Investigating the Formation of Mars Recurring Slope Lineae through Laboratory Experiments

The presence of low-albedo streaks on crater slopes, Recurring Slope Lineae (RSLs), may be evidence for present-day intermittent and repeated flow of water or brine on the surface of Mars. RSLs grow, fade, and can grow again seasonally as surface temperatures change [e.g. 1,2]. Although distinguishable by being darker than the surrounding terrain, they have no diagnostic absorption features [3] with the exception of a ferric feature that may be related to grain size [4] and the notable discovery of hydrated perchlorates at the base of one set of RSLs [5]. To explore liquid-based hypotheses for the formation of RSLs, we have constructed an environmental chamber that can simulate Martian surface conditions. The development of this chamber follows upon the successful completion of preliminary tests under a terrestrial atmosphere [6] to prove the optical design and subsequently under Mars pressure to verify the technical approach [7]. The Mars Analog Reflectance Spectroscopy (MARS) chamber is capable of exposing soils to brines from underneath, simulating possible subsurface wetting that could result in RSL formation. While maintaining Mars pressure and similar oxygen fugacity, the chamber will also allow the collection of spectra from 0.4 to 2.4 microns. Various brine compositions can be investigated, including solutions of iron chlorides. These unique salts can lower the soil albedo without inducing a spectral absorption feature, whereas other salts brighten the surface after drying or retain significant water [8]. Another possible darkening mechanism is also being explored within the MARS chamber. Experiments have shown that evaporation of liquid from palagonitic soils under Mars pressure create ubiquitous grain scale cavities within the surface [7]. This micro-roughness increases shadowing and darkens the surface, indicating it may be a process of darkening that is independent of brine composition.