Coarse Sediment Transport in the Modern Martian Environment

The occurrence of regional and global dust storms, observations of migrating ripples and dunes, and the recognition of aeolian sandstone outcrops demonstrate that aeolian processes have been and continue to be a significant agent of surface modification on Mars. However, the mechanisms of aeolian transport within Mars' low atmospheric pressure surface environment are still not fully understood. This work presents a synthesis of change detection observations conducted with the Mars Science Laboratory Curiosity rover in Gale crater over three Martian years. Sediment mobility during this period was highly variable, consistent with previous orbiter-based observations showing strong seasonal variability, with strongest winds expected during southern summer. Wind activity inferred at each change detection site helps test the accuracy of atmospheric models, as well as constrain the intensity of current atmosphere-surface interactions and the physics of sand transport in the tenuous Martian atmosphere. Results indicate an apparent discrepancy between predicted wind speeds and the wind strengths required by classical incipient-motion models to explain observed sediment motion. Observed mobilization of perched and/or isolated very coarse sand grains and fine pebbles (i.e., grain sizes 1mm d3mm) likely requires an alternate explanation, such as very strong but infrequent wind gusts, drag-induced rolling, or impact-driven creep caused by smaller saltating particles. Plain Language Summary Observations from spacecraft and landed instruments have revealed that modern-day Mars is a cold and dry planet, where aeolian (i.e., wind-driven) processes are the dominant source of surface modification. In general, wind-driven sediment transport is dependent on both the size of the sand grains and the atmospheric density; the Martian atmosphere is around 100 times less dense than the atmosphere on Earth, necessitating much higher winds to generate sediment transport on Mars. This poses a significant challenge for understanding the overwhelming evidence of active aeolian sediment transport occurring on the surface of Mars today. This work presents a synthesis of images taken from the Curiosity rover over almost six Earth years. These images were used to assess wind-driven movement of sediment around the rover and demonstrated that wind-driven transport in Gale crater occurs predominantly around local summer. During this windy season, sand grains as large as 3mm are mobilized, an observation which is inconsistent with the maximum wind speeds that are predicted by models and measured by landed instruments. These findings have overarching implications for understanding transport physics under different atmospheric conditions and for deciphering the paleoenvironments that are recorded in the geologic record on Mars.