[hal-01088832] Interpreting Titan's surface geology from Cassini RADAR observations

Interpreting Titan's geology from Cassini RADAR data presents some unique challenges. The Cassini RADAR acquires data in several modes including SAR, radiometry, and scatterometry. Major geologic units can be identified in the SAR data, but they are only apparent if their backscatter characteristics are sufficiently different from those of surrounding terrains. Structures are more easily recognized when oriented perpendicular to the radar's look direction. At Titan, the SAR mode of the RADAR instrument is used at altitudes under ~4,000 km, resulting in resolution ranging from ~350 m to >2 km, which is in some places insufficient to identify different geologic units on the surface. On Titan, the candidate surface materials (water ice, water-ammonia ice and other ice mixtures, hydrocarbons, tholins) are different from the rocky surfaces more usually imaged with radars; in particular, scattering from the subsurface is thought to be significant. We have been able to identify a wide variety of geologic features on Titan's surface using SAR swaths, which now cover ~45% of the surface. The data are distributed over a wide latitudinal and longitudinal range, enabling some conclusions to be drawn about the global distribution and significance of processes. They reveal a geologically complex surface that has been modified by all the major geologic processes seen on Earth. In terms of global areal distribution, both dunes and mountainous terrains (including the large Xanadu radar-bright region) cover more area than other identified geologic units. In terms of latitudinal distribution, dunes and mountainous terrains are located mostly at low latitudes (less than 30 degrees), with no dunes being present above 60 degrees. Channels formed by fluvial activity are present at all latitudes, but lakes filled with liquid are found at high latitudes only (above 60 degrees). Impact structures are mostly located at low latitudes, with no confidently identified craters above 60 degrees latitude, possibly indicating that more resurfacing has occurred at higher latitudes. Putative cryovolcanic features are not ubiquitous and some could be fluvial in origin. The Sotra Facula region is currently the best candidate for a cryovolcanic region, consisting of a mountain and adjacent pit and flow-like features. We examine temporal relationships between units and conclude that aeolian and fluvial/pluvial/lacustrine processes are the most recent, while tectonic processes that led to the formation of mountainous terrains and Xanadu are likely the most ancient. Mountainous terrains, which along with Xanadu may have at least in part a tectonic origin, are radar-bright and radiometrically distinct from most other areas. They may have been washed clean of organic particulates by methane rains. Preliminary correlations between geologic units and surface properties derived from the radiometry measurements (brightness temperature, effective dielectric constant, and degree of volume scattering) will also be presented.



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