Global characterization of Titan's dune fields by RADAR and VIMS observations

Cassini/RADAR high-resolution images of Titan's surface revealed linear features, geomorphologically similar to longitudinal dunes. Those dunes cover a large portion of the whole surface of Titan, i.e 7.8%, and 13.4% are present on the 58.4% of the surface imaged by the RADAR/SAR from July 2004 to July 2013 (fig.1). 99.6% of the dunes are confined at the equatorial regions (30°N-30°S). Formed and sculpted by the wind, those features represent clues for the understanding of the climatic history on the satellite. By using the joint analysis between RADAR/SAR observations and the infrared VIMS mosaic corrected for atmospheric contributions acquired through July 2013 and June 2010 respectively, we found a very high degree of correlation at global scale (more than 70%) between the RADAR dunes and a specific infrared VIMS spectral unit, the 'dark brown unit'. Some RADAR dunes, less than 2%, also belong in a commonly referenced unit, the 'dark blue unit'. These two units have been delimited by defining for each a specific set of spectral criteria. We have shown that those two units present a spectral behavior different, especially at short wavelengths (below 2 μm) allowing to say that the 'dark brown unit' is dominated by organic sediment, similar to atmospheric aerosols, namely tholins, and the 'dark blue' is most likely enriched in water ice compared to the rest of Titan's surface. Given the strong correlation between RADAR dunes and the infrared 'dark brown unit' we are now able to extrapolate the total surface area of the dunes material to the total surface area of the 'dark brown unit' which correspond to 17% of the Titan's surface. This permits to estimate the volume of sediment of 360,000 km3 (total mass ≈ 290,000 GT). Thus, these estimates based on the RADAR dunes/VIMS units correlation make the dune fields the largest organic reservoir on Titan's surface and characterize more precisely the composition of the dune material over the total extend of the dune regions.