Present-day debris flows on Mars are driven by the sublimation of dry ice (CO2)

Martian gullies are alcove-channel-fan systems that are undistinguishable from debris-flow systems on Earth. Therefore, they have long been hypothesized to be formed by the action of liquid water and brines. However, over the past decade, growing evidence of widespread, extensive, and particularly, seasonal activity in these gully systems, has shifted the formation hypothesis of these landforms away from water-driven processes. The correlation between the spatial and temporal distribution of CO₂ frost on the Martian surface and the formation of new lobes, the movement of meter-scale boulders, and the cutting of new channels has led to a new hypothesis: debris flows on Mars are driven by the seasonal sublimation of dry ice (CO₂ ice). However, the lack of direct observations of these flows hinders our understanding of the exact conditions that lead to these granular flows, their dynamics, and erosional capacity, which hinders our understanding of the formation of these gullies over the last five million years.

Over the last three years, we have conducted three experimental campaigns in two environmental chambers (at the Open University, UK, and Aarhus University, Denmark) with different flume set-ups at varying scales to explore the feasibility of the CO₂-driven granular flow hypothesis. We have quantified the CO₂-driven granular flow dynamics under Martian atmospheric conditions, the physical limits under which these flows can occur, and have determined their erosional capacity. From these results, we conclude that CO₂-driven granular flows can occur on Mars under specific environmental conditions and that the sublimation of very small amounts of CO₂ ice (<0.5% of the flow volume) fluidizes sediment by creating high pore pressures. These high gas pore pressures decrease intergranular friction and make the granular mixture extremely mobile. Although seemingly similar, this process can not directly be compared to increased pore pressure in water-driven debris flows due to the other dynamical effects of the sublimating ice, for example, the grain movement in the flow. The high gas pore pressures under Martian conditions stem from the large CO₂ gas flux created under the thin Martian atmosphere (~8e-3 bar), which is ~100 times larger than it is under Earth's atmosphere (~1 bar).

Furthermore, based on dimensionless analysis (Bagnold, Savage and friction numbers) we show that the dynamics of these CO₂-driven granular flows are similar to terrestrial water-driven debris flows and pyroclastic density currents. In addition, we prove that CO₂-driven granular flows are effective erosive agents, likely more efficient than terrestrial water-driven debris flows.

Combined, these results show that we do not have to evoke a water-driven origin for the Martian gullies as we can explain their formation by CO₂-driven granular processes alone. This has implications for our understanding of the Martian climate, surface conditions and surface processes during the last five million years. Furthermore, these “alien” debris flows allow us to test ideas on terrestrial granular flows outside the confines of our own planet.