>of</strong> Iapetus. Buratti et al. (2002; 2005) suggested that both<br />
966 Hyperion's and Iapetus’ leading hemispheres are impacted by dark, reddish dust from recently<br />
967<br />
968<br />
discovered retrograde satellites exterior to Phoebe. Clark et al. (2008a) <strong>of</strong>fer a solution for the color<br />
discrepancy. Red, fine-grained dust (less than one-half micron in diameter) when mixed in small<br />
969 amounts with water ice produces a Rayleigh scattering effect, enhancing the blue response. This<br />
970 blue enhancement appears just enough on Phoebe to create an approximately neutral spectral<br />
971 response. An examination <strong>of</strong> the spectral signatures <strong>of</strong> Dione, Hyperion, Iapetus and Phoebe by<br />
972 Clark et al (2008a) showed a continuous mixing space on all these satellites, demonstrating variable<br />
973 mixtures <strong>of</strong> fine-grained dark material in the ice. Thus, the colors observed are simply variable<br />
974 amounts <strong>of</strong> dark material, with the redder slopes indicating greater abundance.<br />
975 Ground-based radar observations at 13cm (Black et al., 2004) and Cassini RADAR data at 2.2cm<br />
976 (Ostro et al., 2006) indicate that the dark terrain on Iapetus must be quite thin (one to several<br />
977 decimeters); an ammonia-water ice mixture may be present at a depth <strong>of</strong> several decimeters below<br />
978 the surface on both the leading and trailing hemispheres <strong>of</strong> Iapetus. The RADAR results appear to<br />
979 rule out any theories <strong>of</strong> a thick dark material layer (Matthews, 1992; Wilson and Sagan, 1996) and<br />
980<br />
981<br />
are consistent with the spectral coating indicated by VIMS data as described by Clark et al (2008a).<br />
3<br />
Cassini radio science results (Rappaport et al., 2005) indicate a bulk density for Iapetus <strong>of</strong> 1.1g/cm ,<br />
982 from which it can be inferred that the moon is composed primarily <strong>of</strong> water ice.<br />
983 Because the large craters within the dark terrain appear evenly colored by the dark material in<br />
984 Voyager data, no craters significantly break up the dark material to expose bright underlying terrain<br />
985 (Denk et al., 2000). This suggests that the emplacement <strong>of</strong> dark material by whatever mechanism is<br />
986 relatively new or ongoing.<br />
987 With the arrival <strong>of</strong> Cassini at Saturn and the first Iapetus flyby (Dec 2004), the idea <strong>of</strong> thermal<br />
988 segregation developed (Spencer et al., 2005; Hendrix and Hansen, 2008a). This idea was originally<br />
989<br />
990<br />
mentioned as one option by Mendis and Axford (1974). The September 2007 close flyby <strong>of</strong> Iapetus<br />
permitted testing <strong>of</strong> this hypothesis, resulting in the following theory. A small amount <strong>of</strong> exogenic<br />
991 dust from an unknown source created a 'global color dichotomy': the leading side became slightly<br />
992 redder and darker than the trailing with fuzzy boundaries close to the sub-Saturn and anti-Saturn<br />
993 meridians (Denk et al., 2008). A runaway thermal segregation process might have been initiated<br />
994 primarily at the low and middle latitudes <strong>of</strong> the leading side as well as in some local areas at low<br />
995 latitudes on the trailing side, especially on equatorward-facing slopes (Denk et al., 2008). Indeed,<br />
996 poleward-facing slopes even within the dark terrain (at middle latitudes) are bright. On these slopes<br />
997 the temperature never raises enough to allow the thermal process to act faster than the destruction<br />
998 process by micrometeorite gardening, which transports bright sub-surface material to the surface. In<br />
999 those locations that are now dark, the originally bright surface material becomes warm enough for<br />
1000 volatiles (mainly water ice) on the top part <strong>of</strong> the surface to grow unstable and migrate towards cold<br />
1001 traps, leaving behind residual dark material that was originally intimately mixed with the water ice<br />
1002 as a minor constituent. The slow 79.3day rotation is a crucial boundary condition for this process to<br />
1003 work in the Saturnian system. ISS images suggest that significant cold traps might be located at<br />
1004<br />
1005<br />
high latitudes on the trailing side, where lower-resolution images show bright white polar caps.<br />
19
1006 Compositional data from the UVIS and the VIMS (Hendrix and Hansen, 2009a; Hendrix and<br />
1007 Hansen, 2009b; Clark et al., 2009) indicate variable amounts <strong>of</strong> ice and volatiles in the dark<br />
1008 material. The water-ice absorption edge in the UV at 0.165µm was shown to increase with latitude,<br />
1009 consistent with decreasing temperatures and increasing numbers <strong>of</strong> visibly bright regions (Hendrix<br />
1010 and Hansen, 2009a). This is consistent with thermal segregation as a possible process explaining the<br />
1011 overall appearance <strong>of</strong> Iapetus and, more particularly, the observed correlations <strong>of</strong> the dark/bright<br />
1012 boundary with geographical location and topography (Fig. 15) and the small-scale dramatic<br />
1013<br />
1014<br />
brightness variations (Fig. 15).<br />
1015 Fig. 14 Cassini ISS images from the September 2007 flyby, demonstrating the large- and small-scale<br />
1016 albedo effects <strong>of</strong> thermal segregation. (Left) The central longitude <strong>of</strong> the trailing hemisphere is 24°<br />
1017 to the left <strong>of</strong> the mosaic's center. (Right) The mosaic consists <strong>of</strong> two image footprints across the<br />
1018 surface <strong>of</strong> Iapetus. The view is centered on terrain near 42° southern latitude and 209.3° western<br />
1019 longitude, on the anti-Saturn facing hemisphere. Image scale is approximately 32m/pixel; the crater<br />
1020<br />
1021<br />
in the center is about 8 km across.<br />
1022 The 3µm absorption observed in VIMS data (Clark et al., 2009) might be due to ice, ammonia or<br />
1023 bound water. As the 3µm absorption is not been well matched by those due to ice, bound water and<br />
1024 ammonia, a combination seems to be indicated, but a model showing a good match has not yet been<br />
1025 developed (Clark et al., 2009). The amount <strong>of</strong> ice contained in the dark materials from VIMS data<br />
1026<br />
1027<br />
1028<br />
could be on the order <strong>of</strong> 10%, but is estimated to be
1065 visible wavelength by the Hubble space telescope (Verbiscer et al., 2007). Their brightness changes<br />
1066 with their distance from the E-ring, which is consistent with the assumption that their albedo results<br />
1067 from E-ring coating. Hamilton and Burns (1994) predicted that the relative velocities between the<br />
1068 E-ring particles and the icy satellites might explain the large-scale longitudinal albedo patterns on<br />
1069 the icy satellites. Because <strong>of</strong> their higher relative velocity, the satellites exterior to the densest point<br />
1070 in the E-ring have brighter leading hemispheres, while those interior to it have brighter trailing<br />
1071 hemispheres because <strong>of</strong> the higher velocity <strong>of</strong> the particles. Indeed, Mimas and Enceladus are both<br />
1072 slightly darker on their leading than on their trailing hemispheres, while the leading hemispheres <strong>of</strong><br />
1073 Tethys, Dione and Rhea are brighter than their trailing hemispheres (Buratti et al., 1998). The<br />
1074 visible orbital phase curve <strong>of</strong> Enceladus shows that the leading hemisphere is ~1.2 times darker than<br />
1075 its trailing hemisphere. At visible wavelengths, the leading hemisphere <strong>of</strong> Tethys is ~1.1 times<br />
1076 brighter than its trailing hemisphere, Dione’s leading hemisphere is up to ~1.8 times brighter and<br />
1077 Rhea’s leading hemisphere is ~1.2 times brighter than the trailing hemisphere (Buratti and Veverka,<br />
1078 1984).<br />
1079 The bombardment <strong>of</strong> icy surfaces by charged particles causes both structural and chemical changes<br />
1080 in the ice (Johnson et al., 2004). Effects include the implantation <strong>of</strong> magnetospheric ions, sputtering<br />
1081 and grain size alteration. Low-energy electrons can cause chemical reactions on the surface, while<br />
1082 high-energy electrons and ions tend to cause radiation effects at depth. The latter can result in bulk<br />
1083 chemical changes in composition as well as structural damage. A primary effect <strong>of</strong> energetic<br />
1084 particle bombardment on ice is to cause defects in it (Johnson, 1997; Johnson and Quickenden,<br />
1085<br />
1086<br />
1997; Johnson et al., 1998). Extended defects, in the form <strong>of</strong> voids and bubbles, affect the lightscattering<br />
properties <strong>of</strong> the surface. They also act as reaction and trapping sites for gases, which can<br />
1087 produce spectral absorption features. Incident ions deposit energy over a small region around their<br />
1088 path through the ice, causing local damage. Depending on excitation density and cooling rate, this<br />
1089 can result in either local crystallization or amorphization (Strazzulla, 1998).<br />
1090 Energetic ions and electrons have been seen to brighten laboratory surfaces in the visible by<br />
1091 producing voids and bubbles (Sack et al., 1992). Depending on the size and density <strong>of</strong> these<br />
1092 features, a clear surface can become brighter due to enhanced scattering in the visible. The plasma-<br />
1093 induced brightening competes with annealing, which operates efficiently in regions where the ice<br />
1094 temperature exceeds ~100K and can make equatorial regions less light-scattering despite the plasma<br />
1095 bombardment.<br />
1096 A significant effect <strong>of</strong> the ion bombardment <strong>of</strong> ice is the creation <strong>of</strong> new species through radiolysis.<br />
1097<br />
1098<br />
Bombarding particles can decompose H2O ice, creating molecular hydrogen and oxygen. Miniatmospheres<br />
<strong>of</strong> trapped volatiles have been suggested to form in voids in the ice (Johnson and<br />
1099<br />
1100<br />
Jesser, 1997). As a result, species such as O3, H2O2, O2 and oxygen-rich trace species have been<br />
detected as gases trapped in the regoliths <strong>of</strong> the icy Galilean satellites (Nelson et al., 1987; Noll et<br />
1101 al., 1996; Hendrix et al., 1998, 1999; Spencer et al., 1995; Carlson et al., 1996, 1999; Hibbitts et al.,<br />
