Titanʹs 5-micron lightcurve

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 μm range. These observations, which sample four different positions of Titan around Saturn, clearly indicate a variability of the 5-μm continuum albedo, with Titanʹs geometric albedo decreasing by ∼40% from Titanʹs leading side to the trailing side. This Titan 5-μm “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.