1. Introduction
Cassini-Huygens mission has been in orbit around Saturn since June 2004. The Ku-band (13.8 GHz – 2.2 cm) RADAR instrument onboard the Cassini spacecraft is a combined radiometer/altimeter/scatterometer/imaging radar that has revealed a various and rich surface of Titan through its optically-opaque atmosphere (Elachi et al., 2004). RADAR has allowed the discovery of numerous seas, lakes, rivers, cryo-volcanic structures and vast dune fields (Elachi et al., 2005, Lopes et al., 2007, Lorenz et al., 2006, Radebaugh et al., 2007 and Stofan et al., 2007). Dunes are in particular a major landform on the surface of Titan, since large dune fields cover more than 10 million km2 in equatorial regions. They are typically 1–2 km wide, with 1–4 km spacing, up to 150 m-high, and can reach more than 100 km in length, being mainly east–west oriented and aligned parallel with time-averaged equatorial winds (Lorenz et al., 2006, Radebaugh et al., 2008, Lorenz and Radebaugh, 2009 and Le Gall et al., 2011). Dunes on Titan are the linear type as observed on Earth, the latter ones being used as analogues to infer Titan’s dunes morphology (Neish et al., 2010, Radebaugh et al., 2010 and Paillou et al., 2014). The RADAR instrument shows Titan’s dune fields as dark linear features separated by brighter linear features. Different qualitative interpretations have been proposed for this radar signature: (1) dark lines are sand covered interdunes with brighter features caused by specular reflection over dunes’ crests, as for the linear dunes observed in the Namib Desert (Neish et al., 2010), or (2) dark lines are the smooth dunes with brighter linear features caused by rougher interdunes, where bedrock is exposed, as observed in the Great Sand Sea in Egypt (Paillou et al., 2014). Besides linear dunes, other natural wind-related structures on Earth show a comparable morphology: mega-yardangs are wind-abraded landforms which develop in Earth’s drylands (Sahara, Middle-East, Central Asia), where winds tend to be unimodal in direction (Goudie, 2007). Mega-yardangs are composed of alternating linear ridges and valleys created by wind erosion and sediment deflation, and are often associated with soft deposits of desiccated lake beds. As for linear dunes, radar images of mega-yardang structures show alternating dark linear features (the erosion valleys) and brighter linear features (the ridges): confusing radar images of linear dunes and mega-yardangs is then quite possible, especially at the 300 m resolution of the RADAR instrument.
Comparative planetology is a powerful approach to help understand the geology of remote planetary surfaces. We conducted a comparative study of the radar scattering of both linear dunes and mega-yardangs, based on representative terrestrial analogues. We considered the linear dunes located in the Great Sand Sea in western Egypt and in the Namib Desert in Namibia, and the mega-yardangs located in the Lut Desert in eastern Iran and in the Borkou Desert in northern Chad. We analysed and modelled the radar signatures of both linear structures using high-resolution (18 m) radar images acquired by the X-band (9.6 GHz – 3.1 cm) radar of the TerraSAR-X satellite. We used a simple surface scattering model, whose parameters were derived from the local topography (ASTER Global Digital Elevation Map – GDEM, and Shuttle Radar Topography Mission – SRTM data) and from surface roughness estimates (Bayesian inversion), which accurately reproduces the radar signature for both landforms. We were able to discriminate between two types of dunes: those with bare interdunes in Egypt vs. those with sand-covered interdunes in Namibia; and between two types of mega-yardangs: young ones in Iran vs. older ones in Chad.
We applied our understanding of the radar scattering to the analysis of radar images obtained during the Cassini T8 flyby over the Belet Sand Sea on Titan: we show that the linear dunes there are likely to be of both Egyptian and Namibian types, contradicting previous studies which proposed single-type dune scenarios. We also show that the bright linear structures observed in radar acquisitions during Cassini T64 and T83 flybys are very likely to be mega-yardangs, possible remnants of lake beds at mid-latitude (around 40°N).