A transmission spectrum of titan's north polar atmosphere from a specular reflection of the sun

Cassini/VIMS T85 observations of a solar specular reflection off of Kivu Lacus (87.°4N 241.°1E) provide an empirical transmission spectrum of Titan's atmosphere. Because this observation was acquired from short range (33,000 km), its intensity makes it visible within the 2.0, 2.7, and 2.8 μm atmospheric windows in addition to the 5 μm window where all previous specular reflections have been seen. The resulting measurement of the total one-way normal atmospheric optical depth (corresponding to haze scattering plus haze and gas absorption) provides strong empirical constraints on radiative transfer models. Using those models, we find that the total haze column abundance in our observation is 20% higher than the Huygens equatorial value. Ours is the first measurement in the 2-5 μm wavelength range that probes all the way to the surface in Titan's arctic, where the vast majority of surface liquids are located. The specular technique complements other probes of atmospheric properties such as solar occultations and the direct measurements from Huygens. In breaking the degeneracy between surface and atmospheric absorptions, our measured optical depths will help to drive future calculations of deconvolved surface albedo spectra.