Changes in Soil Cohesion Due to Water Vapor Exchange: A Proposed Dry‐Flow Trigger Mechanism for Recurring Slope Lineae on Mars

Abstract

Recurring slope lineae (RSL) are seasonal flows on steep slopes on Mars. Their formation mechanism is unknown, but dry granular flows are a likely explanation. Any proposed trigger for these flows must be consistent with the observed temperature dependence of RSL: more active in warmer months or when sun‐facing. Here, we use atmospheric modeling and laboratory experiments to explore a potential mechanism that involves both wet and dry processes at Hale Crater, a known RSL location. We propose that dry flows are triggered by changes in soil cohesion due to the loss of water. When surface temperature and humidity were experimentally simulated, salts likely found in the soil only completely dehydrated during the active season for RSL. We propose that the loss of water from soil in warmer months (or when illuminated) lowers soil cohesion and maximum stability angle. Slope failure may occur, exposing darker underlying material and creating RSL.

Mars: Abundant Recurring Slope Lineae (RSL) Following the Planet‐Encircling Dust Event (PEDE) of 2018

“…Additionally, dust lags must be millimeters thick to significantly slow evaporation or sublimation (Schorghofer, 2020), and this is greater than the expected dust deposition in regions that retain a low albedo. Perhaps dust deposition provides salts that aid deliquescence as the driver of RSL growth, but the source for sufficient water to cause downhill flow is still problematic (Gough, Nuding, et al., 2019; Leung, 2020).…”

“…Selected locations may be plausible for deep groundwater discharge (Abotalib & Heggy, 2019; Watkins et al., 2014), but this cannot provide a general explanation for RSL. Highly deliquescent salts are known to exist on Mars and may temporarily trap atmospheric water in extremely small quantities, perhaps sufficient to darken the surface (Heinz et al., 2016), but not sufficient for seepage down slopes (Gough, Nuding, et al., 2019; Gough, Primm, et al., 2019). Some workers have speculated that small quantities of water could trigger granular flows (Dundas et al., 2017; McEwen, 2018; A. Wang et al., 2019).…”

Global Temporal and Geographic Stability of Brines on Present-day Mars

“…Both features were assumed to be associated with liquid-triggered or dominated flows ( Knauth & Burt 2002 ; Malin et al 2006 ; McEwen et al 2011 ; Ojha et al 2015 ), before being reinterpreted as dry flows ( Dundas et al 2017 ; Schmidt et al 2017 ). However, these flow features need an activation mechanism related to some form of phase change, and could include deliquescence ( Gough et al 2011 ; Nuding et al 2014 ), melting of ice and salt mixtures ( Chevrier & Altheide 2008 ; Chevrier & Rivera-Valentín 2012 ), hydration changes ( Gough et al 2020 ), or even boiling ( Masse et al 2016 ). Our results show that equatorial regions are too dry (evaporation plus boiling) and high latitudes too cold (freezing).…”

Martian subsurface cryosalt expansion and collapse as trigger for landslides

“…Tracking of RSL features using morning images from the Colour and Stereo Surface Imaging System on European Space Agency (ESA's) ExoMars Trace Gas Orbiter and afternoon images from HiRISE revealed RSL activity on steep slopes in Hale crater where local temperatures are sufficiently warm for melting of water ice and deliquescence of Cl salts (68). Thermal modeling and remote sensing observations support a hybrid model for RSL formation at Hale crater whereby subsurface deliquescence of salts produces liquid H 2 O, but dry flows on the surface are responsible for the RSL (68,69).…”