Modeling the radar response of Linear Dunes and Mega-Yardangs on Titan: Geological implications

<p>Linear geomorphology features on Earth are well-studied using geo- logical field observations and remote sensing (optical, radar). These structures can be divided into two groups : the wind-deposit and ac- cumulation areas known as linear sand dunes ; and the wind-abraded ridges on eroded cohesive rocks known as mega-yardangs.</p> <p>In this study, we performed a comparative planetology approach to discriminate between these two features on Saturn's moon Titan, by comparing the radar signature of their terrestrial analogues. The Ku- band (13.8 GHz) Synthetic Aperture Radar (SAR) onboard the Cassini spacecraft is able to sense through the optically-opaque atmosphere of Titan, but the resolution of this instrument (200 m) is not sufficient to clearly observe the morphology of linear structures. We then considered orbital radar images of terrestrial structures, provided by the high reso- lution (18 m) X-band radar (9.6 GHz) of the TerraSAR-X mission. We acquired images of the Great Sand Sea (Egypt) and the Namib Desert (Namibia) for the linear dunes ; and of the Lut Desert (Iran) and the Dju- rab Desert (Chad) for the mega-yardangs. We modeled the geometrical effect of the topography using DEM models provided by SRTM and GDEM, and the surface roughness effect was estimated using single scattering models such as Integral Equation model and Geometrical Optics model. We were able to reproduce in a quite faithful way the backscattered radar signal by terrestrial dunes and mega-yardangs.</p> <p>These results were applied on Titan to the analysis of T8 and T49 fly- bys over the Bellet equatorial region, which contains linear dune fields, and to the analysis of T64 and T83 flybys, which show potential mega- yardang structures. Based on our quantitative radar modeling results, we can confirm that linear structures in T64 and T83 are actually mega- yardangs. As on Earth mega-yardang are usually formed over former lakes depressions on homogeneous rocks with a strong and stable uni- directional wind erosion over million years. Their existence on Titan could indicate that lakes were existing in lower latitude regions in the past, providing a new constrain for past climatic models of Titan.