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1457 Dark/Leading Bright/Trailing 1458 2.2-cm 0.32 + 0.09 0.58 + 0.18 1459 13-cm 0.13 + 0.04 0.17 + 0.04 1460 1461 Tab. 5 Total-power radar albedos 1462 Therefore, whereas Iapetus is anomalously bright at 2cm (similar to Callisto) and mimics the optical 1463 hemispheric dichotomy, at 13cm it looks like a typical main-belt asteroid, with minimal evidence of 1464 mimicking the optical hemispheric dichotomy. 1465 The Iapetus results are understandable if (1) the leading side's optically dark contaminant is present 1466 to depths of at least one to several decimeters, so that SL-2 can see the optical dichotomy, and (2) 1467 ammonia and/or some other radar-absorbing contaminant is globally much less abundant within the 1468 upper one to several decimeters than at greater depths, which would explain the wavelength 1469 dependence, that is, the virtual disappearance of the object's 2-cm signature at 13cm. The first 1470 conclusion is consistent with the dark material being a shallow phenomenon, as predicted from the 1471 thermal migration hypothesis discussed elsewhere in this chapter. 1472 Moreover, ammonia contamination may be uniform within the top few meters, so that the 1473 wavelength dependence arises because ammonia's electrical loss is much greater at 13cm 1474 (2.38GHz) than at 2cm (13.8GHz). Unfortunately, the likelihood of this possibility cannot yet be 1475 judged because measuring the complex dielectric constant of ammonia-contaminated water ice as a 1476 function of concentration, frequency and temperature is so difficult that, although needed, it has not 1477 1478 been done yet. This is a pity, because the lack of this information undermines the interpretation of the 2cm to13cm wavelength dependence seen in other Saturnian satellites as well as of the possible 1479 influence of ammonia and its 'modulation' by magnetospheric bombardment (which varies with 1480 distance from Saturn) on the dependence of SL-2 on the distance from Saturn. 1481 Verbiscer et al. (2006) suggested that, while their spectral data do not contain an unambiguous 1482 detection of ammonia hydrate on Enceladus, their spectral models do not rule out the presence of a 1483 modest amount of it on both hemispheres. However, they note that the trailing hemisphere is also 1484 exposed to increased magnetospheric particle bombardment, which would preferentially destroy the 1485 1486 ammonia. In any case, ammonia does not appear to be a factor in Enceladus’ radar albedos. 1487 Iapetus Radar Image 1488 During the Iapetus 49 flyby, RADAR obtained an SAR image at 2km to12km surface resolution 1489 that covers much of the object's leading (optically darker) side. From a comparison with optical 1490 imaging (Fig. 20), we see that, as expected, the SAR reliably reveals the surface features seen in 1491 1492 ISS images. 1493 Fig. 20 RADAR SAR image of the leading side of Iapetus (Sept. 2007, left) and the ISS mosaic 1494 PIA 08406 (Jan. 2008), using similar projections and similar scales (the large crater (on the right in 1495 1496 the ISS image is about 550 km in diameter). 1497 The pattern of radar brightness contrast is basically similar to that seen optically, except for the 1498 large basin at 30°N, 75°W, which has more SL-2-bright than optically bright highlights. This 1499 1500 suggests that the subsurface of these highlights at decimeter scales may be cleaner than in other parts of the optically dark terrain, which is consistent with the idea that the contamination by dark 1501 material might be shallow, at least in this basin. Conversely, many optically bright crater rims are 1502 1503 SL-2-dark, so the exposure of clean ice cannot be very deep. 1504 21.6 Geological Evolution 1505 The densities of the Saturnian satellites constrain their composition to be about 2/3 ice and 1/3 rock. 1506 Accretional energy, tidal despinning and radioactive decay might have heated and melted the 1507 moons. Due to the very low melting temperatures of various water clathrates, it is likely that the 1508 larger bodies differentiated early in their evolution while smaller satellites may still be composed 1509 and structured in their primitive state. For a more detailed discussion of the aspects of origin and 1510 thermal evolution, see Johnson et al. (2009) and Matson et al. (2009). 1511 The heavily cratered solid surfaces in the Saturnian system indicate the role of accretional and post- 1512 accretional bombardement in the geological evolution of the satellites. Observed crater densities are 1513 summarized in Dones et al. (2009). However, the source as well as the flux of bombarding bodies is 1514 still under discussion, and Dones et al. (2009) came to the conclusion that, since no radiometric 1515 sample data exist from surfaces in the outer solar system and the size-frequency distribution of 28
