Compositional and spatial variations in Titan dune and interdune regions from Cassini VIMS and RADAR

Dunes cover about 15% of Titan's visible surface, and represent one of the largest reservoirs of hydrocarbon solids on Titan (Rodriguez, S. et al. [2014]. Icarus 230, 168-179; Lopes, R.M.C. et al. [2016]. Icarus 270, 162-182.). Herein, we use data from the Cassini spacecraft to derive constraints on the compositional and regional variability of Titan's dune and interdune regions by combining spectral information from the Visual and Infrared Mapping Spectrometer (VIMS) and spatial information from Synthetic Aperture RADAR (SAR) data. Using the combined datasets, we extract pure infrared spectra of dune and interdune regions by extrapolating linear correlations between VIMS reflectance and dune area fraction calculated in each VIMS footprint from SAR images. We applied the same method using the Cassini RADAR Radiometer dataset to extract the microwave surface emissivity of the dune and interdune regions. Globally the dune spectra show little variation, but we find that the interdune spectra exhibit several different behaviors. Similarly, we extract from passive radiometry a mean dune emissivity of 0.98 +/- 0.01, while interdune emissivity varies from 0.86 to 0.98. We find that the interdune regions are often spectrally similar to other Titan terrain units, namely Caladan Planitia, the Adiri Mountains, and Sinlap crater, while the dunes are spectrally distinct from all terrain units. Around Sinlap crater, the interdune regions correspond to the dark blue VIMS unit: the dunes could be forming on top of the ejecta, or the material corresponding to the blue unit could be depositing preferentially in the interdunes areas. There was one region in the Belet sand sea where we were unable to extract the dune and interdune spectra and emissivities in spite of high-quality data, which we interpret to result from a thick sand cover in the interdune regions, implying inactive or saturated dune fields. However, the fact that we were able to extract distinct dune and interdune spectra and emissivities in most of Titan's dune fields makes a strong case for sand free interdune areas of varying composition in these regions, on depths from micrometers to decimeters. This would imply that the sand dunes have been active recently on geologic timescales. (C) 2015 Elsevier Inc. All rights reserved.