Angle of repose of Martian wet sand using discrete element method: Implication for the seasonal cycle of recurring slope lineae(RSL) by relative humidity

TL;DR

This study uses discrete element method simulations to show that small amounts of liquid cohesion can significantly increase the angle of repose of Martian sand, explaining the seasonal recurrence of recurring slope lineae (RSL) through humidity-driven wetting and drying cycles.

Contribution

It introduces a model demonstrating how minimal liquid bridges can cause cohesion in Martian sand, influencing RSL formation and disappearance without requiring large water volumes.

Findings

Liquid bridges can raise the repose angle to ~40 degrees.

Humidity cycles induce wetting and drying, triggering RSL avalanches.

Minimal liquid amounts are sufficient for RSL activity.

Abstract

Seasonal cycle of Martian relative humidity shows that, in the southern and the northern RSL sites, humidity decreases when RSL increment while humidity is high on the fading season. We suggest that the recurrence of RSL is caused by the different angle of response between wet and dry grains. 2D DEM simulation indicates that, when the ratio of a liquid bridge volume to a particle volume is the order of 10^{-7}, the repose angle of Martian sand can increase to ~40 degrees due to cohesion, which is near the slope around RSL starting area. Thus, if this amount of liquid bridge is generated by deliquescence or capillary condensation of vapor and if the water evaporates following the humidity and temperature cycles, Martian sand can induce avalanche on RSL slope. When grains are wet again, sand and dust deposit on RSL, which may be the mechanism of RSL disappearance and recharge of grains for the next season. In our model, because liquid induces RSL indirectly (RSL itself is the granular flow), the amount of liquid can be drastically reduced compared with the wet mechanism. In this case, as well as temperature, the seasonal cycle of relative humidity plays an important role in the recurrence of RSL.