1102 2000, 2003). On these satellites, the hydrogen and oxygen produced by radiation decomposition<br />
1103 form tenuous atmospheres; in contrast, atmospheres have not been detected on satellites in the<br />
1104 Saturnian system (with the exception <strong>of</strong> the gaseous plume <strong>of</strong> Enceladus). There are several<br />
1105 possible reasons for this. Lower gravities might support the loss <strong>of</strong> volatiles; temperatures are<br />
1106<br />
1107<br />
1108<br />
lower, which might slow down the breakup <strong>of</strong> H2O and the loss <strong>of</strong> H; the particle environment is<br />
different (discussed below); the electron density and temperature is not effective in exciting and<br />
'lighting up' any gases around the moons; and sputtering is not as intense as in the Jupiter system<br />
1109 due to differences in the net charge and energy <strong>of</strong> the particle environment. On Saturn, the primary<br />
1110 surface alterations found thus far are due to the E-ring, dust (e.g. on Dione) and thermal processing<br />
1111 (especially apparent on Iapetus, as previously discussed).<br />
1112 Neutrals, rather than ions as on Jupiter, dominate the Saturnian system. The energetic heavy ions<br />
1113 (~10keV to MeV) that are critical at Europa and Ganymede are lost by charge exchange to the<br />
1114 neutral cloud <strong>of</strong> the Saturnian system as they diffuse inwards from beyond ~10Saturn radii<br />
1115 (Paranicas et al., 2008). However, although the energy fluxes due to trapped plasma and the solar<br />
1116 UV are relatively low on the icy Saturnian satellites (Paranicas et al., 2008), radiation processing<br />
1117 appears to be occurring. Radiation fluxes are known to cause decomposition in ice, possibly<br />
producing ozone in the icy surfaces <strong>of</strong> Dione and Rhea (Noll et al., 1997) and O2 + 1118<br />
1119<br />
in Saturn’s<br />
plasma (Martens et al., 2007). In addition, the role <strong>of</strong> low energy ions has been recently re-<br />
1120 examined using Cassini CAPS data (Sittler et al., 2008), indicating that sputtering by this<br />
1121<br />
1122<br />
component is not negligible (Johnson et al., 2008). Spencer (1998) searched for but did not find O2<br />
inclusions on Rhea and Dione. Such inclusions have been seen on Ganymede in the low latitudes <strong>of</strong><br />
1123<br />
the trailing hemisphere, and it was suggested that they might be related to charged-particle<br />
21
1124<br />
1125<br />
1126<br />
bombardment or UV photolysis. The lack <strong>of</strong> O2 on Rhea and Dione suggests that low temperatures<br />
preclude the formation <strong>of</strong> O2 bubbles in the ice. Hydrogen peroxide (H2O2) has been tentatively<br />
VIMS data (Newman et al., 2007) <strong>of</strong> the south pole tiger-stripes region <strong>of</strong> Enceladus, although it is<br />
1127 not clear whether exogenic processing is needed to create peroxide.<br />
1128 It is important to distinguish between hemispheric dichotomies in color, albedo, texture and<br />
1129 macroscopic structure because each exogenic alteration mechanism leaves a different signature on<br />
1130 the physical and optical properties <strong>of</strong> the surface. Photometric methods have been employed over<br />
1131 the past two decades to characterize the roughness, compaction state, particle size and phase<br />
1132 function, and single scattering albedo <strong>of</strong> planetary and satellite surfaces, including the Saturnian<br />
1133 satellites (Buratti, 1985; Verbiscer and Veverka, 1989, 1992, 1994; Domingue et al. 1995). An<br />
1134 indication <strong>of</strong> the scattering properties <strong>of</strong> the satellites’ surfaces can be obtained by extracting<br />
1135 photometric scans across their disks at small solar phase angles. Dark surfaces such as that <strong>of</strong> the<br />
1136 Moon, in which single scattering is dominant, are not significantly limb-darkened, whereas bright<br />
1137 surfaces in which multiple scattering is important could be expected to show limb darkening.<br />
1138 Physical properties such as roughness, compaction state and particle size give clues as to the past<br />
1139 evolution <strong>of</strong> the satellites’ surfaces and reveal the relative importance <strong>of</strong> exogenic alteration<br />
1140 processes. Accurate photometric modeling is also necessary to define the intrinsic changes in albedo<br />
1141 and color on a planetary surface; much, and in many cases most <strong>of</strong> the change in intensity is due to<br />
1142 variations in the radiance viewing geometry and is not intrinsic to the surface.<br />
1143 <strong>Planetary</strong> surface scattering models have been developed (e.g. Horak, 1950; Goguen, 1981; Lumme<br />
1144<br />
1145<br />
and Bowell, 1981a, 1981b; Hapke, 1981, 1984, 1986, 1990; Shkuratov et al., 1999, 2005) which<br />
express the radiation reflected from the surface in terms <strong>of</strong> the following physical parameters: single<br />
1146 scattering albedo, single particle phase function, the compaction properties <strong>of</strong> the optically active<br />
1147 portion <strong>of</strong> the regolith, and the scale and extent <strong>of</strong> macroscopically rough surface features.<br />
1148 Observations at small solar phase angles (0-10°) are particularly important for understanding the<br />
1149 compaction state <strong>of</strong> the upper regolith: this is the region <strong>of</strong> the well-known opposition effect, in<br />
1150 which the rapid disappearance <strong>of</strong> shadowing among surficial particles causes a nonlinear surge in<br />
1151 brightness as the object becomes fully illuminated. At the smallest phase angles (40°) are necessary for<br />
1153 investigating the macroscopically rough nature <strong>of</strong> surfaces, ranging in size from clumps <strong>of</strong> several<br />
1154 particles to mountains, craters and ridges. Such features cast shadows and alter the local incidence<br />
1155 and emission angles. Two formalisms have been developed to describe roughness for planetary<br />
1156 surfaces: a mean-slope model in which the surface is covered by a Gaussian distribution <strong>of</strong> slope<br />
1157 angles with a mean angle <strong>of</strong> θ (Hapke, 1984; Helfenstein and Shepard, 1998), and a crater model in<br />
1158 which the surface is covered by craters with a defined depth-to-radius ratio (Buratti and Veverka,<br />
1159 1985). The single particle phase function, which expresses the directional scattering properties <strong>of</strong> a<br />
1160 planetary surface, is an indicator <strong>of</strong> the physical character <strong>of</strong> individual particles in the upper<br />
1161 regolith, including their size, size distribution, shape, and optical constants. Small or transparent<br />
1162 particles tend to be more isotropically scattering because photons survive to be multiply scattered;<br />
1163 forward scattering can exist if photons exit in the direction away from the observer. The single<br />
1164 particle phase function is usually described by a 1 or 2-term Henyey-Greenstein phase function defined<br />
1165 by the asymmetry parameter g, where g = 1 is purely forward-scattering, g = -1 is purely<br />
1166<br />
1167<br />
backscattering, and g = 0 is isotropic. The opposition surge <strong>of</strong> airless surfaces has been described by<br />
parameters that define the compaction state <strong>of</strong> the surface and the degree <strong>of</strong> coherent backscatter <strong>of</strong><br />
1168 multiply scattered photons (Irvine, 1966; Hapke, 1986, 1990). One final physical photometric<br />
1169 parameter is the single scattering albedo (ώ), which is the probability that a photon will be scattered<br />
1170 into 4π steradians after one scattering.<br />
1171 Three other important physical parameters are the geometrical albedo (p), the phase integral (q) and the<br />
1172<br />
1173<br />
Bond albedo (AB). The geometrical albedo is the flux received from a planet at a solar phase angle <strong>of</strong><br />
0º compared with that <strong>of</strong> a perfectly diffusing disk <strong>of</strong> the same size, also at 0º. The phase integral (q) is<br />
1174 the flux, normalized to unity at a solar phase angle <strong>of</strong> 0º, integrated over all solar phase angles. The<br />
1175 Bond albedo is p·q, and when integrated over all wavelengths it yields the bolometric Bond albedo,<br />
1176 which is a measure <strong>of</strong> the energy balance on the planet or satellite. Except for the bolometric Bond<br />
1177 albedo, all these parameters are wavelength-dependent.<br />
1178 Buratti (1984) and Buratti and Veverka (1984) performed photometric analyses <strong>of</strong> visible-<br />
1179 wavelength Voyager data <strong>of</strong> the icy Saturnian moons based on Voyager disk-resolved data sets, and<br />
1180 disk-integrated Voyager observations at large phase angles to supplement existing ground-based<br />
1181 observations at smaller phase angles. These studies investigated limb darkening with implications<br />
1182<br />
for multiple scattering (significant limb darkening suggests that multiple scattering is important).<br />
22
1183 Physical photometric modeling <strong>of</strong> Saturnian moons based on Voyager and ground-based data was<br />
1184 done by Buratti (1985), Verbiscer and Veverka (1989, 1992, 1994), Domingue et al. (1995) and<br />
1185 Simonelli et al. (1999). In general, the surfaces <strong>of</strong> the Saturnian satellites are backscattering and<br />
1186 exhibit roughness comparable to that <strong>of</strong> the Moon, indicating that impact processes produce similar<br />
1187 morphological effects on rocky and icy bodies at low temperatures. The photometric and physical<br />
1188 properties <strong>of</strong> the satellites determined so far from both Voyager and Cassini data are summarized in<br />
1189<br />
1190<br />
Table 4.<br />
Parameter Mimas Enceladus Tethys Dione Rhea Phoebe<br />
Mean slope 30 (2)<br />
13±5 (4) 31±4 (6)<br />
angle θ (º)<br />
Asymmetry<br />
parameter g<br />
30±1 (5) 6±1 (3)<br />
33±3 (7)<br />
Single<br />
scattering<br />
albedo ώ<br />
Phase<br />
integral q<br />
Visual<br />
geometric<br />
albedo pv<br />
Bond<br />
albedo AB<br />
-0.3±0.05 (2)<br />
-0.213±0.005<br />
(5)<br />
0.93±0.03 (2)<br />
0.951±0.002<br />
(5)<br />
0.82±0.05 (1)<br />
0.78±0.05 (5)<br />
-0.35±0.03<br />
(2)<br />
-0.399±0.005<br />
(3)<br />
0.99±0.02 (2)<br />
0.998±0.001<br />
(3)<br />
0.86±0.1 (1)<br />
0.92±0.05 (3)<br />
0.77±0.15 (1) 1.04±0.15 (1)<br />
1.41±0.03 (8);<br />
