[1] We have determined the evaporation rate of brine under simulated martian conditions at temperatures from 0°C to À26.0°C as part of our efforts to better understand the stability of water on Mars. Correcting for the effect of water build-up in the atmosphere and the lower gravity on Mars relative to Earth we observed a factor of almost 30 decrease in evaporation, from 0.88 mm/h at $0°C to 0.04 mm/h at À25.0°C. The results are in excellent agreement with the predictions of Ingersoll's (1970) theoretical treatment, lending support to the theory and our procedures. Thus brine formation will increase the stability of water on Mars not only by extending the liquid temperature range, but also by considerably decreasing the evaporation rate.

[1] We have studied the sublimation of ice buried beneath 200 mm of JSC Mars-1 model regolith under simulated Mars conditions. As expected, even thin layers of regolith cause large decreases in sublimation rate, up to one order of magnitude at 50 mm. When the depth of the regolith was 50 to 200 mm we detected water desorbing from the overlying layers for which we were able to determine a desorption coefficient of 1.45 ± 0.5 Â 10 À3 h À1 . After allowing for the effect of desorption, we found that the diffusion coefficient for water vapor through our regolith is 1.74 ± 0.70 Â 10 À4 m 2 s À1, in excellent agreement with theoretical values. We thus find that a 1-m thick layer of ice buried below a meter of regolith resembling JSC Mars-1 on Mars at 235 K would last $800 years. Citation: Chevrier, V.,

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Results from the Mars Phoenix mission Wet Chemistry Laboratory (WCL) are used to determine the oxidation‐reduction potential (Eh) of the Phoenix WCL Rosy Red sample soil solution. The measured Eh of the Rosy Red sample in the WCL aqueous test solution was 253 ± 6 mV at a pH of 7.7 ± 0.1. Measured solution Eh changes correspond to changes in solution H+ activity, which is controlled mainly by changes in headspace PCO2 and solution CO32−, HCO3−, and CO2 concentrations. If measured at a PCO2 of 8 mbar in water, rather than in WCL test solution, the Eh of the Rosy Red soil solution would be ∼300 mV. The results of laboratory experiments using analog salt mixtures are compatible with the possible presence of low levels (ppm) of metal peroxides or other oxidants and indicate that levels of readily soluble ferrous iron in the soil are below 1 ppm.

2016-12-23T18:52:10