Slope, elevation, and thermal inertia trends of martian recurring slope lineae initiation and termination points: Multiple possible processes occurring on coarse, sandy slopes

Abstract

Recurring slope lineae (RSL) are dark linear features on the surface of Mars that advance incrementally downslope, fading and re-growing annually. Numerous hypotheses have been proposed to explain RSL formation, including “wet” models that involve liquid water or brines and “dry” mechanisms involving liquid-free mass-wasting or solid-vapor phase changes that trigger granular flow. In part, hypotheses to explain RSL formation rely on observations of the physical characteristics of the slopes on which RSL are present. To determine if RSL exhibit physical characteristics typical of one particular formation mechanism, we examined the initiation and termination points of over 10,000 RSL across 16 confirmed RSL sites to determine their slope, elevation, orientation, and thermal inertia. RSL typically form within an envelope of high elevations at each mapping site, are oriented equatorward and west, and have hillslope thermal inertia values consistent with sand and gravel. Notably, the RSL begin and end at slopes above, within, and below the angle of repose required to trigger and sustain dry granular flows. Sensitivity testing indicates that these slope values are not a result of pixel-scale noise. Discontinuous RSL are observed at several study sites. The slope and thermal inertia values of the RSL, as well as these additional geomorphic observations of discontinuous dark features, are not consistent with the characteristics of dry flows in which continuous dust avalanches begin within the dynamic angle of repose and terminate within the static angle of repose. Although RSL are found to be common on coarse sandy and rocky slopes, not all sandy slopes in RSL regions that are close to the angle of repose contain RSL. We conclude that mechanisms other than dry mass wasting are needed to explain all of the observed geomorphic characteristics of RSL source and termination points.