0.6±0.1 (1)<br />
0.5±0.1 (5)<br />
0.9±0.1 (1)<br />
0.91±0.1 (3)<br />
0.7±0.1 (1)<br />
0.80±0.15<br />
(1)<br />
0.6±0.1 (1)<br />
23<br />
0.8±0.1 (1)<br />
0.55±0.15<br />
(1)<br />
0.45±0.1<br />
(1)<br />
-0.287±0.007 (4) -0.24 (6)<br />
-0.3±0.1 (plus<br />
a forward<br />
component)<br />
(7)<br />
0.861±0.008 (4) 0.068 (6)<br />
0.07±0.01 (7)<br />
0.70±0.06 (1) 0.24±0.05 (6)<br />
0.78±0.05 (4) 0.29±0.03 (7)<br />
0.65 (1) 0.081±0.002<br />
(6);<br />
0.082±0.002<br />
(7);<br />
0.45±0.1 (1) 0.020±0.004<br />
0.49±0.06 (4) (6)<br />
0.023±0.007<br />
(7)<br />
1191<br />
1192 Tab. 4 Photometric and physical parameters <strong>of</strong> the icy Saturnian satellites (references: (1) Buratti<br />
1193 and Veverka, 1984; (2) Buratti, 1985; (3) Verbiscer and Veverka, 1994; (4) Verbiscer and Veverka<br />
1194 1989; (5) Verbiscer and Veverka, 1992; (6) Simonelli et al., 1999; (7) Buratti et al., 2008; (8)<br />
1195 Verbiscer et al., 2005<br />
1196<br />
1197 Opposition surge data from 0.2 to 5.1µm were obtained for Enceladus, Tethys, Dione, Rhea and<br />
1198<br />
1199<br />
Iapetus. Fig. 16 shows the opposition curve <strong>of</strong> Iapetus in between 0.35µm and 3.6µm<br />
1200 Fig. 15 (Above) Cassini visual infrared mapping spectrometer (VIMS) disk-integrated observations<br />
1201 <strong>of</strong> the opposition surge on Iapetus between 0.35 and 3.6µm. (Lower left) The slope <strong>of</strong> the solar phase<br />
1202 curve (the 'phase coefficient') between 1º and 8º decreases as the albedo increases. This effect is<br />
1203 expected for shadow hiding, since bright surfaces have partly illuminated primary shadows. (Lower<br />
1204 right) On the other hand, the amplitude <strong>of</strong> the 'spike' under 1º (shown as a fractional increase above<br />
1205 the value <strong>of</strong> the phase curve at 1º) increases with the albedo, suggesting that it is a multiple scattering<br />
1206<br />
1207<br />
effect, such as coherent backscatter.<br />
1208 (Buratti et al., 2009b; Hendrix and Buratti, 2009). The curve is divided into two components: at<br />
1209 phase angles greater than 1º, brightness increases linearly with decreasing phase angle, while at<br />
1210 phase angles smaller than one degree the increase is exponential. Moreover, the albedo-dependence<br />
1211 <strong>of</strong> the two types <strong>of</strong> surge is different. For the first type, the magnitude <strong>of</strong> the surge decreases as the<br />
1212 albedo increases, while for the second type <strong>of</strong> surge the reverse is true. This result suggests that<br />
1213 different phenomena are responsible for the two types <strong>of</strong> surge. At angles greater than 1º, the surge<br />
1214<br />
1215<br />
is caused by mutual shadowing, in which shadows cast among particles <strong>of</strong> the regolith rapidly<br />
disappear as the face <strong>of</strong> the satellite becomes fully illuminated to the observer. Since multiply<br />
1216 scattered photons, which partly illuminate primary shadows, become more numerous as the albedo<br />
1217 increases, this effect becomes less pronounced for bright surfaces. On the other hand, the huge<br />
1218 increase at very small solar phase angles can be explained by coherent backscatter, a phenomenon<br />
1219 in which photons following identical but reversed paths in a surface interfere constructively in exactly<br />
1220<br />
the backscattering direction, causing brightness to increase by up to a factor <strong>of</strong> two (Hapke, 1990).
1221 The Cassini flyby in June 2004 prior to Saturn orbit insertion afforded views at large phase angles –<br />
1222 important for modeling roughness - and over a full excursion in geographical longitudes. Phoebe<br />
1223 exhibits a substantial forward-scattering component to its single particle phase function. This result<br />
1224 is consistent with a substantial amount <strong>of</strong> fine-grained dust on the surface <strong>of</strong> the satellite generated<br />
1225 by particle infall, as suggested by Clark et al. (2005, 2008a). As observed in Cassini images (Porco<br />
1226 et al., 2005a), Phoebe also shows extensive macroscopic roughness, hinting at the violent collisional<br />
1227 history predicted by Nesvorny et al. (2003).<br />
1228 ISS images <strong>of</strong> the low-albedo hemisphere <strong>of</strong> Iapetus were fitted to the crater roughness model<br />
1229 (Buratti and Veverka, 1985), yielding a markedly smooth value for the degree <strong>of</strong> macroscopic<br />
1230 roughness on this side with a mean slope angle <strong>of</strong> only a few degrees (Lee et al., 2009). This result<br />
1231 suggests that small-scale rough features on the dark side have been filled in (features which<br />
1232 probably dominate the photometric effects; see Helfenstein and Shepard, 1998), which is consistent<br />
1233 with the idea <strong>of</strong> Spencer and Denk (2009) that thermal migration <strong>of</strong> water ice leaves behind a fluffy<br />
1234 residue <strong>of</strong> dark material. The low-albedo side <strong>of</strong> Iapetus shows a roughness similar to Enceladus,<br />
1235 which is coated with micron-sized particles from its plume and the E-ring (Verbiscer and Veverka,<br />
1236<br />
1237<br />
1994; see Table 21.4.1). Model-fits for the low albedo hemisphere are shown in Fig. 14.<br />
1238 Fig. 16 A macroscopic roughness model for the low-albedo hemisphere <strong>of</strong> Iapetus, based on the<br />
1239 crater roughness model by Buratti and Veverka (1985). The best-fit roughness function corresponds<br />
1240 to a depth-to-radius <strong>of</strong> 0.14, or a mean slope angle <strong>of</strong> 11º, which is much lower than the ~30º typical<br />
1241<br />
1242<br />
1243<br />
<strong>of</strong> other icy satellites (see Tab. 4). For these fits, a surface phase function f(α) <strong>of</strong> 0.023 was derived.<br />
From Lee et al., 2009<br />
1244 Finally, observations at very large solar phase angles (155º-165º) have been used to search for<br />
1245 cryovolcanic activity on satellites. By comparing the brightness <strong>of</strong> Rhea to Enceladus in this phase<br />
1246 angle range at 2.02µm, Pitman et al. (2008) were able to place an upper limit on water vapor<br />
column density based on a possible plume <strong>of</strong> 1.52 x 10 14 to 1.91 x 10 15 cm -2 1247 , two orders <strong>of</strong><br />
1248<br />
1249<br />
magnitude below the observed plume density <strong>of</strong> Enceladus (Fig. 17).<br />
1250 Fig. 17 Cassini VIMS disk-integrated brightness as a function <strong>of</strong> solar phase angle at 2.23µ.<br />
1251 Symbols: Normalized Rhea and Enceladus VIMS brightness (every fifth data point plotted). Solid<br />
1252 line: Third-order polynomial fit to Rhea data. Dashed lines: (polynomial fit to Rhea data).<br />
1253 Enceladus's plume can be seen as a peak at a solar phase angle <strong>of</strong> 159°. Adapted from Pitman et al.<br />
1254<br />
1255<br />
(2008)<br />
1256 The microphysical structure <strong>of</strong> the E-ring grain coatings <strong>of</strong> the inner icy moons embedded in the E-<br />
1257 ring can be probed by investigating the opposition surge. Verbiscer et al. (2007) found that the<br />
1258 amplitude and angular width <strong>of</strong> the opposition effect on the inner icy moons are correlated with the<br />
1259<br />
1260<br />
moons’ position relative to the E-ring.<br />
1261 21.5 Constraints on the Top-Meter Structure and Composition by Radar<br />
1262 The wavelength <strong>of</strong> Cassini's 13.8GHz RADAR instrument, 2.2cm, is about six times shorter than<br />
1263<br />
1264<br />
the only ground-based radar wavelength available to study the satellites (13cm, at Arecibo) and 22<br />
times longer than the millimeter wavelengths at the limit <strong>of</strong> the CIRS.<br />
1265 The echoes result from volume scattering, and their strength is sensitive to ice purity. Therefore,<br />
1266 they provide unique information about near-surface structural complexity as well as about<br />
1267 contamination with non-ice material. This section summarizes the most basic aspects <strong>of</strong> Cassini-<br />
1268 and ground-based radar observations <strong>of</strong> the satellites, the theoretical context for interpreting the<br />
1269 echoes, and the inferences drawn about subsurface structure and composition.<br />
1270 Cassini measures echoes in the same linear polarization as transmitted, whereas most ground-based<br />
1271 radar astronomy consists <strong>of</strong> Arecibo 13cm or 70cm observations and Goldstone 3.5cm observations<br />
1272 that almost always use transmission <strong>of</strong> a circularly polarized signal and simultaneous reception <strong>of</strong><br />
1273 echoes in same-circular (SC) and opposite-circular (OC) polarizations. A radar target's radar albedo<br />
1274 is defined as its radar cross section divided by its projected area. The radar cross section is the<br />
1275 projected area <strong>of</strong> a perfectly reflective isotropic scatterer which, if observed at the target's distance<br />
1276 from the radar and using the same transmitted and received polarizations, would return the observed<br />
1277 echo power. The total-power radar albedo is the sum <strong>of</strong> the albedos in two orthogonal polarizations,<br />
1278 TP = SL + OL = SC + OC. Here, we will use 'SL-2' to denote the 2cm radar albedo in the SL<br />
1279<br />
polarization, 'TP-13' to denote the 13cm total-power albedo, etc.<br />
24
1280 For a smooth, homogeneous target, single back reflections would be entirely OC when the<br />
1281 transmission is circularly polarized, or entirely SL when the transmission is linearly polarized, so<br />
1282 the polarization ratios SC/OC and OL/SL would equal zero. Saturn's icy satellites and the icy<br />
1283 Galilean satellites not only have radar albedos an order <strong>of</strong> magnitude greater than those <strong>of</strong> the Moon<br />
1284 and most other solar system targets but also unusually large polarization ratios.<br />
1285 These strange radar signatures were discovered in observations <strong>of</strong> Europa, Ganymede and Callisto<br />
1286 in the mid-1970s. It took a decade and a half to realize that the signatures are the outcome <strong>of</strong><br />
1287 'coherent backscattering,' which is phase-coherent, multiple scattering within a dielectric medium<br />
1288 that is both heterogeneous and non-absorbing (e.g. MacKintosh and John, 1989; Hapke, 1990).<br />
1289 During the next decade, Peters (1992) produced a vector formulation <strong>of</strong> coherent backscattering that<br />
