Reprint - Earth & Planetary Sciences - University of California, Santa ...

1693 discussed. This surficial frost redistribution may occur regardless of whether volatiles in the
1694
1695
bedrock of Iapetus sublimate and contribute to bedrock erosion.
1696 21.7 Conclusions
1697 The icy satellites of Saturn show great and unexpected diversity. In terms of size, they cover a range
1698 from ~1500 km (Rhea) to ~270 km (Hyperion) and even smaller 'icy rocks' of less than a kilometer
1699 in diameter. The icy satellites of Saturn offer an unrivalled natural laboratory for understanding the
1700 geology of diverse satellites and their interaction with a complex planetary system.
1701 Cassini has executed several targeted flybys over icy satellites, and more are planned for the future.
1702 In addition, there are numerous other opportunities for observation during non-targeted encounters,
1703 usually at greater distances. Cassini used these opportunities to perform multi-instrument
1704 observations of the satellites so as to obtain the physical, chemical and structural information
1705 needed to understand the geological processes that formed these bodies and governed their
1706 evolution.
1707 All icy satellites exhibit densely cratered surfaces. Although there have been some attempts to
1708 correlate craters and potential impactor populations, no consistent interpretation of the crater
1709 chronology in the Saturnian system has been developed so far (Dones et al., 2009). However, most
1710 of the surfaces are old in a stratigraphical sense. The number of large basins discovered by Cassini
1711 was greater than expected (e.g. Giese et al., 2008). Differences in their relaxation state provide
1712 some information about crustal thickness and internal heat flows (Schenk and Moore, 2007). The
1713
1714
absence of basin relaxation on Iapetus is consistent with a thick lithosphere (Giese et al., 2008),
whereas the relaxed Evander basin on Dione exhibits floor uplift to the level of the original surface
1715 (Schenk and Moore, 2007), suggesting higher heat flow.
1716 The wide variety in the extent and timing of tectonic activity on the icy satellites defies any simple
1717 explanation, and our understanding of Saturnian satellite tectonics and cryovolcanism still is at an
1718 early stage. The total extent of deformation varies wildly, from Iapetus and Mimas (barely
1719 deformed) to Enceladus (heavily deformed and currently active), and there is no straightforward
1720 relationship to predicted tidal stresses (e.g. Nimmo and Pappalardo, 2006; Matsuyama and Nimmo,
1721 2007, 2008; Schenk et al., 2008; Spencer et al., 2009). Extensional deformation is common and
1722 often forms globally coherent patterns, while compressional deformation appears rare (e.g. Nimmo
1723 and Pappalardo, 2006; Matsuyama and Nimmo, 2007, 2008; Schenk et al., 2008; Spencer et al.,
1724 2009). These global patterns are suggestive of global mechanisms, such as despinning or
1725 reorientation, which probably occurred early in the satellites’ histories (e.g. Thomas et al., 2007);
1726 present-day diurnal tidal stresses are unlikely to be important, except on Enceladus (e.g. Spencer et
1727 al., 2009) and (perhaps) Mimas and Dione (e.g. Moore and Schenk, 2007). Ocean freezing gives
1728 rise to isotropic extensional stresses; modulation of these stresses by pre-existing weaknesses (e.g.
1729 impact basins) may explain some of the observed long-wavelength tectonic patterns (e.g. Moore
1730 and Schenk, 2007). Except for Enceladus, there is as yet no irrefutable evidence of cryovolcanic
1731 activity in the Saturnian system, either from surface images and topography or from remote sensing
1732 of putative satellite atmospheres. The next few years will hopefully see a continuation of the flood
1733 of Cassini data, and will certainly see a concerted effort to characterize and map in detail the
1734 tectonic structures discussed here. Understanding the origins of these structures and the histories of
1735
1736
the satellites will require both geological and geophysical investigations and probably provide a
challenge for many years to come.
1737 Although the surfaces of the Saturnian satellites are predominately composed of water ice,
1738 reflectance spectra indicate coloring agents (dark material) on all surfaces (e.g. Fink and Larsen
1739 1975; Clark et al., 1984, 1986; Roush et al., 1995; Cruikshank et al., 1998a; Owen et al., 2001;
1740 Cruikshank et al., 2005; Clark et al., 2005, 2008a, 2009; Filacchione et al. 2007, 2008). Recent
1741 Cassini data provide a greater range in reflected solar radiation at greater precision, show new
1742 absorption features not previously seen in these bodies, and allow new insights into the nature of the
1743 icy-satellite surfaces (Buratti et al., 2005b; Clark et al., 2005; Brown et al., 2006; Jaumann et al.,
1744
1745
2006; Cruikshank et al., 2005, 2007, 2008; Clark et al., 2008a, b, 2009). Besides H2O ice, CO2
absorptions are found on all medium-sized satellites (Buratti et al., 2005b; Clark et al., 2005;
1746
1747
Cruikshank et al., 2007; Brown et al., 2006; Clark et al., 2008a), which make CO2 a common
molecule on icy surfaces as well. Various spectral features of the dark material match those seen on
1748 Phoebe, Iapetus, Hyperion, Dione and Epimetheus as well as in the F-ring and the Cassini Division,
1749 implying that the material has a common composition throughout the Saturnian system (Clark et al.,
1750 2008a, 2009). Dark material diminishes closer to Saturn, which might be due to surface
1751
contamination of the inner moons by E-ring particles and some chemical alteration processes (Clark
32