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652 solid material will follow ballistic trajectories, ultimately forming deposits centered on the eruption 653 vents (Fagents et al., 2000). 654 The plume properties measured on Enceladus by Cassini imply considerably smaller velocities for 655 icy dust grains than for vapor. On hypothesis is that the dust grains condense and grow in vertical 656 vents of variable width. Repeated wall collisions and re-acceleration by the gas in the vents lead to 657 effective friction, which explains the observed plume properties (Schmidt et al., 2008). ). Most dust 658 grains are growing by direct condensation of the ascending water vapor within the ice channels and 659 contain almost pure water ice; they are free of atomic sodium. In 2008 the CDA instrument while 660 analyzing the composition of E-ring particles discovered a second type of icy dust particles. These 661 grains are rich in sodium salts (0.5 - 2% in mass) and there origin is explained by the existence of 662 an ocean of liquid water below the surface of Enceladus and which is in direct contact with its rock 663 core (Postberg et al., 2009). This shows that cryovolcanism can provide valuable information about 664 geological conditions of the interior. 665 The plains of Dione are another candidate for cryovolcanic resurfacing (Fig. 8). Several Voyager- 666 667 era studies concluded that these plains were 668 Fig. 8 Cassini view of smooth plains on Dione. Older heavily cratered terrains lie to the west. Note 669 the prominent north-south ridge and the subtle linear features to the east. Mosaic resolution is 670 671 400m/pixel. 672 673 formed in this way (e.g. Plescia and Boyce, 1982; Plescia, 1983; Moore, 1984). Moore (1984) suggested that several branching broad-rimmed troughs north of the equator at 60°W may have 674 been the source vents. Surprisingly, the plains unit may be topographically higher than the cratered 675 terrain (Moore and Schenk, 2007). This observation seems a little difficult to reconcile with the 676 plains being emplaced as a low-viscosity cryovolcanic flow. One possibility is that the plains 677 material was emplaced as a deformable plastic unit (like terrestrial glaciers), but this, or any other, 678 explanation needs both further evidence and thought. The ridges discussed above bound (or at least 679 occur near the edges) the plains unit in the west and southeast and may be related to their formation. 680 A plains unit on Tethys is located antipodal to the very large Odysseus impact feature. Moore and 681 Ahern (1983) proposed that the plains were formed by cryovolcanism. Alternatively, Moore et al. 682 (2004) considered the possibility that it was extreme seismic shaking caused by energy focused 683 there by the Odysseus impact that formed the antipodal plains. If seismic shaking was responsible, 684 there would probably be a significant transition zone between that unit and the adjacent hilly and 685 cratered terrain. Digital terrain models derived from Cassini images indicate that the transition 686 between the cratered terrain and the plains is abrupt and the plains themselves are level, supporting 687 the hypothesis that they are endogenic (Moore and Schenk, 2007). 688 Both Tethys and Dione show evidence for weak plasma tori (Burch et al., 2007) somewhat 689 analogous to the much more marked feature at Enceladus, which is caused by geysers (Porco et al., 690 2005). It is not yet clear whether surface activity or some other process is generating these tori. 691 Similarly, Clark et al. (2008a) report a possible, but currently unconfirmed, detection of material 692 around Dione, which may also indicate some surface activity. The Cassini magnetometer has also 693 measured small amounts of mass loading near Dione (Burch et al., 2007). 694 695 So far, none of the digital terrain models of Rhea developed in the Cassini era shows any regions that would qualify as plains, supporting the perception that Rhea shows no record of cryovolcanic 696 resurfacing. However, unlike Dione, the diffuse nature of the wisps (the 'wispy terrain') on Rhea has 697 yet to be explained as numerous smaller bright scarps associated with fracture and faulting, and may 698 yet be shown to be an expression of cryo-pyroclastic mantling (Stevenson, 1982). Furthermore, 699 Rhea, surprisingly, exhibits evidence for faint particle ring (Jones et al., 2008), the origin of which 700 is unclear but may be related to endogenic or impact processes. On the other hand, Cassini VIMS 701 observations found no evidence of a water vapor column around Rhea (Pitman et al., 2008). On 702 Iapetus, no evidence of cryovolcanism has been detected thus far. If volcanism ever operated on 703 704 these satellites, it occurred in ancient times and its evidence is now unseen. 705 21.4 Composition and Alteration of Surface Materials 706 The composition of the satellites of Saturn is determined by remote spectroscopy, by sampling 707 materials that were sputtered from the satellite surfaces or, in the case of Enceladus, injected into space 708 where they can be directly sampled by spacecraft instruments. 709 It has long been known that the surfaces of Saturn’s major satellites, Mimas, Enceladus, Tethys, 710 Dione, Rhea, Hyperion, Iapetus and Phoebe, are predominantly icy objects (e.g. Fink and Larsen, 14