1290 led to predictions <strong>of</strong> radar albedos and polarization ratios, and Black (1997) and Black et al. (2001b)<br />
1291 built on Peters' work with models <strong>of</strong> scatterer properties.<br />
1292 Extremely clean ice is insufficient for anomalously large radar albedos and circular polarization<br />
1293 ratios, which arise from multiple scattering within a random, disordered medium whose intrinsic<br />
1294 microwave absorption is very low. Thus, perfectly clean ice that is structurally homogeneous<br />
1295 (constant density at millimeter scales) would not give anomalous echoes. The regoliths on solar<br />
1296 system objects are structurally very heterogeneous, with complex density variations from the<br />
1297 distribution <strong>of</strong> particle sizes and shapes produced by impacts <strong>of</strong> projectiles <strong>of</strong> different sizes and<br />
1298 impact energies.<br />
1299 It is to be expected that the uppermost few meters <strong>of</strong> Saturn's icy satellite regolith are basically<br />
1300<br />
1301<br />
similar to those <strong>of</strong> the icy Galilean satellites or the Moon's in terms <strong>of</strong> particle size distribution,<br />
porosity and density as functions <strong>of</strong> depth, and lateral/vertical heterogeneity. It is the combination<br />
1302 <strong>of</strong> naturally complex density variations and the extreme transparency <strong>of</strong> pure water ice that gives<br />
1303 icy regolith its exotic radar properties.<br />
1304 The icy Galilean satellites were observed in dual-circular-polarization experiments from the ground<br />
1305 at 3.5, 13, and 70cm; experiments measuring SL-13 and OL-13 were done only for Europa,<br />
1306 Ganymede and Callisto. Arecibo obtained estimates <strong>of</strong> SC-13 and OC-13 for Enceladus, Tethys,<br />
1307 Dione, Rhea and Iapetus. The Cassini radar measured SL-2 for Mimas, Hyperion and Phoebe. The<br />
1308 satellites' albedos show different styles and are location- and wavelength-dependent, as discussed<br />
1309 below. The results described in this chapter make use <strong>of</strong> 73 Cassini SL-2 measurements, more than<br />
1310 twice the number reported by Ostro et al. (2006).<br />
1311 If a radar target is illuminated by a uniform antenna beam (that is, if the antenna gain is constant<br />
1312 across the disk), then it is a simple matter to use the appropriate 'radar equation' to convert the<br />
1313 measured echo power spectrum (power as a function <strong>of</strong> Doppler frequency) into a radar cross<br />
1314 section (e.g. Ostro, 1993). For Cassini measurements, except for the SAR imaging <strong>of</strong> Iapetus<br />
1315 discussed below, the antenna beam width is within a factor <strong>of</strong> several times the target's angular<br />
1316 width, so the gain varies across the target's disk, usually significantly. Data reduction yields an<br />
1317 estimate <strong>of</strong> echo power, which is the sum <strong>of</strong> contributions from different parts <strong>of</strong> the surface, with<br />
1318 each contribution weighted by the two-way gain. However, the distribution <strong>of</strong> echo power as a<br />
1319 function <strong>of</strong> location on the target disk is not known a priori. Thus, we do not know how much the<br />
1320 echo power from any given surface element has been 'amplified' by the antenna gain. To estimate a<br />
1321 radar albedo from our data we must make an assumption about the homogeneity and angular<br />
1322<br />
1323<br />
dependence <strong>of</strong> the surface's radar scattering. For the purpose <strong>of</strong> obtaining a radar albedo estimate<br />
from an echo power spectrum, one assumes uniform, azimuthally isotropic, cosine scattering laws,<br />
1324 and uses least squares to estimate the value <strong>of</strong> a single parameter that defines the echo's incidence-<br />
1325<br />
1326<br />
angle dependence. In general, only modest departures from Lambert limb darkening are found.<br />
1327 Radar-Optical Correlations<br />
1328<br />
1329<br />
Fig. 18 shows all measurements <strong>of</strong> the satellites' individual SL-2 albedos along with their<br />
1330 Fig. 18 Individual Cassini measurements <strong>of</strong> SL-2 (small symbols) and average optical geometrical<br />
1331 albedos, plotted for the satellites vs. their distances from Saturn. Optical albedos <strong>of</strong> Mimas,<br />
1332 Enceladus, Tethys, Dione and Rhea are from Verbiscer et al. (2007); those <strong>of</strong> Iapetus and Phoebe<br />
1333 are from Morrison et al. (1986) and involve a V filter; Hyperion’s is from Cruikshank and Brown<br />
1334 (1982). Note the similar distance dependences. Hyperion's radar albedo is large and appears to<br />
1335 break the pattern, perhaps (Ostro et al. 2006) because Hyperion's optical coloring may be due to a<br />
1336 thin layer <strong>of</strong> exogenously derived material that, in contrast to the other satellites, has never been<br />
1337<br />
1338<br />
worked into the icy regolith, which remains very clean and radar-bright.<br />
25
1339 average optical geometrical albedos, plotted vs. distance from Saturn. A very pronounced radar-<br />
1340 optical correlation means that variations in optical and radar albedos involve the same or related<br />
1341 contaminant(s) and/or surface processes. Absolutely pure water ice at satellite surface temperatures<br />
1342 is extremely transparent to radar waves. Contamination <strong>of</strong> water ice with no more than trace<br />
1343 concentrations <strong>of</strong> virtually any other substance (including earth rocks, chondritic meteorites, lunar<br />
1344 soil, ammonia, metallic iron, iron oxides and tholins) can decrease its radar transparency (and hence<br />
1345 the regolith's radar albedo) by one to two orders <strong>of</strong> magnitude. Phenomena that depend on the<br />
1346 distance from Saturn include primordial composition, meteoroid flux and the influx <strong>of</strong> dark material<br />
1347 (perhaps from several sources), the balance between the influx <strong>of</strong> ammonia-containing particles and<br />
1348 the removal <strong>of</strong> ammonia by a combination <strong>of</strong> ion erosion and micrometeoroid gardening, variation<br />
1349 in E-ring fluxes, and the systematics <strong>of</strong> thermal volatiles redistribution.<br />
1350 Modeling <strong>of</strong> icy Galilean satellite echoes (Black et al., 2001b) suggests that penetration to about<br />
1351 half a meter should be adequate to produce the range <strong>of</strong> measured values <strong>of</strong> SL-2.<br />
1352 The explanation by Clark et al. (2008b, 2009) for dark Saturnian system material, which suggests<br />
1353 that it consists <strong>of</strong> sub-micron iron particles from meteorites which oxidized and became mixed with<br />
1354 ice, would also suffice to explain, at least qualitatively, the overall radar-optical albedo correlation<br />
1355 in Fig. 18 if the contamination <strong>of</strong> the ice extends some one to several decimeters below the optically<br />
1356 visible surface, as expected by Clark et al. (2008b, 2009). The tentative detection <strong>of</strong> ammonia in<br />
1357 VIMS spectra by Clark et al. (2008a) opens up the possibility that contamination by this material<br />
1358 may play an important role in inter- and intra-satellite radar albedo variations. If so, this<br />
1359<br />
1360<br />
contamination would affect the radar but not the optical albedo.<br />
Almost all Cassini radar data on the icy satellites consist <strong>of</strong> real-aperture scatterometry, which<br />
1361 means that the beam size determines the resolution. Cassini has no real-aperture resolution <strong>of</strong><br />
1362 Mimas, Enceladus, Hyperion or Phoebe, but does have abundant measurements for Tethys, Dione,<br />
1363 Rhea and Iapetus with beams smaller than, and in many cases less than half, the size <strong>of</strong> the disk<br />
1364 (Ostro et al., 2006).<br />
1365 The SL-2 distributions <strong>of</strong> the individual satellites can be summarized briefly as follows. As regards<br />
1366 Mimas, Herschel may or may not contribute to the highest value <strong>of</strong> the disc-integrated albedo.<br />
1367 Enceladus shows no dramatic leading/trailing asymmetry and no noteworthy geographical<br />
1368 variations except for a low albedo in an observation <strong>of</strong> the Saturn-facing side. Tethys' leading side<br />
1369 is brighter both optically and in SL-2. Dione's highest-absolute-latitude views give the highest radar<br />
1370 albedos. Rhea's leading side has a lower SL-2 in the north, where there is much cratering, and<br />
1371 higher in the south, where there are optically bright rays. Its trailing and Saturn-facing sides, which<br />
1372 look younger, show a relatively high SL-2. Iapetus displays an extremely marked correlation<br />
1373 between optical brightness and SL-2.<br />
1374 In general, a first-order understanding <strong>of</strong> the SL-2 distribution is that higher SL-2 means cleaner<br />
1375 near-surface ice, but the nature <strong>of</strong> the contaminant(s) and the depth <strong>of</strong> the contamination are open<br />
1376 questions. The latter can be constrained by measuring radar albedos at more than one wavelength.<br />
1377 Fortunately, there are Arecibo 13-cm observations <strong>of</strong> Enceladus, Tethys, Dione, Rhea (Black et al.,<br />
1378 2004) and Iapetus (Black et al., 2007).<br />
1379 Also fortunately, the wavelength dependence observed for Europa, Ganymede and Callisto (EGC)<br />
1380 gives us a context for discussing the wavelength dependence observed for Saturn's satellites.<br />
1381<br />
1382 Wavelength Dependence<br />
1383 EGC total-power radar albedos and circular polarization ratios at 3.5cm and 13cm are<br />
1384 indistinguishable. This means that their regolith structure (distribution <strong>of</strong> density variations) and ice<br />
1385 cleanliness look identical at the two wavelengths, for each object. That is, near-surface structures<br />
1386 are 'self-similar' at 3.5cm and 13cm scales; there are no depth-dependent phenomena, like<br />
1387 contamination levels or structural character, perceptible in EGC's 3.5cm and 13cm radar signatures.<br />
1388 At 70cm, moreover, EGC SC/OC ratios show little wavelength dependence, so subsurface multiple<br />
1389 scattering remains the primary source <strong>of</strong> the echo even on this longer scale. However, the radar<br />
1390 albedos (TP-70) plummet for all three objects. The implication is that at 70cm, scattering<br />
