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R.M. Nelson (JPL), R.H. Brown (U of Arizona), B. W. Hapke (U of Pittsburgh), W. D. Smythe, L. Kamp (JPL), M. Boryta (Mt. San Antonio College), K. H. Baines (JPL), G. Bellucci (Inst. Di Astrofisica Spaziale), J. P. Bibring (U. Paris Sud-Orsay), B. J. Buratti (JPL), F. Capaccioni, P. Cerroni (Inst. Di Astrofisica Spaziale), R. N. Clark (USGS Denver), A. Coradini (Inst. Di Astrofisica Spaziale), D.P. Cruikshank (NASA Ames), P. Drossart (Obs. de Paris-Meudon), V. Formisano (Inst. Di Astrofisica Spaziale), R. Jaumann (DLR), Y. Langevin (U. Paris Sud-Orsay), D. L. Matson (JPL), T. B. McCord (U. of Hawaii), V. Menella (Obs. Astron. Capodimonte), P.D. Nicholson (Cornell U.), B. Sicardy (Obs. de Paris-Meudon), C. Sotin (U. Nantes)
On 2 July 2004 Cassini passed within ~400,000 km of Titan. At 2.02 microns where methane is transmitting, the surface is seen by VIMS. The images show features that resemble impact craters. We analyzed two regions measuring reflectance along lines that passed through the sub-solar point, traversing the feature center. We compared our photometric profiles with the profiles expected from a circular depression, a circular depression with a raised rim, and a circular depression with a raised rim and a central peak, using a model based on the bi-directional reflectance equations developed by Hapke (1993). We assumed, 1) The particulate surface scattered isotropically with uniform single scattering albedo, 2) The haze layer was optically thin the surface can be seen, 3) The haze layer does not extend to the surface, and 4) The haze particles are laterally uniformly mixed with the atmospheric gas. Our data do not fit the expected profile for a reasonable crater. In one case the model fit does not agree with the data at large distances from the sub-solar point. In another, the photometric profile expected from the central peak is in the opposite sense to that which we measured. Also, the crater depths required to accommodate these best-fit models are unreasonably large (~50 -100 km.). These features are not caused principally by topographic relief. They are consistent with palimpsests - the remains of ancient impacts that are expressed as units of darker reflectance on a surface where the vertical relief has been lost to lithospheric plastic flow. If these albedo features on the surface are persistent then widespread weathering processes, such as a planet-wide precipitation is limited. This result is consistent with Keck observations at shorter wavelengths by Bouchez. This work carried out at JPL under contract with NASA and with the support of ESA.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.