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417 This effect is much reduced if there are high-pressure ice phases freezing simultaneously (Squyres, 418 1980; Showman et al., 1997). 419 Similar but smaller effects are caused by thermal expansion or contraction (Ellsworth and Schubert, 420 1983; Hillier and Squyres, 1991; Showman et al., 1997; Nimmo, 2004a). A global temperature change of 100K will cause stresses on the order of 100 MPa for a thermal expansivity of 10 -4 K -1 421 . 422 Warming may also lead to silicate dehydration (e.g. Squyres and Croft, 1986), which in turn causes 423 expansion. 424 Satellites are quite likely to have non-axisymmetric structures, in which case some of the above- 425 mentioned global mechanisms may lead to local deformation. For instance, local zones of weakness 426 or pre-existing structures can result in enhanced tidal dissipation and deformation (e.g. Sotin et al., 427 2002; Nimmo, 2004b) and/or the alteration of local stress trajectories. Convection (thermal or 428 compositional) and buoyancy forces due to lateral shell thickness variations can generate local 429 deformation. They are discussed at greater length in Collins et al. (2009). We will not discuss these 430 processes further here, as they do not tend to produce globally organized tectonic structures. 431 Large impact events can also potentially create global fracture networks. This mechanism is implicit 432 in the model presented by E. Shoemaker in Smith et al. (1981) for the catastrophic breakup and re- 433 accretion of the medium-sized icy satellites of Saturn. Presumably, impacts not quite large enough 434 to completely disrupt a satellite could do considerable damage in the form of global fracturing, and 435 post-Voyager studies of these satellites focused in part on attempts to find evidence of an incipient 436 break-up of this kind. For instance, the surface of Mimas is crossed by a number of linear-to- 437 438 439 arcuate, sub-parallel troughs (Fig. 1) that are thought to be the consequence of global-scale fractures during the Herschel impact event (McKinnon, 1985; Schenk, 1989a). 440 Fig. 1 Linear troughs on Mimas, extending east to west. These troughs are interpreted as fractures, 441 possibly related to the Herschel crater. Orthographic map projection at 400m/pixel is centered on 442 the antipode to Herschel, the largest impact basin on Mimas. 443 444 Voyager and Cassini mapping indicates that these features, if related to Herschel, do not form a 445 simple radial or concentric pattern. Nor has it been demonstrated that any of these features form on 446 a plane intersecting Herschel, or whether they should do so. It thus remains possible that these are 447 random fractures formed during freeze expansion of the Mimas interior. Detailed mapping remains 448 to be done to test these hypotheses. Mimas does not otherwise exhibit endogenic landforms and 449 will, therefore, not be discussed further. 450 Enceladus shows the greatest tectonic deformation of any of the Saturnian satellites, and very 451 significant spatial variations in surface age (Smith et al., 1982; Kargel and Pozio, 1996; Porco et al., 452 2006; Jaumann et al., 2008). Spencer et al. (2009) discusses the tectonic behavior of Enceladus in 453 much greater detail; only a very brief summary is given here. 454 Three different terrain types exist on Enceladus. Ancient cratered terrains cover a broad band from 0 to 180 o longitude. Centred on 90 o and 270 o 455 longitude there are younger, deformed terrains roughly 90 o wide at the equator. The southern polar region south of 55 o 456 S is heavily deformed and almost 457 uncratered. The most prominent tectonic features of this region are the linear depressions called 458 ‘tiger stripes’ (Porco et al., 2006). These tiger stripes are typically ~500m deep, ~2 km wide, up to 459 460 ~130 km long and spaced ~35 km apart. Crosscutting fractures and ridges with almost no superimposed impact craters characterize the area between the stripes. The tiger stripes are 461 apparently the source of the geysers observed to emanate from the south polar region (Spitale and 462 Porco, 2007). The energy source of these geysers is unknown, but it may be shear heating generated 463 by strike-slip motion at the tiger stripes (Nimmo et al., 2007). The coherent (but latitudinally 464 asymmetrical) global pattern of deformation on Enceladus is currently unexplained, but it is most 465 probably due to shape changes, perhaps related to a hypothetical episode of true polar wander 466 (Nimmo and Pappalardo, 2006). 467 468 Nearly encircling the globe, Ithaca Chasma is the most prominent feature on Tethys (Fig. 2). 469 Fig. 2 Tethys and Ithaca Chasma. Topography was derived from Cassini stereo images and has a 470 horizontal resolution of 2-5 km and a vertical accuracy of 150-800m. Profiles have superimposed 471 472 model flexural profiles (in red). Te is elastic thickness. (Reproduced from Giese et al. (2007)) 473 It extends approximately 270° around Tethys and is not a complete circle. It is narrowly confined to 474 a zone which lies along a large circle whose pole is only ~20° from the center of Odysseus, the 475 relatively fresh largest basin on the satellite which is 450 km wide (Smith et al., 1982; Moore and 10
