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1006 Compositional data from the UVIS and the VIMS (Hendrix and Hansen, 2009a; Hendrix and 1007 Hansen, 2009b; Clark et al., 2009) indicate variable amounts of ice and volatiles in the dark 1008 material. The water-ice absorption edge in the UV at 0.165µm was shown to increase with latitude, 1009 consistent with decreasing temperatures and increasing numbers of visibly bright regions (Hendrix 1010 and Hansen, 2009a). This is consistent with thermal segregation as a possible process explaining the 1011 overall appearance of Iapetus and, more particularly, the observed correlations of the dark/bright 1012 boundary with geographical location and topography (Fig. 15) and the small-scale dramatic 1013 1014 brightness variations (Fig. 15). 1015 Fig. 14 Cassini ISS images from the September 2007 flyby, demonstrating the large- and small-scale 1016 albedo effects of thermal segregation. (Left) The central longitude of the trailing hemisphere is 24° 1017 to the left of the mosaic's center. (Right) The mosaic consists of two image footprints across the 1018 surface of Iapetus. The view is centered on terrain near 42° southern latitude and 209.3° western 1019 longitude, on the anti-Saturn facing hemisphere. Image scale is approximately 32m/pixel; the crater 1020 1021 in the center is about 8 km across. 1022 The 3µm absorption observed in VIMS data (Clark et al., 2009) might be due to ice, ammonia or 1023 bound water. As the 3µm absorption is not been well matched by those due to ice, bound water and 1024 ammonia, a combination seems to be indicated, but a model showing a good match has not yet been 1025 developed (Clark et al., 2009). The amount of ice contained in the dark materials from VIMS data 1026 1027 1028 could be on the order of 10%, but is estimated to be
1065 visible wavelength by the Hubble space telescope (Verbiscer et al., 2007). Their brightness changes 1066 with their distance from the E-ring, which is consistent with the assumption that their albedo results 1067 from E-ring coating. Hamilton and Burns (1994) predicted that the relative velocities between the 1068 E-ring particles and the icy satellites might explain the large-scale longitudinal albedo patterns on 1069 the icy satellites. Because of their higher relative velocity, the satellites exterior to the densest point 1070 in the E-ring have brighter leading hemispheres, while those interior to it have brighter trailing 1071 hemispheres because of the higher velocity of the particles. Indeed, Mimas and Enceladus are both 1072 slightly darker on their leading than on their trailing hemispheres, while the leading hemispheres of 1073 Tethys, Dione and Rhea are brighter than their trailing hemispheres (Buratti et al., 1998). The 1074 visible orbital phase curve of Enceladus shows that the leading hemisphere is ~1.2 times darker than 1075 its trailing hemisphere. At visible wavelengths, the leading hemisphere of Tethys is ~1.1 times 1076 brighter than its trailing hemisphere, Dione’s leading hemisphere is up to ~1.8 times brighter and 1077 Rhea’s leading hemisphere is ~1.2 times brighter than the trailing hemisphere (Buratti and Veverka, 1078 1984). 1079 The bombardment of icy surfaces by charged particles causes both structural and chemical changes 1080 in the ice (Johnson et al., 2004). Effects include the implantation of magnetospheric ions, sputtering 1081 and grain size alteration. Low-energy electrons can cause chemical reactions on the surface, while 1082 high-energy electrons and ions tend to cause radiation effects at depth. The latter can result in bulk 1083 chemical changes in composition as well as structural damage. A primary effect of energetic 1084 particle bombardment on ice is to cause defects in it (Johnson, 1997; Johnson and Quickenden, 1085 1086 1997; Johnson et al., 1998). Extended defects, in the form of voids and bubbles, affect the lightscattering properties of the surface. They also act as reaction and trapping sites for gases, which can 1087 produce spectral absorption features. Incident ions deposit energy over a small region around their 1088 path through the ice, causing local damage. Depending on excitation density and cooling rate, this 1089 can result in either local crystallization or amorphization (Strazzulla, 1998). 