Titan's Surface Reflectivity and Composition from Empirical Atmospheric Correction of Cassini/VIMS Data

One of the primary challenges to constraining Titan's surface composition from visible and near infrared reflectance measurements is the moon's optically-thick atmosphere. We present a simple radiative transfer model, constrained by atmospheric transmission measurements from solar occultations, which we apply to reflectance data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS). Using an inverse model for absorption and scattering in the atmosphere, we recover corrected reflectance spectra and map the distribution of compositional units. Our results suggest that exposed water ice is concentrated in fluvial channels and along the margins of topographically higher infrared-bright terrain. The latter, exemplified by the anomalously bright (and possibly active) features Tui Regio and Hotei Regio, appear depleted in water ice relative to the darker "bedrock” unit, and their reflectance spectra are consistent with enrichment in cyanoacetylene (HC₃N), ammonia, and/or small quantities of carbon dioxide. A narrow absorption band at 4.92 μm promotes HC₃N over the other two candidates, though CO₂ also has a band within 25 nm of the observed wavelength, so that a shift due to the particular bonding configuration (e.g. with water ice) may be consistent with this component. If Tui Regio and Hotei Regio are enriched in precipitated aerosols such as HC₃N, this implies an unknown concentration mechanism. Dune material is spectrally more neutral than water ice, consistent with a hydrocarbon heteropolymer.