1391 heterogeneities are relatively sparse, and the self-similarity <strong>of</strong> the regolith structure disappears on<br />
1392 the larger scale. On Europa at least, with the most striking albedo drop from TP-13 to TP-70, the<br />
1393 regolith may simply be too young for it to be heterogeneous at 70cm.<br />
1394 Within the framework <strong>of</strong> the Black et al. (2001b) model, this implies that at 70cm 'the scattering<br />
1395 layer is disappearing in the sense that there are too few scatterer at this scale and they are confined<br />
1396 to a layer too shallow to permit numerous multiple scatterings. At even longer wavelengths, <strong>of</strong> the<br />
1397<br />
order <strong>of</strong> several meters, the reflectivity from the surfaces <strong>of</strong> these moons, and Europa in particular,<br />
26
1398 may drop precipitously or even vanish as the layers responsible for the centimeter-wavelength radar<br />
1399 properties become essentially transparent.'<br />
1400 Now, given that the 3.5cm and 13cm polarization properties <strong>of</strong> EGC show no significant<br />
1401 wavelength dependence, and that the 13cm OL/SL ratios <strong>of</strong> those objects are very similar, let us<br />
1402 assume that the linear polarization properties <strong>of</strong> the Saturnian satellites are wavelength-invariant.<br />
1403 Let us then use EGC's mean OL/SL value to estimate total-power 2cm albedos: TP-2 = (1.52 +/-<br />
1404 0.13) SL-2 and compare them to the Arecibo 13cm total-power albedos (TP-13).<br />
1405<br />
1406<br />
Fig. 19 shows relevant Cassini and Arecibo information.<br />
1407 Fig. 19 Wavelength dependence <strong>of</strong> total-power radar albedos. Individual symbols are estimates <strong>of</strong><br />
1408 TP-2 obtained from individual measurements <strong>of</strong> SL-2 using TP-2 = 1.52 SL-2 +/- 0.13 SL. Arecibo<br />
1409<br />
1410<br />
13-cm albedos are horizontal lines. Dashed lines represent EGC.<br />
1411 Moving outward from Saturn, the magnitude <strong>of</strong> 2cm to-13cm wavelength dependence is<br />
1412 unremarkable for Enceladus' leading side, extreme for Enceladus' trailing side, very large for<br />
1413 Tethys, large for Dione, unremarkable for Rhea, large for Iapetus' leading side, and extreme for<br />
1414 Iapetus' trailing side.<br />
1415 Whereas compositional variation (ice cleanliness) almost certainly does not contribute to the<br />
1416 wavelength dependence seen in EGC, it may well play a key role in the wavelength dependence<br />
1417 seen in Saturn's satellites. In this complicated system, the tapestry <strong>of</strong> satellite radar properties may<br />
1418<br />
1419<br />
involve many factors, including inter- and intra-satellite variations in E-ring flux, variations in the<br />
meteoroid or ionizing flux, variations in the concentration <strong>of</strong> radar absorbing materials (including<br />
1420 optically dark material and possibly ammonia) as a function <strong>of</strong> depth, the systematics <strong>of</strong> thermal<br />
1421 volatile redistribution (especially for Iapetus), and/or constituents with wavelength- or temperature-<br />
1422 dependent electrical properties. Here, we will present what we consider reasonable hypotheses<br />
1423 about what the most important factors are.<br />
1424 The unremarkable 2- to-13-cm wavelength dependence seen on Enceladus' leading side and also on<br />
1425 Rhea is reminiscent <strong>of</strong> Europa's 3.5- to-13-cm wavelength invariance. The simplest interpretation is<br />
1426 that at least the top few decimeters are uniformly clean and old enough for meteoroid bombardment<br />
1427 to have created density heterogeneities that are able to produce coherent backscattering and extreme<br />
1428 radar brightness.<br />
1429 Enceladus' leading-side 2cm and 13cmcm TP albedos resemble Europa's 3.5cm and 13cm TP<br />
1430 albedos, whereas Rhea's 2cm and 13cm TP albedos resemble Ganymede's 3.5cm and 13cmcm TP<br />
1431 albedos, reflecting much greater contamination <strong>of</strong> Rhea's upper regolith.<br />
1432 Dione's and Rhea's TP-2 albedos are similar, but Dione is dimmer at 13cm. Dione VIMS data show<br />
1433 evidence <strong>of</strong> a bombardment <strong>of</strong> apparently dark particles on the trailing side (Clark et al., 2008a). If<br />
1434 the dark material contains ammonia, and the ejecta systematics <strong>of</strong> meteoroid bombardment<br />
1435 efficiently distribute the dark material over the surface <strong>of</strong> the object, multiple scattering will be cut<br />
1436 <strong>of</strong>f at a depth that may be too shallow for TP-13 to be enhanced.<br />
1437 The striking wavelength dependence <strong>of</strong> Enceladus' trailing side (TP-2 >>TP-13) mirrors Europa's<br />
1438 (TP-13 >>TP-70). It is easily understood if we assume that the top meter or so <strong>of</strong> ice was deposited<br />
1439 so recently that there has not been time for the regolith to be matured by meteoroid bombardment.<br />
1440<br />
1441<br />
That is, there are small-scale heterogeneities that produce coherent backscattering at 2cm, but none<br />
<strong>of</strong> the larger-scale heterogeneities needed to produce coherent backscattering at 13cm.<br />
1442 This idea is consistent with the fact that Enceladus' trailing side is optically brighter than its leading<br />
1443 side, apparently because <strong>of</strong> the greater E-ring flux it receives (Buratti, 1988; Showalter et al.,<br />
1444 1991b; Buratti et al., 1998). So all <strong>of</strong> Enceladus is very clean, but the uppermost layer <strong>of</strong> one or<br />
1445 several meters on the trailing side is also extremely young, resulting in a striking hemispheric<br />
1446 dichotomy at 13cm that is the most intense seen at that wavelength in the icy-satellite systems <strong>of</strong><br />
1447 either Jupiter or Saturn.<br />
1448 At least some <strong>of</strong> the decrease in average SL-2 (Fig. 18) and the decrease in average wavelength<br />
1449 dependence outward from Enceladus (Fig. 19) is probably due to the outward decrease in E-ring<br />
1450 flux (Verbiscer et al., 2007).<br />
1451 Finally, we get to Iapetus, where TP-2 is strongly correlated with the optical albedo: low for the<br />
1452 optically dark leading-side material and high for the optically bright trailing-side material.<br />
1453 However, Iapetus' TP-13 values show much less <strong>of</strong> a dichotomy and are several times lower than<br />
1454 TP-2, being indistinguishable from the weighted mean <strong>of</strong> TP-13 for main-belt asteroids, 0.15 ± 0.10<br />
1455<br />
1456<br />
(Tab. 5).<br />
27
1457 Dark/Leading Bright/Trailing<br />
1458 2.2-cm 0.32 + 0.09 0.58 + 0.18<br />
1459 13-cm 0.13 + 0.04 0.17 + 0.04<br />
1460<br />
1461<br />
Tab. 5 Total-power radar albedos<br />
1462 Therefore, whereas Iapetus is anomalously bright at 2cm (similar to Callisto) and mimics the optical<br />
1463 hemispheric dichotomy, at 13cm it looks like a typical main-belt asteroid, with minimal evidence <strong>of</strong><br />
1464 mimicking the optical hemispheric dichotomy.<br />
1465 The Iapetus results are understandable if (1) the leading side's optically dark contaminant is present<br />
1466 to depths <strong>of</strong> at least one to several decimeters, so that SL-2 can see the optical dichotomy, and (2)<br />
1467 ammonia and/or some other radar-absorbing contaminant is globally much less abundant within the<br />
1468 upper one to several decimeters than at greater depths, which would explain the wavelength<br />
1469 dependence, that is, the virtual disappearance <strong>of</strong> the object's 2-cm signature at 13cm. The first<br />
1470 conclusion is consistent with the dark material being a shallow phenomenon, as predicted from the<br />
1471 thermal migration hypothesis discussed elsewhere in this chapter.<br />
1472 Moreover, ammonia contamination may be uniform within the top few meters, so that the<br />
1473 wavelength dependence arises because ammonia's electrical loss is much greater at 13cm<br />
1474 (2.38GHz) than at 2cm (13.8GHz). Unfortunately, the likelihood <strong>of</strong> this possibility cannot yet be<br />
1475 judged because measuring the complex dielectric constant <strong>of</strong> ammonia-contaminated water ice as a<br />
1476 function <strong>of</strong> concentration, frequency and temperature is so difficult that, although needed, it has not<br />
1477<br />
1478<br />
been done yet. This is a pity, because the lack <strong>of</strong> this information undermines the interpretation <strong>of</strong><br />
the 2cm to13cm wavelength dependence seen in other Saturnian satellites as well as <strong>of</strong> the possible<br />
1479 influence <strong>of</strong> ammonia and its 'modulation' by magnetospheric bombardment (which varies with<br />
1480 distance from Saturn) on the dependence <strong>of</strong> SL-2 on the distance from Saturn.<br />
1481 Verbiscer et al. (2006) suggested that, while their spectral data do not contain an unambiguous<br />
1482 detection <strong>of</strong> ammonia hydrate on Enceladus, their spectral models do not rule out the presence <strong>of</strong> a<br />
1483 modest amount <strong>of</strong> it on both hemispheres. However, they note that the trailing hemisphere is also<br />
1484 exposed to increased magnetospheric particle bombardment, which would preferentially destroy the<br />
1485<br />
1486<br />
ammonia. In any case, ammonia does not appear to be a factor in Enceladus’ radar albedos.<br />
1487 Iapetus Radar Image<br />
1488 During the Iapetus 49 flyby, RADAR obtained an SAR image at 2km to12km surface resolution<br />
1489 that covers much <strong>of</strong> the object's leading (optically darker) side. From a comparison with optical<br />
1490 imaging (Fig. 20), we see that, as expected, the SAR reliably reveals the surface features seen in<br />
1491<br />
1492<br />
ISS images.<br />
1493 Fig. 20 RADAR SAR image <strong>of</strong> the leading side <strong>of</strong> Iapetus (Sept. 2007, left) and the ISS mosaic<br />
1494 PIA 08406 (Jan. 2008), using similar projections and similar scales (the large crater (on the right in<br />
1495<br />
1496<br />
the ISS image is about 550 km in diameter).<br />
1497 The pattern <strong>of</strong> radar brightness contrast is basically similar to that seen optically, except for the<br />
1498 large basin at 30°N, 75°W, which has more SL-2-bright than optically bright highlights. This<br />
1499<br />
1500<br />
suggests that the subsurface <strong>of</strong> these highlights at decimeter scales may be cleaner than in other<br />
parts <strong>of</strong> the optically dark terrain, which is consistent with the idea that the contamination by dark<br />
