Study of Recurring Slope Lineae Activity in Hale Crater: Wind and Dust Deposition Triggers.

Abstract

Recurring Slope Lineae (hereinafter RSL) are seasonal dark flows on Mars steep slopes. These movements of several meters long appear and grow downwards (more or less incrementally) and fade (partially or totally) more or less progressively. After investigating wet origins, dry processes involving dust are favoured (e.g., dust-removed features, dark sand movements, …) but are not yet precisely understood. One of the main common features between RSL and dust is seasonality, for example major formations are observed during the dust storm season at all latitudes. A specific RSL seasonality composed of three pulses of RSL apparition or lengthening has been found by Stillman et al. (2018) at Hale crater (323.48°E, 35.68°S). Here we assess whether this RSL timing could be related to the three pulses of the dust cycle. So, we reanalyse the observations of Hale to characterise the RSL activity with a dust removal/deposition point of view, trying to constrain the formation triggers (dust deposition, winds, …) and formation scenario for RSL. We analyse consecutive high-resolution images (>0.25 m/pixel) of two Hale areas, taken by HiRISE onboard Mars Reconnaissance Orbiter, for Martian years 31, 32, and 33. We divided the characterisation into two parts: periods of apparition or lengthening of RSL-like features (i.e., when the extent of dark surfaces increases) and periods of RSL fading or disappearing (i.e., when the contrast between dark surfaces and adjacent bright surfaces decreases). In the framework of the “dust-removed” hypothesis for RSL, these two periods correspond respectively to dust removal and deposition periods. Each of those has three levels of intensity: low, intermediate and high. Then, we compare this RSL activity timeline to atmospheric dust optical depth variations over Hale. With this new characterisation, we overall find again the three southern hemisphere spring/summer pulses and we also have identified an RSL formation event occurring near winter solstice (not already noticed). Then, we notice that there are dust depositions before each pulse, which correspond to a long decrease of atmospheric dust optical depth (1st pulse) or two local peaks (2nd and 3rd pulse). This may imply that dust deposition at RSL locations can occur as both progressive fallout or rapid transport associated with storms. This also implies that a certain surface dust deposition seems to be necessary to have a significant level of RSL formation, but it does not seem to be always sufficient to trigger RSL formation. Indeed, local increase in atmospheric dust, which could be related to increased wind activity, seems to be required (1stpulse) or seems to favour RSL formation (3rd pulse). Thus, we can propose an RSL formation scenario consistent with these observations: if there is enough surface dust deposition, a dry avalanche-type formation can be observed (possibly initiated by winds); with less dust deposition or slope unfavourable conditions (not allowing avalanche) the RSL lengthen downward more incrementally (as for the 3rd pulse) under the action of winds. This proposed scenario elaborated using Hale observational constraints will be tested, improved, and confirmed with similar analyses performed at other RSL sites.