Titan's 5-micron lightcurve
Résumé
We report on repeated mid-resolution ( R˜2000) spectroscopic observations of Titan, acquired between November 2002 and January 2003 with ISAAC at the ESO/VLT and covering the 4.84-5.05 mum range. These observations, which sample four different positions of Titan around Saturn, clearly indicate a variability of the 5-mum continuum albedo, with Titan's geometric albedo decreasing by ˜40% from Titan's leading side to the trailing side. This Titan 5-mum "lightcurve" appears to be in phase with the other near-infrared lightcurves. This can be understood in terms of a surface model in which water ice coexists in minor and variable proportions (13-25%, if pure) with a second, dark, component.