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Mars with a Pinch of Salt

Presentation #134.01 in the session Laboratory Astrophysics (LAD) Division Meeting: The Salty Solar System II.

Published onJun 29, 2022
Mars with a Pinch of Salt

Salts are found throughout the Solar System, and are an important factor in the stability of water, as well as potential habitability. Different salts will affect the stability of water to varying degrees. Chlorine salts (chlorides, perchlorates, and chlorates), can suppress the freezing temperature of water down to ~200 K. They also slow down the evaporation rate, extending the lifetime of the liquid water solution. The local relative humidity plays a significant role in the stability of these salts, in both their liquid and solid phases. Salts can undergo deliquescence when the relative humidity exceeds a given threshold, forming liquid droplets. Some halophiles have been known to utilize these droplets for active metabolism. On Mars, chlorides have been found at every landing site, and perchlorates have been detected by Phoenix in the north polar plains, by Curiosity at Gale Crater, and by Perseverance in Jezero Crater. Reanalysis of Viking data suggests perchlorates could have been present there as well. Chlorates, which are almost as stable as perchlorates, are found on Earth everywhere that perchlorates exist, making them likely to be present on Mars too. Other oxidized chlorine salts may be present in limited quantities as precursors to the formation of perchlorate.

Chlorine salts may also play a role in the mechanical properties of the regolith, as well as the stability of subsurface water. High soil cohesion was encountered at the Phoenix landing site making sample analysis challenging; such cohesion may result from hydrated salts and eutectic brines bonding grains together at their contacts by wetting, or from dehydrated salts crystallizing at grain contacts. Changes in hydration state with time (such as diurnally or seasonally) may then result in correlated changes in cohesive properties with time.

It is imperative to study not only how these salts interact with their environment, but also how we detect and distinguish them. Reference spectra of surface materials at relevant temperatures are critical for deriving abundance estimates through spectral modeling. I will present laboratory measurements and modeling of salt, water and/or regolith interactions, as well as spectra of various chlorine-salt hydrates to aid in their identification.

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