Effect of Unstable Water Flow on Grain Transport: Insights From Laboratory Experiments Under Reduced Atmospheric Pressure

Abstract Nowadays, Mars presents an environment characterized by low atmospheric pressure (∼6 mbar), which profoundly alters water‐driven surface processes known from Earth. Under these conditions, water rapidly boils, cools, and stabilizes near the triple point, often freezing, which strongly limits its ability to transport sediment. Unlike today, early Mars likely experienced higher atmospheric pressures that allowed liquid water to persist in a metastable, boilable state for longer periods. Here, we investigate how water instability affects sediment transport efficiency, focusing on phase transitions and mm‐sized grain movement. In our laboratory experiments, water is released onto a shallow inclined plate, producing a sheet‐flow‐like regime that mimics downslope sediment transport by thin transient flows. We find that larger calcite grains (2–4 mm) are transported less efficiently under unstable water conditions. Transport efficiency decreases further as pressure drops. However, near the liquid–vapor phase transition temperature (21°C; 25 mbar), we observed asymmetrical and irregular spreading of grains, reflecting altered flow dynamics caused by bubble formation. At the lowest tested pressure (∼5 mbar), partial water stabilization occurs because rapid evaporation cools the water, slowing the flow and reducing sediment motion without complete freezing. This behavior contrasts with earlier studies on smaller particles (