1501 material might be shallow, at least in this basin. Conversely, many optically bright crater rims are<br />
1502<br />
1503<br />
SL-2-dark, so the exposure <strong>of</strong> clean ice cannot be very deep.<br />
1504 21.6 Geological Evolution<br />
1505 The densities <strong>of</strong> the Saturnian satellites constrain their composition to be about 2/3 ice and 1/3 rock.<br />
1506 Accretional energy, tidal despinning and radioactive decay might have heated and melted the<br />
1507 moons. Due to the very low melting temperatures <strong>of</strong> various water clathrates, it is likely that the<br />
1508 larger bodies differentiated early in their evolution while smaller satellites may still be composed<br />
1509 and structured in their primitive state. For a more detailed discussion <strong>of</strong> the aspects <strong>of</strong> origin and<br />
1510 thermal evolution, see Johnson et al. (2009) and Matson et al. (2009).<br />
1511 The heavily cratered solid surfaces in the Saturnian system indicate the role <strong>of</strong> accretional and post-<br />
1512 accretional bombardement in the geological evolution <strong>of</strong> the satellites. Observed crater densities are<br />
1513 summarized in Dones et al. (2009). However, the source as well as the flux <strong>of</strong> bombarding bodies is<br />
1514 still under discussion, and Dones et al. (2009) came to the conclusion that, since no radiometric<br />
1515<br />
sample data exist from surfaces in the outer solar system and the size-frequency distribution <strong>of</strong><br />
28
1516 comets is less well understood than that <strong>of</strong> asteroids, the chronology <strong>of</strong> the moons <strong>of</strong> the giant<br />
1517 planets remains in a primitive state. Therefore, we refer the reader to Dones et al. (2009) for the<br />
1518 crater chronology discussion. In interpreting the geological evolution <strong>of</strong> the Saturnian satellites, we<br />
1519 will rely on relative stratigraphic relations. Nevertheless, absolute ages are given in Dones et al.<br />
1520 (2009).<br />
1521 In a stratigraphic sense, three major surface unit categories can be distinguished on all icy satellites:<br />
1522 (1) Cratered plains, which are characterized by dense crater populations that are superimposed by<br />
1523 all other geological features and, thus, are the relatively oldest units.<br />
1524 (2) Tectonized zones <strong>of</strong> narrow bands and mostly sub-parallel fractures, faults, troughs and ridges<br />
1525 <strong>of</strong> global extension indicating impact-induced or endogenic crustal stress. These units dissect<br />
1526 cratered plains and partly dissect each other, indicating different relative ages.<br />
1527 (3) Resurfaced units consisting <strong>of</strong> terrain that has undergone secondary exogenic and endogenic<br />
1528 processes such as mass wasting, surface alterations (due to micrometeorite gardening, sputtering,<br />
1529 thermal segregation and crater ray formation by recent impacts), contamination by dark material<br />
1530 and cryovolcanic deposition mostly resulting in smoothed terrain. Superimposed on cratered plains<br />
1531 and tectonized regions, these are the youngest surface units.<br />
1532<br />
1533<br />
Heavily cratered surfaces are found on all the icy rocks and small satellites (Fig. 21) as well as on<br />
1534 Fig. 21 Phoebe, 213 km in average diameter, was the first one <strong>of</strong> the nine classic moons <strong>of</strong> Saturn to<br />
1535 be captured by the ISS cameras aboard Cassini in a close flyby about three weeks prior to Cassini’s<br />
1536<br />
1537<br />
Saturn Orbit Insertion (SOI) in July 2004. The mosaic shows a densely cratered surface. Higherresolution<br />
data revealed that the high frequency <strong>of</strong> craters extends with a steep slope down to craters<br />
1538 only tens <strong>of</strong> meters in diameter (Porco et al., 2005). Image source: http://photojournal.jpl.nasa.gov,<br />
1539<br />
1540<br />
image identification PIA06064.<br />
1541 the medium-sized moons Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus and<br />
1542 Phoebe, indicating an intense post-accretional collision process relatively early in the geological<br />
1543 history <strong>of</strong> the satellites. Cassini observations added to the number <strong>of</strong> large basins and ring structures<br />
1544 mainly on Rhea, Dione and Iapetus. Differences in the topographical expression <strong>of</strong> these impact<br />
1545 structures suggest post-impact processes such as relaxation, which may reflect differences in the<br />
1546 thermal history and/or crustal thickness <strong>of</strong> individual satellites (e.g. Schenk and Moore, 2007; Giese<br />
1547 et al., 2008). Although age models are based on different impactor populations, the cratered<br />
1548 surfaces on Iapetus seem to be the oldest in the Saturnian system, whereas cratered plains on the<br />
1549 other satellites seem to be younger (see Dones et al., 2009). One large (48 km in diameter)<br />
1550<br />
1551<br />
stratigraphically young ray crater was found on Rhea (Wagner et al., 2007) (Fig.22).<br />
1552 Fig. 22 A prominent bright ray crater with a diameter <strong>of</strong> 48 km is located approximately at 12.5° S,<br />
1553 112° W on Saturn’s second-largest satellite Rhea. The crater is stratigraphically young, indicated by<br />
1554 the low superimposed crater frequency on its floor and continuous ejecta (Wagner et al., 2008). The<br />
1555 high-resolution mosaic (right) <strong>of</strong> NAC images, shown in context <strong>of</strong> a WAC frame, was obtained by<br />
1556 Cassini ISS during orbit 049 on Aug. 30, 2007. The global view to the left showing the location <strong>of</strong><br />
1557 the detailed view is a single NAC image from orbit 056.<br />
1558<br />
1559 While the small satellites lack tectonic features, the medium-sized bodies experienced rotation-,<br />
1560 tide- and impact-induced deformation and volume changes, which created extensional or<br />
1561 contractional tectonic landforms. Parallel lineaments on Mimas seem to be related to the Herschel<br />
1562 impact event (McKinnon, 1985; Schenk, 1989a), although freezing expansion cannot be ruled out as<br />
1563<br />
1564<br />
the cause (Fig. 23).<br />
1565 Fig. 23 The anti-Saturnian hemisphere <strong>of</strong> Mimas, centered approximately at lat. 22° S, long. 185°<br />
1566 W, is shown in a single image (NAC frame N1501638509) at a resolution <strong>of</strong> 620 m/pixel, taken<br />
1567 during a non-targeted encounter in August 2005. The densely cratered landscape implies a high<br />
1568<br />
1569<br />
surface age. Grooves and lineaments infer weak tectonic on this geologically unevolved body.<br />
1570 Enceladus exhibits the greatest geological diversity in a complex stress and strain history that is<br />
1571 predominantly tectonic and cryovolcanic in origin, with both processes stratigraphically correlated<br />
1572 in space and time, although the energy source is so far not completely understood (e.g. Smith et al.,<br />
1573 1982; Kargel and Pozio, 1996; Porco et al., 2006; Nimmo and Pappalardo, 2006; Nimmo et al.,<br />
1574<br />
2007; Jaumann et al., 2008). Enceladus is discussed in detail in Spencer et al. (2009).<br />
29
1575 Located in densely cratered terrain, the nearly globe-encircling Ithaca Chasma rift system on Tethys<br />
1576 (Fig. 2) is 100 km wide, consists <strong>of</strong> at least two narrower branches towards the south and was<br />
1577 caused either by expansion when the liquid-water interior froze (Smith et al., 1981) or by<br />
1578 deformation by the large impact which formed Odysseus (Moore and Ahern, 1983). However,<br />
1579 crater densities derived from Cassini images suggest Ithaca Chasma to be older than Odysseus<br />
1580 (Giese et al., 2007), so that the rift system may be an endogenic feature.<br />
1581 Dione exhibits a nearly global network <strong>of</strong> tectonic features such as troughs, scarps, ridges and<br />
1582<br />
1583<br />
lineaments partly superimposed on each other (Fig. 24), which correlate with smooth plains<br />
1584 Fig. 24 This image <strong>of</strong> Tethys, taken by the Cassini ISS NA camera in May 2008 in visible light,<br />
1585 shows the old, densely cratered surface <strong>of</strong> Tethys and a part <strong>of</strong> the extensive graben system <strong>of</strong><br />
1586 Ithaca Chasma. The graben is up to 100 km wide and is divided into two branches in the region<br />
1587 shown in this image. Using digital elevation models infers flexural uplift <strong>of</strong> the graben flanks and a<br />
1588 total topography ranging several kilometers from the flanks to the graben floor (Giese et al., 2007).<br />
1589 Image details: ISS frame number N1589080288, scale 1 km/pixel, centered at lat. 42° S, long. 50°<br />
1590<br />
1591<br />
W in orthographic projection.<br />
1592 (e.g. Plescia, 1983; Moore, 1984; Stephan et al., 2008). The global orientation <strong>of</strong> Dione’s tectonic<br />
1593 pattern is non-random, suggesting a complex endogenic history <strong>of</strong> extension followed by<br />
1594 compression. Cassini images indicate that some <strong>of</strong> the grabens are fresh, which implies that tectonic<br />
1595<br />
1596<br />
activity may have persisted into geologically recent times (Wagner et al., 2006). Cassini also<br />
revealed a crater-lineament relationship on Dione (Moore et al., 2004; Schenk and Moore, 2007),<br />
1597 indicating exogenic-induced tectonic activity in the early geological history <strong>of</strong> the moon. Thus,<br />
1598 Dione’s crust not only experienced stress caused by different exogenic and endogenic processes,<br />
1599 such as ancient impacts, despinning and volume change over a long period <strong>of</strong> time, but also more<br />
1600 recent tidal stress. As on Dione, wispy accurate albedo markings on Rhea are associated with an<br />
1601 extensional fault system (Moore and Schenk et al., 2007; Wagner et al., 2007), although Rhea<br />
1602<br />
1603<br />
seems less endogenically evolved than the other moons (Fig. 25).<br />
1604 Fig. 25 Map <strong>of</strong> Dione’s trailing hemisphere showing an age sequence <strong>of</strong> troughs, graben and<br />
1605 lineaments, inferred from various crosscutting relationships (Wagner et al., 2009). Three age groups<br />
1606 <strong>of</strong> troughs and graben can be distinguished: Carthage and Clusium Fossae are oldest, cut by the<br />
1607 younger systems <strong>of</strong> Eurotas and Palatine Chasmata, which in turn are truncated by Padua Chasmata<br />