711 1975; Clark et al., 1984, 1986; Roush et al., 1995; Cruikshank et al., 1998a; Owen et al., 2001; 712 Cruikshank et al., 2005; Clark et al., 2005, 2008a; Filacchione et al., 2007, 2008). While the 713 reflectance spectra of these objects in the visible range indicate that a coloring agent is present on 714 all surfaces except Mimas, Enceladus and Tethys (Buratti 1984). Only Phoebe and the dark 715 hemisphere of Iapetus display near-IR spectra markedly different from very pure water ice (e.g. 716 Cruikshank et al., 2005; Clark et al., 2005). 717 The major absorptions in icy satellite spectra shown in Figs 9 a-e are due to water ice and can be 718 719 observed at 1.5µm., 2.0µm., 3.0µm., and 4.5µm. 720 Fig. 9 VIMS spectra of the Saturnian icy satellites: (a) The major absorptions in icy satellite spectra 721 are due to water ice and appear at 1.5, 2.0, 3.0, and 4.5µm; (b) Representative (A) bright and (B) 722 dark region Dione spectra (the gaps in the bright region spectra (A) near 1µm are due to sensor 723 724 725 saturation); the inset shows an area of high concentration of CO2 (after Clark et al., 2008a); (c) Spectra of Hyperion showing an increase in blue reflectivity and absorptions of water ice and CO2 (after Clark et al., 2008a); (d) Spectra of Iapetus showing a range of blue peak intensities, 726 727 absorptions of different intensities by water ice and CO2 absorption bands (after Clark et al., 2008a); (e) Spectra of Iapetus showing a range of blue peak intensities and absorptions of different 728 729 intensities by water ice and CO2 absorption bands (after Clark et al., 2008a). However, no CO2 was found on the small satellites Atlas, Pandora, Janus, Epimetheus, Telesto, and Calypso (Buratti et 730 731 al., 2009a). 732 733 Weaker ice absorptions appear at 1.04µm and 1.25µm in the spectra of bright regions where the ice 734 735 is purer. Trapped CO2 was first detected in Galileo near-infrared mapping spectrometer (NIMS) spectra of the Galilean satellites (Carlson et al., 1996) and in the Saturnian system on Iapetus 736 (Buratti et al, 2005), Phoebe (Clark et al., 2005), Hyperion (Cruikshank et al., 2007), and Enceladus 737 738 (Brown et al., 2006). Weak CO2 absorptions were also detected in VIMS data on Dione, Tethys, Mimas and Rhea (Clark et al., 2008a). 739 Organic compounds were detected directly in the Enceladus plume by the INMS (Waite et al., 740 2006) and in trace amounts by the VIMS on Phoebe (Clark et al., 2005), Hyperion (Cruikshank et 741 al., 2007) and Iapetus (Cruikshank et al., 2008), and possibly Enceladus (Brown et al., 2006). While 742 the spectral signatures indicate aromatic hydrocarbon molecules, exact identifications have remained 743 elusive, partly because the detections have low signal-to-noise ratios. Another spectral feature 744 observed in VIMS data of dark material on Phoebe, Iapetus and Dione appears to be an absorption 745 centered near 2.97µm (Fig. 9 d). The detection of this feature is complicated by the fact that VIMS 746 has an order-sorting filter change at this wavelength. Clark et al. (2005) presented evidence that this 747 feature is real and maps to geological patterns on Dione and Iapetus (Clark et al., 2008a, 2009). 748 However, a better understanding of the instrument response is needed to increase confidence in the 749 identification, including confirmation by a different instrument. If correct, the feature strength 750 indicates approximately 1% ammonia in the dark material. 751 The position shift of amorphous ice absorptions from their crystalline counterparts (Mastrapa et 752 al., 2008 and references therein) can be used to determine whether amorphous or crystalline ice is 753 754 present on a surface. Traditionally, amorphous ice displays almost no 1.65µm feature and the 3.1µm Fresnel peak shifts, broadens and weakens. Newman et al. (2008) used variations in the 755 3.1µm peak and the 1.65µm absorption to argue for amorphous ice around the 'tiger stripes' on 756 Enceladus. However, Clark et al. (2009) showed that scattering and sub-micron grain sizes also 757 influence the intensities of the 3.1-µm peak and the 1.65-µm absorption, and can be confused with 758 temperature and crystallinity effects. Further examination of the ice feature positions in spectra of 759 Enceladus and other satellites in the Saturnian system showed that positions and strengths are 760 consistent with those of fine-grained crystalline ice (Clark et al., 2009). Newman et al. (2009) 761 showed that water ice on Dione exists exclusively in crystalline form. 762 The spectra of the satellites in the visible wavelength range are smooth with no sharp spectral features, 763 but all show an ultraviolet absorber affecting the blue end of the visible-wavelength spectral slope. 764 Visible spectra can be classified by their spectral slope: from bluish Enceladus and Phoebe to redder 765 Iapetus, Hyperion and Epimetheus. In the 1µm to1.3µm range, the spectra of Enceladus, Tethys, 766 Mimas and Rhea are characterized by negative slope, consistent with a surface mainly dominated by 767 water ice, while the spectra of Iapetus, Hyperion and Phoebe show considerable reddening, pointing 768 out the relevant role played by the darkening materials present on the surface (Filacchione et al., 2007, 769 2008). In between these two classes are Dione and Epimetheus, which have a relatively flat global 15
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Page 9 and 10: 359 Table 3 shows the diurnal tidal
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Page 23 and 24: 1183 Physical photometric modeling
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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.,
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