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1 1 2 3 Icy Satellites 1 Saturn after Cassini/Huygens 2 Revised Version April 10, 2009 3 4 Icy Satellites: Geological Evolution and Surface Processes 5 6 Ralf Jaumann1,2,*, Roger N. Clark3, Francis Nimmo4, Amanda R. Hendrix5, Bonnie J. Buratti5, 7 Tilmann Denk2, Jeff M. Moore6, Paul M. Schenk7, Steve J. Ostro✝5 and Ralf Srama8 8 9 1DLR, Institute of Planetary Research. Rutherfordstrasse 2, 12489 Berlin, Germany; 10 2Freie Universität Berlin, Institute of Geological Sciences, Malteserstr. 74-100, 12249 Berlin, Germany; 11 3U.S. Geological Survey, Denver Federal Center, Denver CO 80225; 12 4Dept. Earth & Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, USA; 13 5Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA; 14 6NASA Ames Research Center, MS 245-3 Moffett Field, CA 94035-1000, USA; 15 7Lunar and Planetary Institute Houston TX 77058, USA; 16 8Max Planck Institut für Kernphysik, 69117, Heidelberg, Germany. 17 18 *Corresponding author (Fax :+493067055 402 ; Email address: [email protected] 19 ✝deceased 20 21 Content 22 Abstract 23 21.1 Introduction 24 21.2 Cassini’s Exploration of Saturn’s Icy Satellites 25 21.3. Morphology, Geology and Topography 26 21.4 Composition and Alteration of Surface Materials 27 21.5 Constraints on top-meter Structure and Composition by Radar 28 21.6 Geologic Evolution 29 21.7 Conclusions 30 31 Abstract 32 The sizes of the Saturnian icy satellites range from ~1500 km in diameter (Rhea) to ~20 km 33 (Calypso), and even smaller 'rocks' of only a kilometer in diameter are common in the system. All 34 these bodies exhibit remarkable, unique features and unexpected diversity. In this chapter, we will 35 mostly focus on the 'medium-sized icy objects' Mimas, Tethys, Dione, Rhea, Iapetus, Phoebe and 36 Hyperion, and consider small objects only where appropriate, whereas Titan and Enceladus will be 37 described in separate chapters. Mimas and Tethys show impact craters caused by bodies that were 38 almost large enough to break them apart. Iapetus is unique in the Saturnian system because of its 39 extreme global brightness dichotomy. Tectonic activity varies widely - from inactive Mimas 40 through extensional terrains on Rhea and Dione to the current cryovolcanic eruptions on Enceladus 41 – and is not necessarily correlated with predicted tidal stresses. Likely sources of stress include 42 impacts, despinning, reorientation and volume changes. Accretion of dark material originating from 43 outside the Saturnian system may explain the surface contamination that prevails in the whole 44 satellite system, while coating by Saturn’s E-ring particles brightens the inner satellites. 45 So far, among the surprising Cassini discoveries are the volcanic activity on Enceladus, the sponge- 46 like appearance of Hyperion and the equatorial ridge on Iapetus - unique features in the solar 47 system. The bright-ray system on Rhea was caused by a relatively recent medium impact which 48 formed a ~40 km crater at 12°S latitude, 112°W longitude, while the wispy streaks on Dione and 49 Rhea are of tectonic origin. Compositional mapping shows that the dark material on Iapetus is 50 composed of organics, CO2 mixed with H2O ice, and metallic iron, and also exhibits possible 51 signatures of ammonia, bound water, H2 or OH-bearing minerals, and a number of as-yet 52 unidentified substances. The spatial pattern, Rayleigh scattering effect, and spectral properties 53 argue that the dark material on Iapetus is only a thin coating on its surface. Radar data indicate that 54 the thickness of the dark layers can be no more than a few decimeters; this is also consistent with 55 the discovery of small bright-ray and bright-floor craters within the dark terrain. Moreover, several 56 spectral features of the dark material match those seen on Phoebe, Iapetus, Hyperion, Dione and 57 Epimetheus as well as in the F-ring and the Cassini Division, implying that throughout the 58 Saturnian system. All dark material appears to have a high content of metallic iron and a small 59 content of nano-phase hematite. However, the complete composition of the dark material is still 1 60 unresolved, and additional laboratory work is required. As previously concluded for Phoebe, the dark 61 material appears to have originated external to the Saturnian system. 