LINKING COBBLE-RICH FLUVIAL ENVIRONMENTS ON EARTH AND TITAN

Besides the Earth, Saturn's moon Titan is the only world in the solar system that supports extant liquids at its surface. Fluvial activity has left its geomorphological footprint in various forms across much of Titan’s surface. Based on numerical modeling, the high radar brightness of some fluvial features has been interpreted to be a consequence of the presence of fluvially rounded cobble-sized sediment acting as spheroidal retroreflectors within the river valleys. However, the modeled efficiency of backscatter at the 2.2-cm wavelength of Cassini’s RADAR instrument depends on grain size, shape, and composition. Studying the effect of these controls on radar backscatter for terrestrial rivers, in addition to further complications such as sediment sorting, can inform interpretations of Titan’s fluvial environments. By analogy to terrestrial rivers, sediment in Titan rivers is expected to round and fine with transport. Thus, changes in radar brightness are expected with distance downstream. The physical weathering of water-ice clasts at Titan temperatures (~95 K) is being tested in a roller mill, dubbed the Titan Tumbler. The rate of rounding and comminution in this end-member high-abrasion scenario of rolling without cushioning liquids indicates logarithmic trends of increasing roundness and decreasing grain size as a function of distance rolled. For 3.2 cm pure H2O clasts, high roundness indices (>0.95) are achieved after ~30 km. Tuning experiments have been conducted with variable speed, temperature, number of input clasts, and barrel lining material. Ongoing experiments explore the response to changes in clast shape and ice crystal grain size. The resulting comminution trends will be analyzed in comparison to changes in grain properties suggested by downstream radar brightness trends. Sieving of icy sand and silt produced via laboratory abrasion helps constrain Titan’s sediment cycle, with initial tests indicating little link between input grain sizes and output fines. Radar image analyses, when compiled with experimentation in a cryogenic roller mill, will improve understanding of flow conditions, rate of erosion, and sediment sources/sinks at Titan’s surface, adding additional depth to the relatively sparse dataset available for this distant world.