Using terrestrial analogs to test alluvial fan formation mechanisms on Titan

Synthetic Aperture Radar (SAR) data of the Saturnian moon Titan show widespread fluvial and alluvial features, including hypothesized alluvial fans. On Earth, sediment delivery to alluvial fans occurs via two mechanisms. ... [ view full abstract ]

Synthetic Aperture Radar (SAR) data of the Saturnian moon Titan show widespread fluvial and alluvial features, including hypothesized alluvial fans. On Earth, sediment delivery to alluvial fans occurs via two mechanisms. Where catchments contain predominately fine-grained material, e.g., clays, those sediments increase the water viscosity, producing debris flows. The resultant surface is mantled with cobbles and other poorly-sorted sediments. Where catchments are clay-poor and sand-rich, the water flow is lower viscosity, and moves sediment largely by sheetfloods. The resultant surface is draped with sand. Based on evidence for a higher proportion of fine-grained sediment at the poles than at the equator, we hypothesize that potential fans on Titan are dominated by debris flows at high latitudes and sheetfloods at low- and mid-latitudes.
To test these hypotheses, we compared trends in the normalized radar cross sections (σ0) of 23 potential fans on Titan, spanning ~55° S to ~68° N, to those of terrestrial fans formed by debris flows and sheetfloods. Our terrestrial analogs include fourteen fans in Death Valley, California. A lithological contact between clay-rich sedimentary rocks and clay-poor igneous rocks in the catchments separates these fans into seven debris flow fans and seven sheetflood fans, respectively. We delineated the planform geometries of the terrestrial and Titan fans and calculated the mean σ0 for each fan. We also took profile data for each fan, sampling σ0 in the downfan direction.
In the terrestrial SAR images, the debris flow fans are significantly brighter than the sheetflood fans. We interpret this enhanced brightness to result from the cobbles mantling the debris flow fan surfaces, and the sheetflood fans having a drape of sandy sediment.
On Titan, the potential fans are significantly brighter in the low- and mid-latitudes and darker near the poles. This comparison suggests that the low- and mid-latitude potential fans on Titan are also mantled with coarse sediment from debris flows and that the near-polar fans are draped with finer sediments from sheetfloods.
Based on these results, our hypothesis for the Titan fans is not supported. The enhanced backscatter from the equatorial fans suggests a mantling of cobbles, consistent with debris flow deposits, whereas the lower radar brightness of polar fans on Titan suggests sediment delivery by sheetfloods. These data could indicate that the relative proportion of fine-grained sediments available for transport is greater at low latitudes than at Titan’s poles, or that sand is available at the higher latitudes.