Recent liquid water flow on Mars has been proposed based on geomorphological features, such as gullies. Recurring slope lineae — seasonal flows that are darker than their surroundings — are candidate locations for seeping liquid water on Mars today, but their formation mechanism remains unclear. Topographical analysis shows that the terminal slopes of recurring slope lineae match the stopping angle for granular flows of cohesionless sand in active Martian aeolian dunes. In Eos Chasma, linea lengths vary widely and are longer where there are more extensive angle-of-repose slopes, inconsistent with models for water sources. These observations suggest that recurring slope lineae are granular flows. The preference for warm seasons and the detection of hydrated salts are consistent with some role for water in their initiation. However, liquid water volumes may be small or zero, alleviating planetary protection concerns about habitable environments.
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Observation planning was funded by the MRO project, and analysis by NASA grants NNX13AK01G and NNX14AO21G. We thank NASA/JPL/University of Arizona and the MRO/HiRISE project for collecting and processing data, the University of Arizona for producing DTMs, and NASA for supporting extended mission investigations. D. Stillman provided helpful comments.
Electronic supplementary material
Supplementary Table 1; Supplementary Figures S1–S4; and caption information for the supplementary Movies 1–2 animations provided as separate files. (Animations S1–S2)
Animated GIF showing avalanche features on a sand dune slipface, to be compared with RSL
Animated GIF showing upslope movement of ripples at one RSL location
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