Global Mapping of the Surface of Titan Using VIMS Infrared Images - Geodynamical Implications

The VIMS imaging spectrometer onboard Cassini spacecraft has been acquiring images of Titan in 352 spectral channels from 0.3 to 5.2 microns since 2004. The presence of a thick atmosphere makes the observation of surface spectral properties quite difficult to interpret since the aerosols induce a strong blurring of the images, and the atmospheric gas (in particular the few percent of methane) strongly absorb the flux, except in 7 narrow spectral windows which can be used to accurately map surface features. Merging data acquired at different flybys, sometimes with very different viewing geometries (incidence, emergence and phase angles), is challenging due to the strong effects of the atmosphere (additive and multiplicative components) and also to photometric effects. This makes uncorrected mosaics appear with seams, which can have about the same level than the surface spectral heterogeneities. Our goal is to retrieve homogeneous global maps of the different surface units, using a heuristic approach to correct the atmospheric and photometric effects. We show in particular that VIMS observations at high resolution (lower than 1 km/pixel) of dune fields can be used to evaluate the additive component of the signal, which is mostly due to the scattering by the aerosols. The multiplicative contribution of the atmosphere can then be removed, once the additive component has been empirically corrected. This process results in maps of surface heterogeneities, which emphasize surface variations that were not detected by previous analysis. Geological implications are being studied. We have already reported on possible flows that suggest that the outer icy layer of Titan has been active in a recent past.