Evaluation of grainflow mechanisms for martian recurring slope lineae (RSL)

Abstract

Recurring slope lineae (RSL) are dark narrow features that incrementally lengthen and fade at least once each Mars year. Their origin has remained enigmatic. We report quantitative modeling of three hypotheses related to triggering of over-steepened sand caches that result in dry-grain flow. First, sand could be supplied by either wind blowing upslope from within the crater or by wind blowing into the crater from the outside. Without cementation, grain flows should be directly correlated with temporal variations in potential sediment transport. Second, deliquesced perchlorate salts could increase soil cohesion; with the loss of water this soil cohesion may be reduced. The timing of accumulation versus release can be distinct from each other. Third, seasonal water frost could act in a similar cementation capacity. We assess these hypotheses using atmospheric modeling to determine wind speed, wind direction, temperature, and relative humidity at three craters (Rauna, Krupac, and Palikir) that host confirmed RSL and that cover a large latitudinal range. Overall, we find no convincing support for any of these hypotheses. Deliquescence is predicted to occur only at Rauna crater and its formation does not correlate with RSL activity. The occurrence of frost is inconsistently correlated with RSL activity among the three craters. Upslope winds at Palikir crater transport a significant amount of sediment only when RSL are active. However, the largest sediment flux into the crater is also during periods of RSL activity, thus supporting both internal and external contributions. Sediment transport into Krupac crater to S- and SW-facing slopes is also maximized when RSL are active, but there is no correlated upslope transport. This supports the hypothesis that RSL formation is dominated by external sediment supply alone. On the other hand, W- and NW-facing Krupac and all RSL at Rauna show no correlations with any directional sediment transport. Nonetheless, we suggest that the mixed success of the external sediment transport model is still quantitatively better than any competitor (including water), and that we simply lack the model and data resolution to treat RSL at the required meter scales. In all of the aeolian models, favorably-directed potential sediment flux greatly exceeds the volume of sand that must be displaced to form RSL. We conclude that RSL may require a particular combination of favorable strong winds, local sediment traps, and a supply of sand grains that easily saltate on Mars (~100 μm in diameter).