1516 comets is less well understood than that of asteroids, the chronology of the moons of the giant 1517 planets remains in a primitive state. Therefore, we refer the reader to Dones et al. (2009) for the 1518 crater chronology discussion. In interpreting the geological evolution of the Saturnian satellites, we 1519 will rely on relative stratigraphic relations. Nevertheless, absolute ages are given in Dones et al. 1520 (2009). 1521 In a stratigraphic sense, three major surface unit categories can be distinguished on all icy satellites: 1522 (1) Cratered plains, which are characterized by dense crater populations that are superimposed by 1523 all other geological features and, thus, are the relatively oldest units. 1524 (2) Tectonized zones of narrow bands and mostly sub-parallel fractures, faults, troughs and ridges 1525 of global extension indicating impact-induced or endogenic crustal stress. These units dissect 1526 cratered plains and partly dissect each other, indicating different relative ages. 1527 (3) Resurfaced units consisting of terrain that has undergone secondary exogenic and endogenic 1528 processes such as mass wasting, surface alterations (due to micrometeorite gardening, sputtering, 1529 thermal segregation and crater ray formation by recent impacts), contamination by dark material 1530 and cryovolcanic deposition mostly resulting in smoothed terrain. Superimposed on cratered plains 1531 and tectonized regions, these are the youngest surface units. 1532 1533 Heavily cratered surfaces are found on all the icy rocks and small satellites (Fig. 21) as well as on 1534 Fig. 21 Phoebe, 213 km in average diameter, was the first one of the nine classic moons of Saturn to 1535 be captured by the ISS cameras aboard Cassini in a close flyby about three weeks prior to Cassini’s 1536 1537 Saturn Orbit Insertion (SOI) in July 2004. The mosaic shows a densely cratered surface. Higherresolution data revealed that the high frequency of craters extends with a steep slope down to craters 1538 only tens of meters in diameter (Porco et al., 2005). Image source: http://photojournal.jpl.nasa.gov, 1539 1540 image identification PIA06064. 1541 the medium-sized moons Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus and 1542 Phoebe, indicating an intense post-accretional collision process relatively early in the geological 1543 history of the satellites. Cassini observations added to the number of large basins and ring structures 1544 mainly on Rhea, Dione and Iapetus. Differences in the topographical expression of these impact 1545 structures suggest post-impact processes such as relaxation, which may reflect differences in the 1546 thermal history and/or crustal thickness of individual satellites (e.g. Schenk and Moore, 2007; Giese 1547 et al., 2008). Although age models are based on different impactor populations, the cratered 1548 surfaces on Iapetus seem to be the oldest in the Saturnian system, whereas cratered plains on the 1549 other satellites seem to be younger (see Dones et al., 2009). One large (48 km in diameter) 1550 1551 stratigraphically young ray crater was found on Rhea (Wagner et al., 2007) (Fig.22). 1552 Fig. 22 A prominent bright ray crater with a diameter of 48 km is located approximately at 12.5° S, 1553 112° W on Saturn’s second-largest satellite Rhea. The crater is stratigraphically young, indicated by 1554 the low superimposed crater frequency on its floor and continuous ejecta (Wagner et al., 2008). The 1555 high-resolution mosaic (right) of NAC images, shown in context of a WAC frame, was obtained by 1556 Cassini ISS during orbit 049 on Aug. 30, 2007. The global view to the left showing the location of 1557 the detailed view is a single NAC image from orbit 056. 1558 1559 While the small satellites lack tectonic features, the medium-sized bodies experienced rotation-, 1560 tide- and impact-induced deformation and volume changes, which created extensional or 1561 contractional tectonic landforms. Parallel lineaments on Mimas seem to be related to the Herschel 1562 impact event (McKinnon, 1985; Schenk, 1989a), although freezing expansion cannot be ruled out as 1563 1564 the cause (Fig. 23). 1565 Fig. 23 The anti-Saturnian hemisphere of Mimas, centered approximately at lat. 22° S, long. 185° 1566 W, is shown in a single image (NAC frame N1501638509) at a resolution of 620 m/pixel, taken 1567 during a non-targeted encounter in August 2005. The densely cratered landscape implies a high 1568 1569 surface age. Grooves and lineaments infer weak tectonic on this geologically unevolved body. 1570 Enceladus exhibits the greatest geological diversity in a complex stress and strain history that is 1571 predominantly tectonic and cryovolcanic in origin, with both processes stratigraphically correlated 1572 in space and time, although the energy source is so far not completely understood (e.g. Smith et al., 1573 1982; Kargel and Pozio, 1996; Porco et al., 2006; Nimmo and Pappalardo, 2006; Nimmo et al., 1574 2007; Jaumann et al., 2008). Enceladus is discussed in detail in Spencer et al. (2009). 29
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 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: 1398 may drop precipitously or even
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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