A case study of recurring slope lineae (RSL) at Tivat crater: Implications for RSL origins

The origin or origins of recurring slope lineae (RSL) on Mars are enigmatic. These seasonally dark, linear features correlate with temperatures appropriate for liquid water or brine. They also progress slowly downslope and fade (disappear) when inactive, similar to shallow subsurface groundwater flows that wick moisture to the surface. If liquid water or brine does indeed flow so near the surface and is sufficiently dilute, it could have profound implications for the modern habitability of Mars. We present the results of a case study at Tivat crater in the southern midlatitudes that provide new insights into RSL, including their potential origins at Tivat. These results include comprehensive maps of RSL extents in four Mars years, detailed RSL evolution across one growth season, rockfall markings, and dust devil tracks. In two well-constrained years, RSL and rockfall markings-which are more widely distributed than RSL-fade at the same time, suggesting a fading mechanism that acts across Tivat. Based on relative albedo analyses, RSL and rockfall markings at Tivat fade because their host slopes darken, which is most easily explained by the widespread removal of dust. Such a dry fading mechanism would favor a dry origin for RSL. We also present a wide range of other RSL observations, including highly variable growth, pooling-like behavior where slopes shallow to roughly 30°, and total areal extents that decrease each year across a span of five years. We discuss the implications of these observations for various RSL formation models, including a new conceptual model that primarily mobilizes dust. Taken together, our observations seem to favor a dry mass movement origin for RSL at Tivat, but significant challenges remain for each examined model.