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534 Cassini-era images of the tectonic troughs (mostly grabens) show them to be morphologically fresh, 535 implying that extension continued into geologically recent times (Wagner et al., 2006). 536 After Voyager, Rhea was held to be the least endogenically evolved satellite larger than 1000 km in 537 diameter. Moore et al. (1985) pointed out a few parallel lineaments and noted several large ridges or 538 scarps (which they termed ‘megascarps') and speculated that these features were formed by a period 539 of extension followed by compression. Cassini images, and digital terrain models derived from 540 them, of the trailing hemisphere of Rhea seen from both middle northern and far southern latitudes 541 clearly show that the wispy arcuate albedo markings seen in Voyager images are associated with a 542 graben/extensional fault system dominantly trending north-south (Moore and Schenk, 2007; 543 544 Wagner et al. 2007) (Fig. 5a). 545 Fig. 5 (a) Cassini image of the northern trailing hemisphere of Rhea showing the bright scarps of 546 the northern extension of the graben system and their relationship to 'wispy' terrain. (b) Mosaic of 547 Cassini images showing curvilinear grabens (black arrows) and an orthogonal set of ridges and 548 549 troughs (white arrow). Base mosaic resolution is 1 km/pixel. 550 This is the first identification of unambiguous regional endogenic activity on Rhea. The digital 551 terrain models of Rhea derived from Cassini images also show a set of roughly north-south trending 552 553 ridges (Figs. 5b, and 6) that correspond to the 'megascarps' of Moore et al. (1985). 554 555 556 Fig. 6 Preliminary stereo topography map, from Schenk and Moore (2007). White arrows indicate graben systems. The DEM has a dynamic range of 10 km (red is highest, magenta is lowest). 557 If these ridges are in fact due to compressional tectonics, their geometrical relation to the 558 graben/scarp system may also indicate a genetic relationship. However, the poor morphological 559 presentation of these ridges implies that they are old, while the 'wispy terrain' grabens are fresh and 560 presumably much more recently formed. 561 While the troughs and ridges suggest mainly extensional and (minor) compressional tectonics 562 (Thomas, 1988), the en-echelon pattern of the scarps and troughs on the trailing hemisphere 563 suggests shear stress (Wagner et al., 2007). This pattern may have been influenced by the possible 564 presence of a large, degraded impact basin (basin C of Moore et al., 1985). 565 566 The equatorial ridge of Iapetus (Fig. 7) is the most enigmatic feature on any of the medium-sized 567 Fig. 7 Topography of Iapetus as derived from stereo Cassini imaging. Note the sharply defined 568 equatorial ridge. The DEM at left is referenced to a sphere, the DEM at right to a biaxial ellipsoid. 569 570 (Adapted from Giese et al., 2008) 571 icy satellites (Porco et al., 2005a; Giese et al., 2008); it extends across more than half of Iapetus' 572 circumference (Giese et al., 2008). Different parts show very different morphologies, from 573 trapezoidal to triangular cross-sections with steep walls and a sharp rim to isolated mountains with 574 moderate slopes and rounded tops (Giese et al., 2008). Although the ridge is variable in height, it 575 rises up to 10 km above the local mean but also has a gently sloping flank. The ridge is abundantly 576 577 cratered, indicating that it is comparable in age to other terrains on Iapetus (Schmedemann et al., 2008). Although the equatorial location of the ridge might be due to despinning (Castillo-Rogez et 578 al., 2007), it represents such a large mass that, if it had formed in another location, it would almost 579 certainly have reoriented to the equator anyway. However, as already mentioned above, irrespective 580 of whether a satellite is spinning down or spinning up, equatorial tectonic features can be expected 581 to be oriented north-south, making it difficult to explain the ridge with despinning. One possibility 582 is that despinning stresses combined with lithospheric thickness variations due to convection may 583 have resulted in equatorial extension and diking (Roberts and Nimmo, 2009; Melosh and Nimmo, 584 2009). Other proposals suggest that the morphology of the ridge indicates that the surface was 585 warped up by tectonic faulting (Giese et al., 2008), that it is a result of extensional forces acting 586 above an ascending current of solid-state convection from a two-cell convection pattern 587 (Czechowski and Leliwa-Kopystyński, 2008), or that it was formed by the impact of an ancient 588 debris disk on Iapetus (Ip, 2006). The great apparent elastic thickness suggested by the survival of 589 the ridge may be used to place constraints on the thermal evolution of Iapetus (Castillo-Rogez et al., 590 2007). 591 Although the global shape of Iapetus does suggest an earlier, more rapid spin rate (Castillo-Rogez et 592 al., 2007), despinning cannot explain the equatorial ridge (see above): stresses in the north-south 12
