High resolution VIMS images of Titan's surface: implications for its composition, internal structure and dynamics

With a field of view of 0.5 mrad per pixel, the VIMS (Visual and Infrared Mapping Spectrometer) onboard the Cassini spacecraft can acquire images with a resolution of 500 m per pixel at closest approach during a typical Titan flyby. This resolution is comparable to the resolution of the radar instrument and allows comparisons between the radar images and optical images in the six infrared windows where the surface can be observed. Such opportunities were not set up for the nominal tour before Saturn insertion. The opportunity was offered during the TA flyby [Sotin et al., Nature, 2005] and the results lead the Cassini program to give VIMS the prime observations during closest approach at the T24 and T38 flybys. Two different implementations were experienced. During the T24 flyby (01/29/2007), we used a push-broom mode allowing VIMS to image a long path before pointing to a specific site at the limit between the light and dark terrains. This observation allowed us to see the dunes and to infer some information on their composition [Barnes et al., Icarus, 2008], to image channels and to infer information of erosion processes of the bright equatorial regions [Jaumann et al., Icarus, in press] and to observe the strong correlation between radar images and the VIMS images over a bright area interpreted as a flow feature [Lopes et al., Icarus, 2007]. During the T38 flyby over Ontario Lacus (12/05/2007), it was decided to point to the lake and get different images which provide us with a set of observations obtained with different emergence angles. This observation allowed us to infer the liquid nature of the lake and the composition of the lake [Brown et al., Nature, 2008]. In addition, this mode gives good information on the atmospheric component and will help us remove that component to get better spectra of Titan's surface. During the extended mission, two observations are forecasted at the beginning and at the end of the Cassini Equinox Mission. The first one will happen on November 19, 2008. The VIMS has been programmed to observe the Huygens landing site area at a resolution of 1 km/pixel. Before and after this observation, the push-broom mode will be used in order to cross-cut some of the radar paths. Because Titan's spin rate may be different from synchronous [Stiles et al., 2007; Lorenz et al., 2008], there is some uncertainty on the pointing. This study will report on the results of this flyby. This work has been carried out at the JPL, Caltech, under contract with NASA.