476 Ahern, 1983). Smith et al. (1981) suggested that Ithaca Chasma was formed by freeze-expansion of 477 Tethys's interior. They noted that if Tethys was once a sphere of liquid water covered by a thin solid 478 ice crust, freezing in the interior would have produced an expansion of the surface comparable to 479 the area of the chasm (~10% of the total satellite surface area). This hypothesis fails to explain why 480 the chasm occurs only within a narrow zone; besides, it is difficult to account for the geophysical 481 energy needed for the presence of a molten interior. Expansion of the satellite's interior should have 482 caused fracturing over the entire surface in order to effectively relieve stresses in a rigid crust 483 (Moore and Ahern, 1983). 484 The nearly concentric geometrical relationship between Odysseus and Ithaca Chasma prompted 485 Moore and Ahern (1983) to suggest that the trough system was an immediate manifestation of the 486 impact event, perhaps caused by a damped, whole-body oscillation of the satellite. Based on the 487 deep topography of Odysseus, Schenk (1989b) also suggested that it had not undergone substantial 488 relaxation, and that Ithaca Chasma was the equivalent of a ring graben formed during the impact 489 event by a prompt collapse of the floor involving a large portion of the interior. In a Cassini-era 490 study, Giese et al. (2007) doubted the Odysseus-related hypothesis (Moore and Ahern, 1983; 491 Schenk, 1989b) on the basis of crater densities within Ithaca Chasma relative to Odysseus, from 492 which they concluded that Ithaca Chasma is older than Odysseus and therefore could not have 493 influenced its formation. They went on to measure photogrammetrically the raised flanks of Ithaca 494 Chasma, which they attributed to flexural uplift. Assuming that these raised flanks were indeed due 495 to flexural uplift, they derived a mechanical lithospheric thickness of ~20 km and surface heat fluxes of 18-30 mW/m 2 at assumed strain rates of 10 -17 to 10s -1 496 497 . They concluded that Ithaca Chasma is an endogenic tectonic feature but could not explain why it is confined to a narrow zone 498 approximating a large circle. 499 Several Voyager-era studies of Dione (Plescia, 1983; Moore, 1984) noted a near-global network of 500 tectonic troughs and the presence of a smooth plain crossed by both ridges and troughs located near 501 the 90°W longitude and extending eastward. Cassini coverage revealed the westward extension of 502 this plain (Wagner et al., 2006) and demonstrated that the fractured and the cratered dark plains are 503 spectrally distinct (Stephan et al., 2008). Like the eastern portion observed by Voyager, the western 504 plain also exhibits both ridges and troughs (Fig. 3). 505 Fig. 3 (a) Image mosaic of Dione, centered on 180 o 506 longitude. Red and white arrows point to Amata 507 and Turnus impact structures, respectively. (b) Topography of Dione, from Schenk and Moore 508 (2007). An ancient degraded impact basin centered just west of Amata can be identified by the two 509 concentric rings in the DEM (white arrows). DEM has a dynamic range of 8 km (red is highest, 510 511 magenta is lowest). 512 However, the ridge system is much more pronounced. The western ridges are planimetrically 513 narrower and morphologically better expressed (e.g. their relative lack of degradation). These 514 western ridges also conform much more closely to the boundary between the plain and the older 515 cratered rolling terrain to the west. The linear to arcuate troughs interpreted to have been created by 516 tectonic extension form a complex network that divides Dione’s surface into large polygons (Smith 517 et al., 1981, 1982; Plescia, 1983; Moore, 1984). The troughs often form parallel to sub-parallel sets, 518 519 somewhat reminiscent of the grooved terrain on Ganymede. The global orientation of Dione’s tectonic fabric is non-random (Moore, 1984) and exhibits patterns consistent with a decline in 520 oblateness due to either despinning or orbital recession (Melosh 1977, 1980). 521 The topography derived from Cassini images of Dione shows one clear case of tectonic orientation 522 regionally influenced by a preexisting but very degraded large impact basin centered on the smaller 523 crater Amata (Schenk and Moore, 2007 (Figs. 3 and 4)). 524 Fig. 4 Fracture zones on Dione. Global base map of Cassini imaging has a resolution of 400m. The 525 circle represents the outer ridge corresponding to the rim of the degraded basin near Amata shown 526 527 in Fig. 3. 528 Moore et al (2004) identified another example of tectonic lineament orientation associated with an 529 unambiguously identified impact feature, 90 km Turnus. Such crater-focused tectonics has been 530 identified on Ganymede from Galileo data (e.g. Pappalardo and Collins, 2005). This crater- 531 lineament relationship on Dione is the most pronounced among any of the middle-sized icy 532 satellites except Enceladus (Spencer et al., 2009). Moore (1984) proposed a tectonic history of 533 extension followed by (at least regional) compression, the latter being responsible for the ridges. 11
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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: 359 Table 3 shows the diurnal tidal
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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.,
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