1090 Energetic ions and electrons have been seen to brighten laboratory surfaces in the visible by 1091 producing voids and bubbles (Sack et al., 1992). Depending on the size and density of these 1092 features, a clear surface can become brighter due to enhanced scattering in the visible. The plasma- 1093 induced brightening competes with annealing, which operates efficiently in regions where the ice 1094 temperature exceeds ~100K and can make equatorial regions less light-scattering despite the plasma 1095 bombardment. 1096 A significant effect of the ion bombardment of ice is the creation of new species through radiolysis. 1097 1098 Bombarding particles can decompose H2O ice, creating molecular hydrogen and oxygen. Miniatmospheres of trapped volatiles have been suggested to form in voids in the ice (Johnson and 1099 1100 Jesser, 1997). As a result, species such as O3, H2O2, O2 and oxygen-rich trace species have been detected as gases trapped in the regoliths of the icy Galilean satellites (Nelson et al., 1987; Noll et 1101 al., 1996; Hendrix et al., 1998, 1999; Spencer et al., 1995; Carlson et al., 1996, 1999; Hibbitts et al., 1102 2000, 2003). On these satellites, the hydrogen and oxygen produced by radiation decomposition 1103 form tenuous atmospheres; in contrast, atmospheres have not been detected on satellites in the 1104 Saturnian system (with the exception of the gaseous plume of Enceladus). There are several 1105 possible reasons for this. Lower gravities might support the loss of volatiles; temperatures are 1106 1107 1108 lower, which might slow down the breakup of H2O and the loss of H; the particle environment is different (discussed below); the electron density and temperature is not effective in exciting and 'lighting up' any gases around the moons; and sputtering is not as intense as in the Jupiter system 1109 due to differences in the net charge and energy of the particle environment. On Saturn, the primary 1110 surface alterations found thus far are due to the E-ring, dust (e.g. on Dione) and thermal processing 1111 (especially apparent on Iapetus, as previously discussed). 1112 Neutrals, rather than ions as on Jupiter, dominate the Saturnian system. The energetic heavy ions 1113 (~10keV to MeV) that are critical at Europa and Ganymede are lost by charge exchange to the 1114 neutral cloud of the Saturnian system as they diffuse inwards from beyond ~10Saturn radii 1115 (Paranicas et al., 2008). However, although the energy fluxes due to trapped plasma and the solar 1116 UV are relatively low on the icy Saturnian satellites (Paranicas et al., 2008), radiation processing 1117 appears to be occurring. Radiation fluxes are known to cause decomposition in ice, possibly producing ozone in the icy surfaces of Dione and Rhea (Noll et al., 1997) and O2 + 1118 1119 in Saturn’s plasma (Martens et al., 2007). In addition, the role of low energy ions has been recently re- 1120 examined using Cassini CAPS data (Sittler et al., 2008), indicating that sputtering by this 1121 1122 component is not negligible (Johnson et al., 2008). Spencer (1998) searched for but did not find O2 inclusions on Rhea and Dione. Such inclusions have been seen on Ganymede in the low latitudes of 1123 the trailing hemisphere, and it was suggested that they might be related to charged-particle 21
Page 1 and 2: 1 Saturn after Cassini/Huygens 2 3
Page 3 and 4: Siarnaq 17531 895.53 0.2960 46.002
Page 5 and 6: 193 Target Orbit Flyby date Altitud
Page 7 and 8: 251 Comparison with craters on larg
Page 9 and 10: 359 Table 3 shows the diurnal tidal
Page 11 and 12: 476 Ahern, 1983). Smith et al. (198
Page 13 and 14: 593 direction are always smaller th
Page 15 and 16: 711 1975; Clark et al., 1984, 1986;
Page 17 and 18: 829 dark regions of Iapetus. The pr
Page 19: 947 late 2004 suggest external mate
Page 23 and 24: 1183 Physical photometric modeling
Page 25 and 26: 1280 For a smooth, homogeneous targ
Page 27 and 28: 1398 may drop precipitously or even
Page 29 and 30: 1516 comets is less well understood
Page 31 and 32: 1634 evidence of cryovolcanism on I
Page 33 and 34: 1752 et al., 2008a). Although the g
Page 35 and 36: 1869 Buratti, B. J., 1985, Applicat
Page 37 and 38: 1985 Collins, G.C., McKinnon, W.B.,
Page 39 and 40: 2103 and Newmann, S., 2007, Saturn
Page 41 and 42: 2219 Hoppa, G., Tufts, B.R., Greenb
Page 43 and 44: 2333 Kempf, S., Srama, R., Postberg
Page 45 and 46: 2450 Moore, J.M., Asphaug, E., Morr
Page 47 and 48: 2568 2569 Ostro, S.J., West, R. D.;
Page 49 and 50: 2686 2687 Schenk P.M., Moore, J.M.,
Page 51 and 52: 2804 2805 Squyres, S.W., 1980, Volu
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