Martian linear dune gullies are landforms consisting of parallel and often sinuous channels with distinct levees and pit-shaped endings that occur in the mid-latitudes on Mars. Recent observations of ongoing activity link their formation to sliding blocks of CO2 ice in early spring. Here we combine laboratory experiments in which we release CO2-ice blocks on sandy slopes under Martian atmospheric pressure with morphometric observations of linear dune gullies on Russell crater mega dune to test this hypothesis. Our work shows that linear dune gullies are carved by burrowing blocks of CO2 ice that induce explosive ballistic sediment transport, resulting in gullies with all morphological characteristics we observe on Mars; high levees, deep channels, and extreme channel sinuosity. Our findings highlight the specific climatological, topographical, and grain size conditions under which these landforms originate, making it possible to use them as a microclimate proxy on present-day and potentially ancient Mars.

Martian gullies are landforms consisting of an erosional alcove, a channel, and a depositional apron. A significant proportion of Martian gullies at the mid-latitudes is active today. The seasonal sublimation of CO2 ice has been suggested as a driver behind present-day gully activity. However, due to a lack of in-situ observations, the actual processes causing the observed changes remain unresolved. Here, we present results from flume experiments in environmental chambers in which we created CO2-driven granular flows under Martian atmospheric conditions. Our experiments show that under Martian atmospheric pressure, large amounts of granular material can be fluidized by the sublimation of small quantities of CO2 ice in the granular mixture (only 0.5% of the volume fraction of the flow) under slope angles as low as 10°. Dimensionless scaling of the CO2-driven granular flows shows that they are dynamically similar to terrestrial two-phase granular flows, i.e. debris flows and pyroclastic flows. The similarity in flow dynamics explains the similarity in deposit morphology with levees and lobes, supporting the hypothesis that CO2-driven granular flows on Mars are not merely modifying older landforms, but they are actively forming them. This has far-reaching implications for the processes thought to have formed these gullies over time. For other planetary bodies in our solar system, our experimental results suggest that the existence of gully-like landforms is not necessarily evidence for flowing liquids but that they could also be formed or modified by sublimation-driven flow processes.

A document by Lonneke Roelofs. Click on the document to view its contents.