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Tethys 132 June 3, 2010 52600 154 / 99 / 45 nT 194 195 196 Tab. 2 Mission characteristics of the Cassini icy satellites flybys; C/A ... closest approach 197 Three of the targeted flybys were dedicated to Enceladus, two in 2005 and one in 2008. In addition, 198 a very close but non-targeted Enceladus flyby happened in February 2005. As the data provided by 199 the MAPS instruments in March 2005 had raised suspicions about potential internal activity (Spahn 200 et al., 2006; Waite et al., 2006), the July 2005 flyby was lowered from 1000 km to 173 km to allow 201 for increased sensitivity in MAPS measurements. This flyby supplied overwhelming evidence from 202 multiple instruments of current activity on Enceladus, eventually confirmed by direct images of 203 plumes taken during a distant flyby in November 2005. The locations of the plumes were found to 204 coincide with linear tectonic features near the south pole, dubbed 'tiger stripes'. The CIRS 205 instrument also identified these 'tiger stripes' as unusually warm linear features (Spencer et al., 206 2006); the tiger strips contain relatively large ice particles identified by VIMS (Brown et al., 2006; 207 208 Jaumann et al., 2008). All other moons had only one targeted flyby during the nominal mission, if any. The Tethys, 209 Hyperion, and Dione flybys in the late summer/early fall of 2005 were performed within 3 weeks of 210 each other during the same orbit of Cassini around Saturn. Due to Hyperion's chaotic rotation, it 211 was impossible to predict which part of its surface would be visible and how the surface might look. 212 By coincidence, the viewing perspective during the approach was almost identical with the best 213 Voyager 1 views. It showed a giant crater, dark material within numerous smaller craters and a very low mean density of only 0.54g/cm 3 214 . The 'wispy streaks' of Dione were observed at very oblique 215 viewing angles during the October 2005 flyby, and were confirmed to be tectonic in origin (Wagner 216 et al., 2006). The images from this targeted Dione flyby are among the most spectacular pictures of 217 the mission. The Rhea flyby in November 2005 was dedicated to radio science, and the results from 218 that flyby are reported in Matson et al. (2009). However, while the spacecraft itself was inertially 219 pointed with its antenna towards Earth, the cameras slewed across the surface, obtaining the 220 highest-resolved images of Rhea to date. 221 In 2006, no targeted flybys took place, primarily because the spacecraft spent most of its time in 222 highly inclined orbits. The very close but non-targeted Rhea flyby in August 2007 was dedicated to 223 remote sensing and revealed small details of the prominent bright-ray crater and its ejecta 224 environment. This might be the youngest large crater in the Saturnian system. The targeted Iapetus 225 flyby in September 2007 over the anti-Saturnian and trailing hemispheres yielded very high- 226 resolution observations of the highest parts of the equatorial ridge, the transition zone between the 227 dark and the bright terrain, and the bright trailing side. Combined with data from a more distant, 228 non-targeted flyby on December 31, 2004, and other distant observations, these data permitted 229 developing a model of the formation of the global and unique brightness dichotomy of this unusual 230 moon (Denk and Spencer, 2008). For more detailed descriptions of the Cassini mission, see 231 232 Dougherty et al. (2009), Orton et al. (2009) and Seal et al. (2009). 233 21.3. Morphology, Geology and Topography 234 Voyager’s partial global mapping at 1 km resolution suggested that the morphology of the 235 Saturnian satellites was dominated by impact craters (Smith et al., 1981). Indeed, Voyager’s best 236 237 images, 500 m resolution views of Rhea, showed a landscape saturated with craters. Voyager also found evidence of limited volcanic and tectonic activity in the form of relatively smooth plains and 238 linear features (Smith et al., 1981, 1982; Plescia and Boyce, 1982, 1983; Moore et al., 1985; Moore 239 and Ahren, 1983), especially on Enceladus. The Enceladus discoveries aside, Cassini’s considerably 240 improved resolution and global mapping coverage confirmed the abundance of impact craters on all 241 satellites (Dones et al., 2009) and also revealed surprisingly varied degrees of past endogenic 242 activity on Tethys, Rhea and especially Dione, indicating that these satellites were more dynamic 243 than originally thought. Observed deformations range from linear grooves (Mimas) and nearly 244 global-scale grabens (Tethys, Rhea) to repeated episodes of tectonics and resurfacing (Dione, 245 246 Enceladus). To date, little or no direct evidence of past or present endogenic activity has been identified on Hyperion, Phoebe or Iapetus (other than its potentially endogenic equatorial ridge). 247 This diversity is in itself a puzzle that has not yet been explained. In the following sections, we 248 249 examine the features observed, their morphology, and implications for internal evolution. 250 Craters 6
