VIMS Evidence for Palimpsests on Titan as a Constraint on Widespread Precipitation.

Details VIMS Evidence for Palimpsests on Titan as a Constraint on Widespread Precipitation. VIMS Evidence for Palimpsests on Titan as a Constraint on Widespread Precipitation.

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p41b..08n&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #P41B-08

Other

5420 Impact Phenomena (Includes Cratering), 5455 Origin And Evolution, 5470 Surface Materials And Properties, 5475 Tectonics (8149), 6275 Saturn

Scientific paper

The Cassini spacecraft passed about 400,000 km of Titan on 2 July 2004. Titan's surface is seen in Visual and Infrared Mapping Spectrometer (VIMS) images at infrared wavelengths where methane, the principal atmospheric absorber, is transmitting. At 2.02 microns VIMS images show several circular features on Titan. These bear a striking similarity to circular features exhibiting topographic relief caused by impact events on a wide range of solar system objects. We undertook a photometric analysis of two circular regions using 2.02 micron images taken near the time of closest approach. We measured the reflectance along lines that passed through the sub-solar point on Titan's surface and traversed the center of each feature. The extracted reflectance profiles enabled us to search for vertical relief by comparing 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 widely used bi-directional reflectance equations developed by Hapke (1993). We assumed: 1) the particulate surface scattered isotropically and had uniform single scattering albedo, 2) the haze was optically thin, did not extend to the surface, and was uniformly mixed laterally with the atmosphere. Despite our best effort to adjust the depression parameters to fit the data our data do not fit that expected for a craterlike depression. In one case the model fit does not agree with the data at large distances from the sub-solar point. In the other, the photometric profile expected from the central peak is in the opposite sense to that which we measured. In both cases the crater depths required to accommodate these best-fit models are extremely, if not unreasonably, large (~50 -100 km.) with diameters of 1000 and 2000 km.). We find it unusual to have two craters of such size on Titan because major cratering events are principally associated with the early bombardment period of solar system system-the first 1 billion years.Therefore, we suggest that these features are not caused by topographic relief and are not true craters. They are consistent with palimpsests-expressions of darker reflectance on a surface where the vertical relief has been lost to lithospheric plastic flow. If these features are palimpsests and are the remains of ancient impacts then their persistence on the surface suggests that widespread weathering processes, such as a planet-wide precipitation of aerosols, on Titan are severely limited. This result is consistent with Keck observations at shorter wavelengths by Bouchez.

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