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888 but the Cassini Division displays spectra that closely match those of Iapetus in Iapetus’ transition 889 zone from dark to light material, implying mixtures of the same materials. The ring spectra are 890 much redder in the 0.3µm to 0.6µm region than the spectra of dark material on Phoebe, Iapetus and 891 Hyperion. Cosmic rays impacting the huge surface area of the rings break up water molecules, thus 892 creating an atmosphere of hydrogen and oxygen. The hydrogen escapes, leaving an atmosphere of 893 oxygen around the rings (Johnson et al., 2006). Oxygen ions are very reactive, and when they 894 895 encounter iron particles, they oxidize them and turn them into nano-phase hematite (Fe2O3). The spectra of nano-phase hematite mixed with ice and dark particles closely match the UV red slope 896 seen in the spectra of Dione (Clark et al., 2008a) (Fig. 11). To explain the dark material in the 897 Saturnian system, Clark et al. (2008b, 2009) suggest that oxidized sub-micron iron particles from 898 meteorites, mixed with ice, might explain all the colors and spectral shapes observed in the system. 899 A small amount of carbon might also be identified in the mixture, which would change the slope of 900 iron and further lower the albedo. Sub-micron particles also explain the origin of the observed 901 902 Rayleigh scattering. Trace amounts of CO2, ammonia and organics would not significantly influence the visible and UV spectra down to 0.3µm. Finally, the inner satellites may be exposed to 903 as much iron as the outer satellites, but E-ring ice particles mix with and cover up the iron particles. 904 Species found by INMS, CAPS and MIMI in the magnetosphere are candidates for surface 905 components as the magnetospheric material originates either directly from satellite surfaces or from 906 ring particles. Compositional analyses of Enceladus’ plume are described in Spencer et al. (2009). 907 The idea of iron in the Saturn system is supported by the Cassini CDA instrument, which has 908 909 910 discovered small (
947 late 2004 suggest external material emplacement (e.g. dark material on ram-facing crater walls at 948 high latitudes) (Porco et al., 2005a), but this was later found to be due to photometric effects (Denk 949 et al., 2008). The initial theory of an exogenically-created dark pattern (Cook and Franklin, 1970) 950 suggested that pre-existing dark material was uncovered by meteoritic bombardment; this idea was 951 extrapolated by Wilson and Sagan (1996). Researchers theorized (Soter, 1974) that the dark 952 material is exogenically emplaced on Iapetus’ leading hemisphere as material is lost from the moon 953 Phoebe (Burns, 1979). Retrograde Phoebe dust from a distance of 215 Saturn radii would travel 954 inward from the effects of Poynting-Robertson drag and impact the leading hemisphere of Iapetus 955 orbiting at 59 Saturn radii. However, Phoebe is spectrally neutral at visible wavelengths, while the 956 Iapetus dark material is reddish (Cruikshank et al., 1983; Squyres et al., 1984). If the material does 957 come from Phoebe, then some sort of chemistry or impact volatilization must occur to change the 958 color and darken the material (Cruikshank et al., 1983; Buratti and Mosher, 1995). Another 959 possibility is that the exogenic source of the dark material is Hyperion (Matthews, 1992; Marchi et 960 al., 2002), Titan (Wilson and Sagan, 1995; Owen, 2001), Iapetus itself (Tabak and Young, 1989) or 961 the debris remnants of a collision between a former outer moon and a planetocentric object (Denk 962 and Neukum, 2000). Matthews (1992) suggested that the hypothetical impact that disrupted 963 Hyperion created a debris cloud that subsequently hit Iapetus. Both Hyperion and Titan dark 964 materials are spectrally reddish (Thomas and Veverka, 1985; McDonald et al., 1994; Owen et al., 965 2001), though not as dark as those of Iapetus. Buratti et al. (2002; 2005) suggested that both 966 Hyperion's and Iapetus’ leading hemispheres are impacted by dark, reddish dust from recently 967 968 discovered retrograde satellites exterior to Phoebe. Clark et al. (2008a) offer a solution for the color discrepancy. Red, fine-grained dust (less than one-half micron in diameter) when mixed in small 969 amounts with water ice produces a Rayleigh scattering effect, enhancing the blue response. This 970 blue enhancement appears just enough on Phoebe to create an approximately neutral spectral 971 response. An examination of the spectral signatures of Dione, Hyperion, Iapetus and Phoebe by 972 Clark et al (2008a) showed a continuous mixing space on all these satellites, demonstrating variable 973 mixtures of fine-grained dark material in the ice. Thus, the colors observed are simply variable 974 amounts of dark material, with the redder slopes indicating greater abundance. 975 Ground-based radar observations at 13cm (Black et al., 2004) and Cassini RADAR data at 2.2cm 976 (Ostro et al., 2006) indicate that the dark terrain on Iapetus must be quite thin (one to several 977 decimeters); an ammonia-water ice mixture may be present at a depth of several decimeters below 978 the surface on both the leading and trailing hemispheres of Iapetus. The RADAR results appear to 979 rule out any theories of a thick dark material layer (Matthews, 1992; Wilson and Sagan, 1996) and 980 981 are consistent with the spectral coating indicated by VIMS data as described by Clark et al (2008a). 3 Cassini radio science results (Rappaport et al., 2005) indicate a bulk density for Iapetus of 1.1g/cm , 982 from which it can be inferred that the moon is composed primarily of water ice. 983 Because the large craters within the dark terrain appear evenly colored by the dark material in 984 Voyager data, no craters significantly break up the dark material to expose bright underlying terrain 985 (Denk et al., 2000). This suggests that the emplacement of dark material by whatever mechanism is 986 relatively new or ongoing. 987 With the arrival of Cassini at Saturn and the first Iapetus flyby (Dec 2004), the idea of thermal 988 segregation developed (Spencer et al., 2005; Hendrix and Hansen, 2008a). This idea was originally 989 990 mentioned as one option by Mendis and Axford (1974). The September 2007 close flyby of Iapetus permitted testing of this hypothesis, resulting in the following theory. A small amount of exogenic 991 dust from an unknown source created a 'global color dichotomy': the leading side became slightly 992 redder and darker than the trailing with fuzzy boundaries close to the sub-Saturn and anti-Saturn 993 meridians (Denk et al., 2008). A runaway thermal segregation process might have been initiated 994 primarily at the low and middle latitudes of the leading side as well as in some local areas at low 995 latitudes on the trailing side, especially on equatorward-facing slopes (Denk et al., 2008). Indeed, 996 poleward-facing slopes even within the dark terrain (at middle latitudes) are bright. On these slopes 997 the temperature never raises enough to allow the thermal process to act faster than the destruction 998 process by micrometeorite gardening, which transports bright sub-surface material to the surface. In 999 those locations that are now dark, the originally bright surface material becomes warm enough for 1000 volatiles (mainly water ice) on the top part of the surface to grow unstable and migrate towards cold 1001 traps, leaving behind residual dark material that was originally intimately mixed with the water ice 1002 as a minor constituent. The slow 79.3day rotation is a crucial boundary condition for this process to 1003 work in the Saturnian system. ISS images suggest that significant cold traps might be located at 1004 1005 high latitudes on the trailing side, where lower-resolution images show bright white polar caps. 19
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: 829 dark regions of Iapetus. The pr
Page 21 and 22: 1065 visible wavelength by the Hubb
Page 23 and 24: 1183 Physical photometric modeling
Page 25 and 26: 1280 For a smooth, homogeneous targ
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Page 29 and 30: 1516 comets is less well understood
Page 31 and 32: 1634 evidence of cryovolcanism on I
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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
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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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