Detection and Likelihood of Subsurface Liquid Brines on Mars

Stable brine forms when the temperature, relative humidity, and salt-mass percentage are within bounds that are dependent on the salt type. Mars is dry and temperature and relative humidity are anti-correlated. Making surface brine challenging. However, subsurface conditions are more favorable for brine formation if deliquescence can occur within 10s of minutes. Previous laboratory experiments have found different answers to the kinetics of deliquescence on Mars. These experiments used Raman spectroscopy, which is a powerful tool to determine the phase of water. However, if the resolution of the instrument or if the sample size is greater than the laser beam penetration depth, ambiguity in your measurements is more likely. Thus, we designed a new Mars chamber that can measure the electrical properties of Mars regolith analogs under martian conditions with Mars-like concentrations of salt. Here, electrical properties of materials are represented by the real part of the relative dielectric permittivity e' that represents charge storage in bound charges, and whose imaginary part e" represents energy loss. Briefly, the dielectric permittivity drastically changes when liquid water is present (e.g., ice e'=3.1 and liquid water e'~80). Through this measurement, we are able to unambiguously determine if liquid water is within our sample. In addition to determining if liquid water is present, we are able to determine the volume of brine in the sample. We also calculate the DC (direct current) conductivity via e² and can use an empirical relationship (Archie's law) to determine the salinity and water activity of the brine. Our laboratory data combined with atmospheric data or modeling can then be used to better understand if liquid solutions are possible via deliquescence. Additionally, our research will help better understand the results from the Brine Observation Transition to Liquid Experiment (BOTTLE) onboard the ExoMars rover (Rosalind Franklin), that will expose Mars-relevant salts to the atmosphere of Mars and measure the electrical properties. Additionally, we will discuss other geophysical methods that can be used to detect brine within the shallow subsurface.