CONNECTING FIELD, LAB, AND MISSION DATA TOWARD AN UNDERSTANDING OF FLUVIAL PROCESSES ON TITAN (Invited Presentation)

Rounded cobbles observed at Titan's surface and inferred from flyby radar data indicate abrasion during fluvial transport. Interpreting the comminution of these cobbles can inform the geologic history of this sedimentologically active world. However, the breakdown of water ice sediment at Titan temperatures is poorly understood. To more precisely link measured downstream changes in radar brightness to sedimentary processes of abrasion and hydrodynamic sorting we integrate field analog study with laboratory experiments.

The alluvial fans of Death Valley, CA provide an abundant variety of dry sediment to analyze in the field for comparison to satellite radar data. By characterizing radar brightness of terrestrial sedimentary deposits in terms of grain size/shape distributions and inverting existing numerical models for Titan, we provide a range of reasonable sedimentological interpretations for the varying radar brightness of Titan's fluvial deposits. Initial field study indicates increasing radar brightness with grain size but no statistically significant effect from roundness. Confounding factors to be addressed include meter-scale topography, compositional variability, and radar penetration to unknown subsurface scatterers. Upcoming imaging campaigns will increase the number of sample sites beyond that possible with hand measurements.

To constrain the processes that control grain properties, we use a cryogenic abrasion mill—the Titan Tumbler—to quantify sediment breakdown and estimate possible transport distances for clasts on Titan. We roll water ice clasts at Titan temperatures (~100 K) and record changes in mass and shape across grain sizes ranging from several cm down to silt. Recent tumbler experiments show temperature-dependent fracturing, with exponential mass-loss but relatively low rounding rates across a wide range of initial conditions. The broad grain size distributions produced in the tumbler may indicate a source, in part, for sand-sized fines that are abundant near Titan's equator. When linked to measurements from Cassini radar images, abrasion estimates of the Titan Tumbler, in combination with terrestrial analog study, demonstrate the possible lifetime and cycles of fluvial sediment on Titan.