Evaluation of wet and dry recurring slope lineae (RSL) formation mechanisms based on quantitative mapping of RSL in Garni Crater, Valles Marineris, Mars

Recurring slope lineae (RSL) are narrow (0.5-5 m) low-albedo features that incrementally lengthen down steep slopes during warm seasons, fade in colder seasons, and recur each Mars year. To reduce the effort involved in manually mapping and analyzing each RSL in Garni crater, we developed Mapping and Automated Analysis of RSL (MAARSL) to analyze a set of orthorectified High Resolution Imaging Science Experiment (HiRISE) images and a digital elevation map. MAARSL along with manual mapping allowed us to detect RSL, compute descriptive statistics, and characterize changes over time. We mapped 2910 RSL in 22 orthoimages, from Mars Year (MY) 31 solar longitude (L-s) 133.0 degrees to MY32 L-s 323.7 degrees. The MAARSL results confirmed that RSL lengthening and fading occur concurrently on slopes with the same orientation and many times within some individual RSL. Slope angles of RSL and a slope slump show that some RSL start, stop, and have mean slope angles that are below the angle of repose. Our analysis shows that RSL are actively lengthening on at least one slope-facing direction in all HiRISE observations of the crater. We also found that NE-, N-, and NW-facing RSL in Garni crater lengthened during times of increasing shortwave insolation, while S- and SW-facing RSL lengthened during increasing and decreasing shortwave insolation. A (non-orthorectified) HiRISE image acquired shortly after the MY34 dust storm and shows RSL on every slope-facing direction, which is anomalous with respect to observations prior to the dust storm. We find that dust removal and deposition could explain the darkening and fading (respectively) of RSL, and could also explain the apparent lack of material being transported. Observations of RSL lengthening and fading occur concurrently on slopes could suggest overprinting of dry granular flows. Dry flows could also explain the significant lengthening activity of every slope-facing direction after a fresh layer of dust was deposited via the MY34 dust storm. Alternatively, briny shallow subsurface flows are consistent with observations of RSL on slopes below the angle of repose and those that exhibit concurrent lengthening and fading. However, the most significant problem with briny RSL flows is accessing a source of briny water and removing excess salt from the regolith.