1608 which are the youngest. Several systems <strong>of</strong> lineaments with various trends, either parallel or radial<br />
1609 (the latter previously assumed to be a bright ray crater named Cassandra) can be mapped which<br />
1610<br />
1611<br />
appear to be younger than the troughs and graben.<br />
1612 Parallel north-south trending lineaments are thought to be the result <strong>of</strong> extensional stress followed<br />
1613 by compression (Moore et al., 1985) and appear in Cassini observations as poorly developed ridges.<br />
1614 This suggests that they are older than the grabens identified in the wispy terrain that appear to have<br />
1615 formed more recently (Wagner et al., 2006).<br />
1616 Iapetus’ equatorial ridge (Porco et al., 2005a; Giese et al., 2008) (Fig. 7) is densely cratered, making<br />
1617<br />
1618<br />
it comparable in age to any other terrains on this moon (Schmedemann et al., 2008). Models explain<br />
the origin <strong>of</strong> the ridge either by endogenic shape and/or spin state changes (e.g. Porco et al., 2005a;<br />
1619 Giese et al., 2008) or exogenic causes, with the ridge accumulating from ring material left over<br />
1620 from the formation <strong>of</strong> proto-Iapetus (Ip, 2006). On Iapetus, diurnal tidal stress is rather low,<br />
1621 indicating that the ridge is old, and its preservation suggests the formation <strong>of</strong> a thick crust early in<br />
1622 the moon's history (Castillo-Rogez et al., 2007).<br />
1623 Enceladus, where cryovolcanism is definitely active (Porco et al., 2006), is discussed in Spencer et<br />
1624 al. (2009). Titan, where some evidence <strong>of</strong> cryovolcanism exists (Lopes et al., 2007a; Nelson et al.,<br />
1625 2009a,b) is discussed in Jaumann et al. (2009) and Sotin et al. (2009).<br />
1626 There is no direct evidence <strong>of</strong> cryovolcanic processes (Geissler, 2000) on the other Saturnian<br />
1627 satellites. Although it has been suggested in pre-Cassini discussions that the plains on Dione<br />
1628 (Plescia, and Boyce, 1982; Plescia, 1983; Moore, 1984) and Tethys (Moore and Ahern, 1984) are<br />
1629 endogenic in origin, Cassini has not directly observed any cryovolcanic activity on these satellites.<br />
1630 The plains correlate with tectonic features and exhibit distinct boundaries with more densely<br />
1631 cratered units (Moore and Schenk, 2007), which indicate that the plains are a stratigraphically<br />
1632 younger formation. In addition, there is evidence <strong>of</strong> material around Dione, Tethys and even Rhea<br />
1633<br />
(Burch et al., 2007; Jones et al., 2008; Clark et al., 2008a), but its origin is unclear. There is no<br />
30
1634 evidence <strong>of</strong> cryovolcanism on Iapetus. Although the plains on Dione and Tethys are younger than<br />
1635 the surrounding terrain, cryovolcanic activity must have occurred in ancient times, assuming that<br />
1636 such processes caused these surface units.<br />
1637 All medium-sized icy satellites appear to have a debris layer, as may be inferred from (1) the non-<br />
1638 pristine morphology <strong>of</strong> most craters; (2) the presence <strong>of</strong> groups <strong>of</strong> large boulders and debris lobes<br />
1639 seen in the highest-resolution ISS images; and (3) derived thermal-physical properties consistent<br />
1640 with loose particulate materials. With the exception <strong>of</strong> Enceladus, which is addressed in Spencer et<br />
1641 al. (2009), these debris layers were generated by impacts and their ejecta on the other satellites. The<br />
1642 upper limiting case for a given satellite is crustal disruption from extreme global impact-induced<br />
1643 seismic events. A way to evaluate the depth <strong>of</strong> fracture in such events is to assume that these events<br />
1644 were energetic enough to open fractures in the brittle ice (brittle on the timescale <strong>of</strong> these events)<br />
1645 composing the interiors <strong>of</strong> the target satellite, and that, during the impact event, the lithostatic<br />
1646 pressure everywhere was less than the brittle tensile strength <strong>of</strong> cold ice. Brittle fracture caused by<br />
1647 global seismicity, all else being equal, extends deeper into smaller satellites, given that larger<br />
1648 satellites are sheltered from dynamic rupture by their interior pressure. These being transient events,<br />
1649 a simple estimate <strong>of</strong> the realm where brittle fracture may be expected to be induced, and visibly<br />
1650 sustained, can be obtained from global seismicity by applying an expression (e.g. Turcott and<br />
1651<br />
1652<br />
Schubert, 2002) <strong>of</strong> the lithostatic pressure at radius Rp, and comparing this to the brittle tensile<br />
strength <strong>of</strong> water ice at timescales in which extreme global impact-induced seismic events produce<br />
1653 energy sufficient to break ice mechanically. The determination <strong>of</strong> the fracture depth is based on<br />
1654<br />
1655<br />
laboratory-derived dynamic values <strong>of</strong> ~1MPa (Lange and Ahrens, 1987; Hawkes and Mellor, 1972).<br />
-3<br />
With ρ the (bulk) density <strong>of</strong> the body (approximated as 1000kg/m ), G the gravitational constant,<br />
1656 and R the radius <strong>of</strong> the satellite, the depth <strong>of</strong> the fracture can be constrained. Based on this, the<br />
1657<br />
1658<br />
value <strong>of</strong> R - Rp for Mimas is ~20 km (~10% <strong>of</strong> its radius), ~10 km (~2% <strong>of</strong> radius) for Tethys, and<br />
~5 km (~0.5% <strong>of</strong> radius) for Rhea. This is a measure <strong>of</strong> the depth to which it is reasonable to expect<br />
1659 the surface layer (i.e., the megaregolith) to be intensely fractured. Of course, a body may be<br />
1660 fractured to greater depths by large impacts, and the resulting pore space may not easily be<br />
1661 eliminated. Deep porosity is most easily maintained on smaller and presumably colder bodies such<br />
1662 as Mimas (Eluszkiewicz, 1990). Thus, for Mimas we expect that relatively recent major impact<br />
1663 events, such as Herschel, might exploit pre-existing fracture patterns in the 'megaregolith' which<br />
1664 might penetrate deep into the body rather than focus energy, as has been suggested for the<br />
1665 formation <strong>of</strong> the plains <strong>of</strong> Tethys (e.g. Bruesch and Asphaug, 2004; Moore et al., 2004).<br />
1666 Consequently, the lack <strong>of</strong> antipodal focusing or axial symmetry about a radius through Herschel in<br />
1667 the lineament pattern on Mimas may be unsurprising, if that pattern is indeed an expression <strong>of</strong><br />
1668 tectonic movements initiated by the Herschel impact.<br />
1669 The impact-gardened regolith <strong>of</strong> most middle-sized icy satellites can be assumed to be many meters<br />
1670 thick if the regolith in the lunar highlands is any guide. Calculations <strong>of</strong> impact-generated regolith,<br />
1671 based on crater densities observed in Voyager images, yielded mean depths <strong>of</strong> 500m for Mimas,<br />
1672 740m for Dione, 1600m for Tethys and 1900m for Rhea (Veverka et al., 1986).<br />
1673 This regolith will be further distributed by gravity-driven mass wasting (non-linear creep, i.e.<br />
1674 disturbance-driven sediment transport that increases nonlinearly with the slope due to granular<br />
1675 creep (e.g. Roering et al., 2001; Forsberg-Taylor et al., 2004)) which reduces high-standing relief on<br />
1676<br />
1677<br />
craters (e.g. lowering <strong>of</strong> crater rims and central peaks) and tectonic features (e.g. the brinks <strong>of</strong><br />
scarps) as well as filling depressions (e.g. Howard, 2007). The process <strong>of</strong> non-linear creep, when<br />
1678 not competing with other processes, produces a landscape, which has a repetitive consistent<br />
1679 appearance. Impact-induced shaking and perhaps diurnal and seasonal thermal expansion facilitate<br />
1680 such mass wasting. Note that this common expression <strong>of</strong> mass wasting on regolith-covered slopes<br />
1681 assumes no topography-maintaining precipitation <strong>of</strong> frosts on local topographical highs, as<br />
1682 discussed below.<br />
1683 Bedrock erosion might be caused by the sublimation <strong>of</strong> mechanically cohesive interstitial volatiles.<br />
1684 The icy bedrock would crumble, move down slope through nonlinear creep and form mantling<br />
1685 slopes, perhaps leaving only topographical highs (crests) composed <strong>of</strong> bedrock exposed. This<br />
1686 situation would occur if the initial bedrock landscape disaggregates to form debris at a finite rate, so<br />
1687 that exposure <strong>of</strong> bedrock gradually diminishes, lasting longest at local crests. Such a situation might<br />
1688 explain the landscape <strong>of</strong> Hyperion. If there is re-precipitation <strong>of</strong> volatiles in the form <strong>of</strong> frosts on<br />
1689 local topographical highs, these frosts would armor the topographical highs against further<br />
1690 desegregation. It has been proposed that landscapes evolved in this fashion on Callisto (Moore et al,<br />
1691 1999; Howard and Moore, 2008). Thermally driven global and local frost migration and<br />
1692<br />
redistribution may occur on Iapetus (Spencer et al., 2005; Giese et al., 2008), as previously<br />
31
1693 discussed. This surficial frost redistribution may occur regardless <strong>of</strong> whether volatiles in the<br />
1694<br />
1695<br />
bedrock <strong>of</strong> Iapetus sublimate and contribute to bedrock erosion.<br />
1696 21.7 Conclusions<br />
1697 The icy satellites <strong>of</strong> Saturn show great and unexpected diversity. In terms <strong>of</strong> size, they cover a range<br />
1698 from ~1500 km (Rhea) to ~270 km (Hyperion) and even smaller 'icy rocks' <strong>of</strong> less than a kilometer<br />
1699 in diameter. The icy satellites <strong>of</strong> Saturn <strong>of</strong>fer an unrivalled natural laboratory for understanding the<br />
1700 geology <strong>of</strong> diverse satellites and their interaction with a complex planetary system.<br />
1701 Cassini has executed several targeted flybys over icy satellites, and more are planned for the future.<br />
1702 In addition, there are numerous other opportunities for observation during non-targeted encounters,<br />
1703 usually at greater distances. Cassini used these opportunities to perform multi-instrument<br />
1704 observations <strong>of</strong> the satellites so as to obtain the physical, chemical and structural information<br />
