CO2-driven granular flows as erosional forces on present-day Mars

Martian gullies are alcove-channel-fan systems which have been hypothesized to be formed by the action of liquid water and brines, the effects of sublimating CO₂ ice or a combination of these processes. Recent activity and new flow deposits in these systems have shifted the leading hypothesis from water-based flows to CO₂-driven flows. This shift in thinking is supported by the low availability of atmospheric water under present Martian conditions and the observation that gully activity occurs at times when CO₂ ice is present. We recently performed novel experiments that have shown that this hypothesis holds; sediment can be mobilized and fluidized by sublimating CO₂ ice under Martian atmospheric pressure. However, if these flows are able to erode the underlying surface and can explain the formation of Martian gully systems over the long term remains unknown. Therefore, we present an additional series of experiments that test the capacity of CO₂-driven granular flows under Martian atmospheric conditions to erode sediment. These experiments were conducted in a 4 m long flume in the Aarhus Mars Simulation Wind Tunnel. Our experiments show that CO₂-driven granular flows can erode loose sediment under a range of different slopes and CO₂-ice fractions. The results also show that incorporation of warmer sediment increases fluidization of the mixture, reflected by an increase in gas pore pressure in the flow. These results thus prove that morphological evolution in the gully systems on Mars can be explained by CO₂-driven granular flows.