Laboratory Measurements of Resistance to Fluvial Incision in Polycrystalline Water-Ice Under Titan Conditions

The morphological similarities of Earth and Titan are extensive. Both bodies have dissected ridge-and-valley topography, branching channel networks, and meandering river valleys, suggestive of similar fluvial incision processes. This raises the question: Can terrestrial incision models be applied to the alien conditions of Titan, and in particular, how do ice material properties vary with temperature?

Here we apply a well-established theory for the rate of rock detachment by low-velocity impacts, where the energy required to erode a unit volume of material depends on the tensile strength, elastic modulus and a non-dimensional rock-resistance coefficient. Previous experiments have shown that ice erodes much more readily than terrestrial bedrock of a similar tensile strength. To build on those results, we are eroding polycrystalline water-ice disks made by adding near-freezing, boiled and distilled water to a 2-4 mm ice matrix in a rounded mold and chilling it for 24 hours at approximately 253 K. We measure tensile strength using Brazilian tensile strength splitting tests. The energy required to erode a unit volume of erosion is measured using drop tests, where an ice clast is repeatedly dropped from a constant height onto an ice disk and volume lost is calculated from measured ice density and mass difference. We vary temperature over a wide range, from 273 K to 89 K, using dry ice and liquid nitrogen. We find a power-law temperature dependence for all ice material properties and find that polycrystalline water ice at Titan temperatures (~94 K) erodes approximately 15x faster than terrestrial bedrock of comparable strength. The results of these experiments will lay the groundwork for future physical modeling experiments of Titan channels in an ice flume, where we will investigate the relationship between discharge and channel width to help interpret images of Titan's drainage basin-scale channel morphology.