37th DPS Meeting, 4-9 September 2005
Session 47 Icy Satellites
Poster, Wednesday, September 7, 2005, 6:00-7:15pm, Music Lecture Room 5

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[47.10] Ice Grain Size Distribution: Differences Between Jovian and Saturnian Icy Satellites from Galileo and Cassini Measurements

G. Hansen (Univ. Washington), T. McCord (Space Sci. Inst.), R. Clark (USGS Denver), D. Cruikshank (NASA Ames), R. Brown (Univ. Arizona), K. Baines (Jet Prop. Lab.), G. Bellucci (IFSI, CNR Rome), B. Buratti (Jet Prop. Lab.), F. Capaccioni, P. Cerroni (IAS, CNR Rome), M. Combes (Obs. Paris-Meudon), A. Coradini (IAS, CNR Rome), P. Drossart (Obs. Paris-Meudon), V. Formisano (IFSI, CNR Rome), R. Jaumann (DLR Berlin), Y. Langevin (IAS Orsay), D. Matson (Jet Prop. Lab.), V. Mennella (INAF-OAC Napoli), R. Nelson (Jet Prop. Lab.), P. Nicholson (Cornell Univ.), B. Sicardy (Obs. Paris-Meudon), C. Sotin (LPG, Univ. Nantes), L. Soderblom (USGS Flagstaff), C. Hibbitts (Appl. Phys. Lab.)

Most of the satellites of the outer solar system have surfaces covered by water ice. The near-infrared (1-5 \mum) spectrum of snow has several absorption bands below 2.7 \mum, and stronger absorption beyond 3 \mum (for fine-grained ice, a reflection peak near 3.6 \mum occurs). For grain sizes larger than a micrometer, a Fresnel reflection peak is found near 3.1 \mum. The reflection spectrum of the icy Galilean satellites of Jupiter (Callisto, Ganymede and Europa), as returned by the Galileo Near Infrared Mapping Spectrometer, are well fitted by ice models using a single, or at most a closely-spaced bimodal, grain size in the range from 20 to several 100 \mum. These spectra all have a strong 3.1-\mum peak. The reflection spectrum of the Saturnian icy satellites Mimas, Tethys, Rhea, and Enceladus, as returned by the Cassini Visual and Infrared Mapping Spectrometer instrument are very different, as there seems to be a large sub-micron component of the ice that creates a broad, low region from 2.8 to 3.2 \mum and a highly variable and often indistinct 3.1-\mum reflection peak as if masked by purely absorbing sub-micron grains. The size of this peak is found to diminish as phase angle increases. Other parts of the spectrum are matched by grain sizes of several 10's to 100 \mum, consistent with telescopic observations over the last two decades. This behavior can be modeled as a coarse-sized distribution covered by sub-micron grains. Higher radiation fluxes and enhanced thermal grain metamorphisis exist on the Jovian satellites relative to satellites in the Saturnian system. The grain size distribution, including a submicron component, on the Saturnian satellites may indicate surfaces in a different thermal and radiation environment that results in an accumulation of sputtered and redeposited grains.

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Bulletin of the American Astronomical Society, 37 #3
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