1705 needed to understand the geological processes that formed these bodies and governed their<br />
1706 evolution.<br />
1707 All icy satellites exhibit densely cratered surfaces. Although there have been some attempts to<br />
1708 correlate craters and potential impactor populations, no consistent interpretation <strong>of</strong> the crater<br />
1709 chronology in the Saturnian system has been developed so far (Dones et al., 2009). However, most<br />
1710 <strong>of</strong> the surfaces are old in a stratigraphical sense. The number <strong>of</strong> large basins discovered by Cassini<br />
1711 was greater than expected (e.g. Giese et al., 2008). Differences in their relaxation state provide<br />
1712 some information about crustal thickness and internal heat flows (Schenk and Moore, 2007). The<br />
1713<br />
1714<br />
absence <strong>of</strong> basin relaxation on Iapetus is consistent with a thick lithosphere (Giese et al., 2008),<br />
whereas the relaxed Evander basin on Dione exhibits floor uplift to the level <strong>of</strong> the original surface<br />
1715 (Schenk and Moore, 2007), suggesting higher heat flow.<br />
1716 The wide variety in the extent and timing <strong>of</strong> tectonic activity on the icy satellites defies any simple<br />
1717 explanation, and our understanding <strong>of</strong> Saturnian satellite tectonics and cryovolcanism still is at an<br />
1718 early stage. The total extent <strong>of</strong> deformation varies wildly, from Iapetus and Mimas (barely<br />
1719 deformed) to Enceladus (heavily deformed and currently active), and there is no straightforward<br />
1720 relationship to predicted tidal stresses (e.g. Nimmo and Pappalardo, 2006; Matsuyama and Nimmo,<br />
1721 2007, 2008; Schenk et al., 2008; Spencer et al., 2009). Extensional deformation is common and<br />
1722 <strong>of</strong>ten forms globally coherent patterns, while compressional deformation appears rare (e.g. Nimmo<br />
1723 and Pappalardo, 2006; Matsuyama and Nimmo, 2007, 2008; Schenk et al., 2008; Spencer et al.,<br />
1724 2009). These global patterns are suggestive <strong>of</strong> global mechanisms, such as despinning or<br />
1725 reorientation, which probably occurred early in the satellites’ histories (e.g. Thomas et al., 2007);<br />
1726 present-day diurnal tidal stresses are unlikely to be important, except on Enceladus (e.g. Spencer et<br />
1727 al., 2009) and (perhaps) Mimas and Dione (e.g. Moore and Schenk, 2007). Ocean freezing gives<br />
1728 rise to isotropic extensional stresses; modulation <strong>of</strong> these stresses by pre-existing weaknesses (e.g.<br />
1729 impact basins) may explain some <strong>of</strong> the observed long-wavelength tectonic patterns (e.g. Moore<br />
1730 and Schenk, 2007). Except for Enceladus, there is as yet no irrefutable evidence <strong>of</strong> cryovolcanic<br />
1731 activity in the Saturnian system, either from surface images and topography or from remote sensing<br />
1732 <strong>of</strong> putative satellite atmospheres. The next few years will hopefully see a continuation <strong>of</strong> the flood<br />
1733 <strong>of</strong> Cassini data, and will certainly see a concerted effort to characterize and map in detail the<br />
1734 tectonic structures discussed here. Understanding the origins <strong>of</strong> these structures and the histories <strong>of</strong><br />
1735<br />
1736<br />
the satellites will require both geological and geophysical investigations and probably provide a<br />
challenge for many years to come.<br />
1737 Although the surfaces <strong>of</strong> the Saturnian satellites are predominately composed <strong>of</strong> water ice,<br />
1738 reflectance spectra indicate coloring agents (dark material) on all surfaces (e.g. Fink and Larsen<br />
1739 1975; Clark et al., 1984, 1986; Roush et al., 1995; Cruikshank et al., 1998a; Owen et al., 2001;<br />
1740 Cruikshank et al., 2005; Clark et al., 2005, 2008a, 2009; Filacchione et al. 2007, 2008). Recent<br />
1741 Cassini data provide a greater range in reflected solar radiation at greater precision, show new<br />
1742 absorption features not previously seen in these bodies, and allow new insights into the nature <strong>of</strong> the<br />
1743 icy-satellite surfaces (Buratti et al., 2005b; Clark et al., 2005; Brown et al., 2006; Jaumann et al.,<br />
1744<br />
1745<br />
2006; Cruikshank et al., 2005, 2007, 2008; Clark et al., 2008a, b, 2009). Besides H2O ice, CO2<br />
absorptions are found on all medium-sized satellites (Buratti et al., 2005b; Clark et al., 2005;<br />
1746<br />
1747<br />
Cruikshank et al., 2007; Brown et al., 2006; Clark et al., 2008a), which make CO2 a common<br />
molecule on icy surfaces as well. Various spectral features <strong>of</strong> the dark material match those seen on<br />
1748 Phoebe, Iapetus, Hyperion, Dione and Epimetheus as well as in the F-ring and the Cassini Division,<br />
1749 implying that the material has a common composition throughout the Saturnian system (Clark et al.,<br />
1750 2008a, 2009). Dark material diminishes closer to Saturn, which might be due to surface<br />
1751<br />
contamination <strong>of</strong> the inner moons by E-ring particles and some chemical alteration processes (Clark<br />
32
1752 et al., 2008a). Although the general composition <strong>of</strong> the dark material in the Saturnian system has not<br />
1753<br />
1754<br />
yet been determined, it is known that it at least contains CO2, bound water, organics and, tentatively,<br />
ammonia (Clark et al., 2008a). In addition, a blue scattering peak that dominates the dark spectra<br />
1755 (Clark et al., 2008a, 2009) can be modelled by nano-phase hematite mixed with ice and dark<br />
1756 particles. This led Clark et al. (2008b, 2009) to infer that sub-micron iron particles from meteorites,<br />
1757 which oxidize (explaining spectral reddening) and become mixed with ice could best explain all the<br />
1758 colors and spectral shapes observed in the system. Iron has also been detected in E-ring particles<br />
1759 and in particles escaping the Saturnian system (Kempf et al., 2005). Previous explanations for the<br />
1760 dark material were based on tholins (e.g. Cruikshank et al., 2005), but this is inconsistent with the<br />
1761 lack <strong>of</strong> marked absorptions due to CH in the 3μm to 4μm region as well as other major absorptions at<br />
1762 wavelengths longer than 2μm. Iron particles are consistent with previous Cassini studies that<br />
1763 suggested an external origin for the dark material on Phoebe, Iapetus and Dione (Clark et al., 2005,<br />
1764 2008a,b, 2009). Another remarkable issue relating to dark material is the albedo dichotomy on Iapetus,<br />
1765 whose leading hemisphere is the darkest surface in the Saturnian system. In contrast to the outermost<br />
1766 satellite, Phoebe, also covered by dark material, Iapetus' dark hemisphere is spectrally reddish, making<br />
1767 it doubtful whether Phoebe is the source <strong>of</strong> the dark material. Clark et al. (2008a, 2009) explained the<br />
1768 color discrepancy by red fine-grained dust (
1810<br />
1811<br />
1812<br />
1813<br />
1814<br />
1815<br />
1816<br />
1817<br />
1818<br />
material has been observed throughout the system, and there is some evidence that this<br />
contamination might also be a relatively recent process.<br />
Acknowledgements: We gratefully acknowledge the long years <strong>of</strong> work by the entire Cassini team that allowed these<br />
data <strong>of</strong> the Saturnian satellites to be obtained. We should also like to thank NASA, ESA, ASI, DLR, CNES and JPL for<br />
providing support for the international Cassini team. We thank K. Stephan, R. Wagner and M. Langhans for data<br />
processing. The discussions <strong>of</strong> reviewers C. Chapman and D. Domingue are highly appreciated. Part <strong>of</strong> this work was<br />
performed at the DLR Institute <strong>of</strong> <strong>Planetary</strong> Research, with support provided by the Helmholz Alliance ’<strong>Planetary</strong><br />
Evolution and Life’ and the Jet Propulsion Laboratory under contract to the National Aeronautics and Space<br />
1819 Administration.<br />
We dedicate this work to Steve Ostro, who provided outstanding scientific contributions to the exploration <strong>of</strong> the<br />
1820<br />
1821<br />
1822<br />
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2939<br />
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2940<br />
2941<br />
Figures<br />
2942 Chap.21_Icy Sat_Revised Version March 31, 2009<br />
2943<br />
2944<br />
2945<br />
2946<br />
2947<br />
Fig. 1<br />
53
2948<br />
2949<br />
2950<br />
2951<br />
2952<br />
Fig. 2<br />
54
2953<br />
2954<br />
2955<br />
2956<br />
2957<br />
2958<br />
2959<br />
2960<br />
2961<br />
2962<br />
2963<br />
Fig. 3<br />
Fig. 4<br />
Fig. 5<br />
55
2964<br />
2965<br />
2966<br />
2967<br />
2968<br />
2969<br />
2970<br />
2971<br />
Fig. 6<br />
Fig. 7<br />
56
2972<br />
2973<br />
2974<br />
2975<br />
2976<br />
2977<br />
2978<br />
2979<br />
Fig. 8<br />
Fig. 9a<br />
57
2980<br />
2981<br />
2982<br />
2983<br />
2984<br />
2985<br />
2986<br />
2987<br />
Fig. 9b<br />
Fig. 9c<br />
58
2988<br />
2989<br />
2990<br />
2991<br />
2992<br />
2993<br />
2994<br />
2995<br />
Fig. 9d<br />
Fig. 9e<br />
59
2996<br />
2997<br />
2998<br />
2999<br />
3000<br />
3001<br />
3002<br />
3003<br />
Fig. 10<br />
Fig. 11<br />
60
3004<br />
3005<br />
3006<br />
3007<br />
3008<br />
3009<br />
3010<br />
3011<br />
3012<br />
Fig. 12<br />
Fig. 13<br />
61
3013<br />
3014<br />
3015<br />
3016<br />
3017<br />
3018<br />
3019<br />
3020<br />
Fig. 14 (left)<br />
Fig. 14 (right)<br />
62
3021<br />
3022<br />
3023<br />
3024<br />
3025<br />
3026<br />
3027<br />
3028<br />
3029<br />
3030<br />
Fig. 15 (above)<br />
Fig. 15 (lower left)<br />
63
3031<br />
3032<br />
3033<br />
3034<br />
3035<br />
3036<br />
3037<br />
3038<br />
Fig. 15 (lower right)<br />
Fig. 16 (left)<br />
64
3039<br />
3040<br />
3041<br />
3042<br />
3043<br />
3044<br />
3045<br />
3046<br />
3047<br />
3048<br />
3049<br />
Fig. 16 (right)<br />
Fig. 17<br />
65
3050<br />
3051<br />
3052<br />
3053<br />
3054<br />
3055<br />
3056<br />
Fig. 18<br />
Fig. 19<br />
66
3057<br />
3058<br />
3059<br />
3060<br />
3061<br />
3062<br />
3063<br />
3064<br />
Fig. 20<br />
Fig. 21<br />
67
3065<br />
3066<br />
3067<br />
3068<br />
3069<br />
3070<br />
3071<br />
3072<br />
Fig. 22<br />
Fig. 23<br />
68
3073<br />
3074<br />
3075<br />
3076<br />
3077<br />
3078<br />
3079<br />
3080<br />
3081<br />
Fig. 24<br />
Fig. 25<br />
69