THE TITAN TUMBLER: INITIAL RESULTS FROM LABORATORY SIMULATION OF ICY SEDIMENT COMMINUTION AND ROUNDING

Titan’s pervasive fluvial landforms appear to be commonly floored by fluvially rounded coarse-grained sediments. This inference is based on visible imaging of ~10-cm-diameter rounded cobbles at the Huygens landing site, inferred rounded sediments with diameters >2.2 cm in two radar-bright features interpreted to be outflow channels, and the radar-bright appearance of other fluvial features. Initial measurements of radar-bright fluvial features show decreasing brightness with distance downstream, suggesting comminution during transport. Rounding and comminution hold information on the duration, distance, and intensity of the flow that produced them. Accessing this information for Titan sediments requires quantifying rounding and comminution rates for water ice. The resultant grain size distribution may also inform our understanding of Titan’s extensive aeolian sand, which might be composed either of organics or of organic-coated ice grains. We are undertaking experiments in a cryogenic clast tumbling system – ‘The Titan Tumbler’ – to measure the rates of change of ice particle mass, rounding, and sphericity as a function of crystal grain size, rolling rate, and distance. Initial experiments used 3-cm-cubes of ice having three grain size distributions and abraded under three transport regimes. Changes to mass, sphericity, and roundness of the clast population were measured at different transport distances. The minimum experimental transport distance is the shortest likely distance for the Huygens landing site clasts, ~2-2.5 km. The maximum distance approximates the longest mapped fluvial networks, ~100-200 km. Initial results show that after 70 km of transport, the resultant grain size distribution was largely pebbles with some silt but no sand. The minimal change in the largest grain raises questions as to how non-fluvial processes might be abrading sediment, whereas the lack of sand supports an aggregation formation for the aeolian sediment.