OBSERVATIONAL AND EXPERIMENTAL EVIDENCE FOR THE COMMINUTION OF FLUVIAL SEDIMENT ON TITAN

At the cryogenic temperatures (~94 K) on Titan, rivers are composed of liquid methane and transport sediments of water ice and organic material. The radar brightness of fluvial features on Titan’s surface has been interpreted to indicate the presence of natural retroreflectors—cobbles rounded via physical weathering during fluvial transport. We test the hypothesis that, as on Earth, fluvial sediments on Titan fine with transport downstream. Our study consists of two parts: 1) deriving brightness trends of fluvial features in synthetic aperture radar (SAR) images, and 2) conducting cryogenic roller mill experiments. Modeling demonstrates that transparent spheroidal cobbles (presumably composed of water ice) produce significant backscatter at wavelengths approximating that of the Cassini RADAR instrument (2.18 cm) and reflect a decreasing signal with decreasing grain size. We measure SAR brightness as a function of distance downstream, with initial results indicating a range of both decreasing and relatively constant downstream trends. Rates of fining with distance will constrain minimum transport distances to help identify possible source regions for clasts. Although there may be multiple contributions to grain size trends (e.g., selective transport/deposition, stream confluence/divergence, etc.), the degree of comminution and rounding due to abrasion can be tested with a common sedimentological tool—the roller mill. In our cryogenic roller mill, titled the Titan Tumbler, we run experiments on two 11-cm diameter barrels in a -20 C chamber. A future setup will utilize a custom mill, cooled to Titan-like conditions. In our initial setup, we have tested variations in the number of input clasts and clast grain size. We remove clasts and fines at regular intervals to sieve, weigh, and image (for calculation of roundness parameters). Trends of mass-loss, rounding, and produced grain size distributions are examined. From these data, we can form an understanding of how Titan’s fluvially transported ice may be processed by clast-clast collisions with implications for fine sediment production, such as the sand-size particles identified in Titan’s expansive dune belts. By linking laboratory experiments to Titan observations, we can constrain the source-to-sink modification of icy sediment on Titan’s surface.