Titan's Surface Albedo From Near-infrared CFHT/FTS And VLT Spectra
Résumé
We have observed Titan in a series of campaigns from 1991 to 2005 between 0.8 and 2.5 microns with the Fourier Transform Spectrometer (FTS) at the Canada France Hawaii Telescope (CFHT), and the NACO system at the Very Large Telescope (VLT). The CFHT dataset allows us to explore five methane windows at 0.94, 1.08, 1.28, 1.58 and 2 microns at different longitudes and resolutions. The effect of the methane absorption coefficients in the modeling and its influence on the retrieval of surface albedos was investigated by performing a comparative study of the methane absorption coefficients currently available from different sources. The obtained surface albedos seem to be very dependent on the coefficients used, with large consequences to the analysis of Titan's surface composition. Our 2005 VLT spectra, taken two days after Huygens's landing, using adaptive optics, allowed us to recover spatially resolved spectra of Titan at different latitudes and longitudes on Titan, including at the Huygens probe landing site. By applying a microphysical and radiative transfer model, with fractal haze particles, we were able to retrieve some atmospheric and surface information about these different areas. Tests on the dependence of the surface albedo on the haze profile used were also performed. Finally a comparison was made between the retrieved surface albedo in all the windows and some possible surface constituents. A mixture of water ice and tholins showed to give a compatible fit, but leaves room for additional components.