Changes in Soil Cohesion Due to Water Vapor Exchange: A Proposed Dry‐Flow Trigger Mechanism for Recurring Slope Lineae on Mars. Issue 11 (4th June 2020)

• Record Type: Journal Article
• Authors: Gough, R. V.; Nuding, D. L.; Archer, P. D.; Fernanders, M. S.; Guzewich, S. D.; Tolbert, M. A.; Toigo, A. D.
• Is Part Of: Geophysical research letters. Volume 47:Issue 11(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-06-04
• Subjects: Mars -- recurring slope lineae -- water vapor -- salt -- perchlorate -- humidity; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL087618
• Languages: English
• ISSNs: 0094-8276

Abstract

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. Plain Language Summary: Some steep hillsides on Mars have dark streaks that appear in warm months and disappear in cold months, year after year. These "recurring slope lineae, " or RSL, could be flowing liquid but are most likely dust avalanching. RSL appear in warmer months, which could be a clue as to what causes the features. In this study, we use an atmospheric model to predict relative humidity and temperature at the surface at one RSL location, Hale Crater. We then simulate these conditions in the laboratory. We find that a type of salt found on Mars can take up and release water from the atmosphere every day, but the salt only loses all of its water when the RSL active season was simulated. We propose that the dry grain flows are triggered in the summer when the soil loses water and becomes less "sticky." When this happens, dust and sand may slide downhill, exposing the darker material underneath, and create RSL. Key Points: Temperature and relative humidity conditions at Martian recurring slope lineae locations were modeled and experimentally simulated When surface conditions at an RSL location were simulated, Mars‐relevant salts only completely dehydrated during the RSL active season RSL on Mars may be a dry grain flow triggered by seasonal changes in water content and resulting changes in soil cohesion