593 direction are always smaller than those in the east-west direction, and will result in N-S oriented 594 features (Melosh, 1977). Thus, the origin of the equatorial ridge is currently a mystery. 595 The deformation of Saturnian satellites varies wildly, from currently active and heavily deformed 596 Enceladus to nearby Mimas, which is tectonically uninteresting. Nor does the degree of deformation 597 correlate in any straightforward way with predicted tidal stresses. However, there are at least two 598 global yet somewhat contradictory aspects, which may be identified. On the one hand, coherent 599 global tectonic patterns are evident on Dione, Tethys and Enceladus and to a lesser extent on Rhea 600 and Iapetus. On the other, several satellites, especially Tethys and Enceladus, show very 601 pronounced spatial variations in the extent of deformation. These two aspects suggest that global 602 sources of stress are common, but also that lateral variations in the mechanical properties of the 603 satellites’ ice shells play an important role. 604 Regarding this second point, localization of deformation (e.g. at the south pole of Enceladus or at 605 Ithaca Chasma) may arise naturally in ice shells (e.g. Sotin et al., 2002; Nimmo, 2004b) but is 606 currently poorly understood and makes modeling tectonic deformation much harder. As far as 607 global deformation patterns are concerned, several satellites (Enceladus, Iapetus, perhaps Dione) 608 show patterns with simple symmetries about the rotational and tidal axes. This symmetry may be 609 due either to feature reorientation or the operation of mechanisms (tides, despinning etc.), which 610 have a natural symmetry. On Dione, Rhea and Iapetus at least, diurnal tidal stresses are rather small 611 (Tab. 3), and most of the tectonic features are apparently ancient, which suggests that despinning, 612 volume changes or reorientation are more likely to be responsible for the observed deformation than 613 614 present-day tidal stresses. As in the Galilean satellites, extensional features are generally more common than compressional 615 features on the other Saturnian satellites. This observation is readily explained if we assume that the 616 satellites once contained subsurface oceans which progressively froze (e.g. Smith et al., 1981; 617 Nimmo, 2004a); alternative explanations are that compressional features such as folds (Prockter and 618 Pappalardo, 2000) are less easy to recognize than extensional features, or that larger stresses are 619 620 required to form compressional features (see Pappalardo and Davis, 2007). 621 Cryovolcanism 622 Cryovolcanism can be defined as 'the eruption of liquid or vapor phases (with or without entrained 623 solids) of water or other volatiles that would be frozen solid at the normal temperature of the icy 624 satellite’s surface' (Geissler, 2000). Cryovolcanism therefore includes effusive eruptions of icy 625 slurries and explosive eruptions consisting primarily of vapor (Fagents, 2003). Voyager-era data 626 suggested that such activity might be common in the outer solar system, resulting in a flurry of 627 theoretical papers (e.g. Stevenson, 1982; Allison and Clifford, 1987; Crawford and Stevenson, 628 1988; Kargel. 1991; Kargel et al., 1991). Higher-resolution images from Galileo generally revealed 629 tectonic rather than cryovolcanic resurfacing processes, although some features may be due to 630 cryovolcanic activity (e.g. Schenk et al., 2001; Fagents et al., 2000; Miyamoto et al., 2005). In the 631 following, we discuss the current state of knowledge about cryovolcanism in the Saturnian 632 satellites. Enceladus, which is definitely cryovolcanically active (Porco et al., 2006), and Titan, 633 which may be (Lopes et al., 2007; Nelson et al., 2009a,b), are discussed by Spencer et al. (2009), 634 Jaumann et al. (2009) and Sotin et al. (2009). 635 636 The principal difference between cryovolcanism and silicate volcanism is that in the former, the melt is denser than the solid (by ~10%). This means that special circumstances must exist for an 637 eruption of non-vapor cryovolcanic materials to occur (though of course on Earth it is not 638 uncommon for denser basaltic lavas to erupt through lighter granitic material). Such circumstances 639 include one or more of the following: (1) the melt includes dissolved volatiles, which exsolve and 640 reduce the bulk melt density (Crawford and Stevenson, 1988); (2) the melt is driven upwards by 641 pressure differences caused by tidal effects (Greenberg et al., 1998), surface topography (Showman 642 et al., 2004) or freezing (Fagents, 2003; Manga and Wang, 2007); (3) compositional differences 643 render the ice shell locally dense or the melt less dense (e.g. by the addition of ammonia) (Kargel 644 1992, 1995). The latter mechanism can be nothing more than a local effect, since if the bulk ice 645 shell is denser than the underlying material; it will sink if the ice shell is disrupted. 646 Erupted material consisting primarily of water plus solid ice will simultaneously freeze and boil, the 647 latter process ceasing once a sufficiently thick ice cover is formed (Allison and Clifford, 1987). The 648 resulting flow velocity will depend on the viscosity of the ice-water slurry (Kargel et al., 1991) as 649 well as on flow thickness and local slope (Wilson et al., 1997; Miyamoto et al., 2005). If the erupted 650 material is dominated by the vapor phase, the vapor plume will expand outwards (Tian et al., 2007) 651 and escape entirely if the molecular thermal velocities exceed the escape velocity. Any associated 13
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: 476 Ahern, 1983). Smith et al. (198
Page 15 and 16: 711 1975; Clark et al., 1984, 1986;
Page 17 and 18: 829 dark regions of Iapetus. The pr
Page 19 and 20: 947 late 2004 suggest external mate
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
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
Page 53 and 54: 2922 Wilson, P. D., and Sagan, C.,
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