62 The icy satellites of Saturn offer an unrivalled natural laboratory for understanding the geological 63 diversity of different-sized icy satellites and their interactions within a complex planetary system. 64 65 21.1 Introduction 66 Covering a wide range of diameters, the satellites of Saturn can be subdivided into three major 67 classes: ‘icy rocks,’ with radii < 10 km, 'small satellites' with radii <100 km, such as Prometheus, 68 Pandora, Janus, Epimetheus, Telesto, Calypso, and Helene, and 'medium-sized satellites' with radii 69 <800 km: Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus and Phoebe. The largest 70 satellite, Titan, has a radius of 2575 km. We focus here on the ‘medium-sized satellites,’ 71 considering the ‘small satellites’ where appropriate. Enceladus is discussed in a separate chapter 72 (Spencer et al., 2009) and Titan in a dedicated book (Brown et al., 2009; Jaumann et al., 2009); 73 therefore, these two satellites will not be discussed in detail here. The main orbital and physical 74 characteristics of the medium and small satellites of Saturn are shown in Table 1. 75 Mean a i Mean V or Satellite P [days] e radius GM 0 p [103 m] [deg] Density R [km] 0.00033 Pan 134 0.575 0.0000 0.001 12.8 0.56 ± 19.4 0.5 0.00005 Atlas 138 0.602 0.0012 0.003 10. 0.5 0.00014 19.0 0.4 0.01246 46.8 ± 0.435 ± Prometheus 139 0.613 0.0022 0.008 ± 15.8 0.6 5.6 0.159 0.00083 0.00995 40.6 ± 0.530 ± Pandora 142 0.629 0.0042 0.050 ± 16.4 0.5 4.5 0.194 0.00155 90.4 ± 0.612 ± 0.1266 ± Janus 151 0.695 0.0068 0.163 14.4 0.6 3.0 0.062 0.0017 58.3 ± 0.634 ± 0.0351 ± Epimetheus 151 0.694 0.0098 0.351 15.6 0.5 3.1 0.102 0.0004 2.530 ± Mimas 186 0.942 0.0202 0.001 198.2 1.150 12.8 0.6 0.012 7.210 ± Enceladus 238 1.370 0.0045 0.001 252.1 1.608 11.8 1.0 0.011 41.210 ± Tethys 295 1.888 0.0000 0.002 533 0.973 10.2 0.8 0.007 12. ± Telesto 295 1.888 0.0002 1.180 1.0 0.00048 18.5 1.0 3.0 9.5 ± Calypso 295 1.888 0.0005 1.499 1.0 0.00024 18.7 0.7 1.5 73.113 ± Dione 377 2.737 0.0022 0.005 561.7 1.476 10.4 0.6 0.003 16. ± Helene 377 2.737 0.0071 0.213 1.5 0.0017 18.4 0.6 4.0 154.07 ± Rhea 527 4.518 0.0010 0.029 764.3 1.233 9.6 0.6 0.16 133.0 0.569 ± 0.37 ± Hyperion 1501 21.28 0.0274 0.461 14.4 0.3 ± 8.0 0.108 0.02 120.50 ± Iapetus 3561 79.33 0.0283 14.968 735.6 1.083 11 0.6 0.03 0.081 106.6 1.633 ± 0.5531 ± Phoebe 12948 550.31 0.1635 26.891 16.4 ± ± 1.1 0.049 0.0006 0.002 Paaliaq 15200 686.95 0.3630 45.084 11.0 2.3 0.00055 21.3R 0.06 Albiorix 16182 783.45 0.4770 34.208 16 2.3 0.0014 20.5R 0.06 2 Siarnaq 17531 895.53 0.2960 46.002 20 2.3 0.0026 20.1R 0.06 76 77 Tab. 1 Physical and orbital characteristics of the small (<100 km radius) and medium-sized moons 78 (<800 km radius) of Saturn (from JPL at http://ssd.jpl.nasa.gov/?satellites). 79 a - mean value of the semi-major axis 80 P - sidereal period P 81 e - mean eccentricity 82 i = mean inclination with respect to the reference plane ecliptic, ICRF, or local Laplace 83 V0 = mean opposition magnitude or R = red magnitude 84 p = geometric albedo (geometrical albedo is the ratio of a body's brightness at zero phase angle to 85 the brightness of a perfectly diffusing disk of the same position and apparent size as the body). 86 (After Thomas et al., 1983; Morrison et al., 1984; Thomas, 1989; Showalter, 1991; Jacobson and 87 French, 2004; Jacobson et al., 2004; Spitale et al., 2006; Jacobson 2006; Porco et al., 2005b; 88 Jacobson et al., 2005; Jacobson, 2007) 89 90 Common properties of the Saturnian moons include a very large amount of water ice on the surface 91 and in the interior, resulting in low mean densities between ~1.0g/cm3 and ~1.6g/cm3, and 92 numerous impact craters scattered over the surface. Tectonic features and less heavily cratered 93 plains are restricted to only a few satellites and indicate endogenic geological activity. The Voyager 94 missions (e.g. Smith et al., 1981, 1982) changed our perception of the medium-sized Saturnian 95 moons from small dots in large telescopes into real worlds with alien landscapes. Impact craters 96 were recognised as the dominant landforms, except for parts of Enceladus that appeared to be 97 crater-free. It was found that instead of cratering widespread tectonics seems to have shaped the 98 surface of Enceladus, and most craters showed evidence of viscous relaxation.
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