251 Comparison with craters on larger icy satellites (e.g. Ganymede) is illuminating because surface 252 gravities differ by a factor of 5 to 10. Central-peak craters are common on Saturnian satellites. 253 Central peaks in larger craters can be as high as 10 km, many of them rising above the local mean 254 elevation. Cassini has revealed little evidence of terrace formation, and floor uplift dominates clearly 255 over rim collapse in the formation of complex craters (Schenk and Moore, 2007). Pancake (formerly 256 called pedestal) ejecta have been observed in smooth plains on Dione. This kind of ejecta deposit, 257 once only known as a feature of the Galilean satellites, now appears to be common on icy satellites 258 in general. The fact that it is not obvious on other satellites is most likely due to the difficulty of 259 identifying it on these rugged, heavily cratered surfaces. 260 A transition to larger, more complex crater landforms (e.g. central pit craters, multi-ring basins) on 261 these satellites is predicted to occur at much larger diameters (>150 km) than those seen on 262 Ganymede and Callisto (Schenk, 1993), due to their weaker gravity. True central-pit craters like 263 those on Ganymede or Callisto (Schenk, 1993) have not been observed, although the central 264 complex of Odysseus features a central depression and closely resembles its counterparts on large 265 icy moons. The dominant morphology at larger diameters (>150 km) is peak-ring or similar, with a 266 prominent central peak surrounded by a ring or an elevated circular plateau. The number of such 267 basins discovered by Cassini on Iapetus and Rhea was greater than expected. Indeed, at least 10 268 basins larger than 300 km across have been seen on Iapetus (e.g. Giese et al., 2008). 269 The most ancient large basins on Rhea and Tethys have been modified by relaxation and long-term 270 impact bombardment (e.g. Schenk and Moore, 2007). The younger basins are generally deep and 271 272 unrelaxed. However, the floor of Evander, is the largest basin identified on Dione, has been relaxed and uplifted to the level of the original surface, and although its rim and peak remain prominent 273 (e.g. Schenk and Moore, 2007). Relaxation of this basin is consistent with elevated heat flows on 274 Dione, as indicated by the relaxation of smaller (diameters of 25-50 km) craters within the extensive 275 smooth plains to the north of Evander. Relaxation in this basin, which has a low superposed crater 276 density, is consistent with elevated heat flows, as indicated by the smaller relaxed craters nearby and 277 the extensive smooth plains to the north. The relaxation state of impact basins can be determined by 278 comparing their depth to that of unmodified craters and examining their shape. The most ancient 279 large basins on Rhea (e.g. Tirawa) and Tethys are roughly 10 to 20% shallower than predicted and 280 feature prominent domical or conical central uplifts that approach or exceed the local elevation of 281 the satellite (Schenk and Moore, 2007), which suggests they have been modified to some degree by 282 relaxation. The youngest basins appear to be deeper and possibly unrelaxed. The most prominent 283 examples include Herschel on Mimas (diameter ~120 km, depth ~11 km) and Odysseus (diameter 284 ~450 km, depth >8 km). In neither case do the central uplifts rise above the local topography. A 285 prominent exception is the relatively young 400 km wide Evander basin on Dione, which may be 286 either ~3Gyr (Wagner et al., 2006) or less than 2Gyr old (Kirchoff and Schenk, 2009). For a more 287 detailed discussion of surface ages, see Dones et al. (2009). 288 Iapetus’ leading side shows more relief than the other medium-sized Saturnian satellites (Thomas et 289 al., 2007b; Giese et al., 2008). The lack of basin relaxation on Iapetus is consistent with the 290 presence of a thick (50–100 km) lithosphere in Iapetus’ early history. Such a lithosphere might also 291 support Iapetus’ prominent equatorial ridge. The ridge shows a complex morphology, probably the 292 result of impact erosion in some places and of post-formation tectonic modifications in others. 293 294 The abundance of large impact basins, even in Voyager’s limited view, suggested that such basins could have disruptive effects on these satellites (Smith et al., 1981). Several attempts to discover 295 evidence of these effects (e.g. seismic shaking, global fracturing) have met with limited success (e.g. 296 297 Moore et al., 2004), but Cassini-based studies are now in progress. 298 Tectonics 299 The Voyager 1 and 2 spacecraft revealed that the medium-sized icy Saturnian satellites have 300 undergone varying degrees of tectonic deformation (Smith et al. 1981; 1982; Plescia and Boyce 301 1982, 1983). Although such deformation can provide constraints on the interior structure and 302 evolution of a satellite, the imaging resolution was often insufficient to permit sustainable 303 conclusions. Post-Voyager advances include a vastly better understanding of tectonic deformation 304 and thermal evolution in the Jovian system thanks to Galileo, and greatly improved experimental 305 measurements of relevant ice properties (Schmitt et al., 1998). In this section, we combine this 306 improved understanding with the results of the ongoing Cassini mission to discuss the current state 307 of knowledge of icy-satellite tectonics in the Saturnian system. We also examine the extent to which 308 cryovolcanic processes may be relevant. Most of the topics covered in this section are treated in 309 greater detail in Collins et al. (2009). 7
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Page 37 and 38: 1985 Collins, G.C., McKinnon, W.B.,
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Page 49 and 50: 2686 2687 Schenk P.M., Moore, J.M.,
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