CASSINI VIMS UNVEILS TITAN'S SURFACE

The Cassini Visual Infrared Mapping Spectrometer (VIMS) is an imaging spectrometer that obtains spectra in the 0.35 to 5.2 micron spectral region. This spectral region is ideally suited to detecting ices, volatiles, minerals, and organics. The instrument operates in a “whiskbroom” fashion, gathering a 64X64 image of spatial picture elements at each of 352 spectral elements distributed over the wavelength range. By mid-October, the instrument will have obtained observations from seven close flybys of Titan, ranging in distance at closest approach from about 1000 km to 4000 km. VIMS images obtained during closest approach offer spatial resolutions as high as 1 km, and are particularly valuable for identifying geologic features. Titan is the only planetary satellite with a dense atmosphere, composed mainly of nitrogen with a few percent methane. This atmosphere has shrouded the surface of Titan at visible wavelengths, but VIMS obtains data at infrared atmospheric windows where the atmospheric opacity is low. Data from the early flybys have been analyzed to show intriguing geologic features including possible cryovolcanos, ridge and groove topography on the scale of a few hundred meters, eolian deposits, drainage patterns and pits, and substantial changes in albedo, macroscopic roughness, and surface particle size. No evidence for global-scale deposits of liquid exist, although we cannot yet eliminate the possibility of ponds or lakes on Titan. The general shape of Titan's spectrum through the atmospheric windows in the infrared region is consistent with a surface composition of water ice (or clathrate/hydrate) mixed with organics, although the details in the 2.7 micron region may not match this simple model. Comparison of the Cassini VIMS images to lower-resolution infrared data from Earth-based telescopes shows no evidence for large scale changes on Titan's surface during the past 5 years.