Rapid rounding of icy clasts during simulated fluvial transport in the Titan Tumbler

Abstract

Surface imaging of subrounded ice clasts and anomalously bright radar backscatter properties provide evidence for rounding of cobbles on Titan. By analogy with sediment on Earth, sediment on Titan is expected to degrade during transport, decreasing in size and increasing in roundness as a function of distance. Terrestrial sedimentology includes a long history of investigating such modification via analog lab experiments. Here we report results from the first cryogenic tumbler designed to reproduce some aspects of fluvial transport on Titan. Following pilot study experiments conducted at warmer temperatures, water ice clasts of various initial size, shape, and ice type were tumbled at Titan-like temperatures of ~100 K, including various levels of liquid nitrogen in the barrel. The application of an exponential decay commonly used to characterize downstream fining (Sternberg's law) fails to fully capture the breakdown behavior of ice in these tumbler experiments, whose data are better described by a more recent, multi-component formula. Rapid fragmentation and attrition of ice clasts led to abrasion rates comparable to previous tumbler and field studies of only the weakest terrestrial lithologies, two orders of magnitude greater than abrasion rates of quartzite. Ice clasts underwent more frequent fragmentation via splitting than is seen in most terrestrial tumbler studies. However, roundness indices similar to those measured for clasts at the Huygens landing site can be achieved after just a few kilometers of tumbling, suggesting that Titan ice clasts may round faster than water-transported silicate rocks on Earth. The relatively small quantities of sand-sized sediment produced during tumbling suggest breakdown of icy clasts may not be a major contribution to Titan's equatorial sand dunes. Although these experiments reproduce only some aspects of natural sediment abrasion, data from the Titan Tumbler provide a useful starting point in the experimental study of Titan's fluvial processes.