Water Desorption as a Mechanism for Recurring Slope Lineae Formation

Abstract:

Understanding water in Martian regolith is important because water affects surface morphology, chemistry, and habitability, and is a valuable resource for future human exploration. Recurring slope lineae (RSL)–seasonal dark streaks that propagate down Martian slopes in the summer and fade in the winter–are strongly linked to water because of their seasonality, but their precise formation mechanism remains unclear. RSL are morphologically similar to dry flows because they terminate on slopes that are similar to the angle of repose of sand. However, even dry RSL hypotheses require some role for water to explain their seasonal activity.

One explanation for seasonal mass wasting is that slopes destabilize in low relative humidity (RH) conditions as they release adsorbed water. In high RH winter conditions, slopes hydrate as the regolith adsorbs water from the atmosphere. Adsorbed water increases the slope’s angle of repose by strengthening cohesive forces between grains. In low RH summer conditions, water desorbs from the regolith and the angle of repose decreases. If the angle of repose falls below the actual steepness of the slope, the slope will fail.

To explore this hypothesis, we measure water adsorption and angle of repose in a Mars regolith simulant at RH from 0-100%. In a fixed-RH chamber, dry regolith adsorbs water as it equilibrates. We then use a transparent tumbler (Fig 1A) to measure the angle of repose of the hydrated regolith. The tumbler rotates slowly, gradually increasing the slope of the regolith inside. Eventually, the slope exceeds the angle of repose and fails. We film hundreds of slope failures and analyze the video to calculate the maximum angle reached. From the distribution of measured angles (Fig 1B), we determine the angle of repose (black circles) for each RH. We find that regolith hydration and angle of repose increase with increasing RH conditions, which supports the hypothesis that RSL result from mass wasting triggered by seasonal water desorption. Unlike other mechanisms, the desorption model explains RSL seasonality while requiring relatively little water, which is more consistent with Mars’ arid surface conditions. This has implications for astrobiology and planetary protection, because RSL in this model are unlikely to contain enough water to sustain life.