36th DPS Meeting, 8-12 November 2004
Session 4 Phoebe and Iapetus
Oral, Monday, November 8, 2004, 1:30-3:00pm, Lewis

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[4.03] The Roughness of Phoebe as Determined by Cassini VIMS

D. P. Simonelli (Senior NRC Research Associate/JPL), B. J. Buratti, M. D. Hicks (JPL/Caltech), R. H. Brown (U. Arizona), R. N. Clark (USGS), D. P. Cruikshank (NASA Ames), R. Jaumann (DLR), T. B. McCord (PSI), R. M. Nelson (JPL/Caltech)

The Cassini spacecraft's June flyby of the saturnian satellite Phoebe revealed a rough, heavily cratered object: craters and significant topography were seen at all spatial scales observed, consistent with earlier suggestions that Phoebe has a rough surface (i.e., Voyager images that indicated a non-spherical shape and which, upon Hapke-function photometric modeling, produced a macroscopic roughness ``mean slope angle" of more than 30 degrees; Simonelli et al. 1999, Icarus 138, 249).

During the Cassini flyby, the Visual and Infrared Mapping Spectrometer (VIMS) instrument observed Phoebe at a cruder spatial resolution than Cassini's camera but with a much wider wavelength range (0.4 to 5.1 microns) and much finer spectral resolution (typical wavelength spacing of 16 to 17 nanometers). The VIMS disk-resolved photometric data are now being analyzed with various radiative-transfer models, including a crater-roughness model (Buratti and Veverka 1985, Icarus 64, 320) that has previously been used to study the roughness of different terrains on the nucleus of Comet Borrelly (Buratti et al. 2004, Icarus 167, 16).

Preliminary results of this analysis indicate that the surface is quite rough at scales smaller than a VIMS pixel, consistent with Phoebe's appearance during the Cassini flyby and with the earlier Voyager modeling work. The preliminary VIMS results also suggest important variations in roughness on different parts of the satellite. Clearly, Phoebe---with its retrograde orbit, and status as a probable captured object---has had an eventful impact history at a wide range of spatial scales.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.