An Experimental Study on Liquid Brine Formation at Gale Crater
Abstract:Here we present experiments conducted in the Michigan Mars Environmental Chamber  to test the possibility of the formation of liquid brines from calcium perchlorate salts at Gale Crater. We tested bulk samples of Ca(ClO4)2 using Raman spectroscopy to observe spectral changes in the perchlorate band (930-990 cm-1) and the O-H vibrational stretching band (3000-3700 cm-1) of the samples. Our results indicate that brine formation by deliquescence (absorption of water vapor from the atmosphere) does not occur at Gale Crater within the time (< 2 hours)  when the ground temperature is above the calcium perchlorate’s eutectic temperature (199 K)  and the relative humidity is above the deliquescence threshold (26%) . On the contrary, we show that bulk liquid brine of calcium perchlorate salt forms within minutes if the salt is in direct contact with water ice. However, water ice is not expected in the shallow (tens of cm) subsurface of Gale Crater  and, on the sols during which frost events might have occurred at the surface, the calculated frost point (~190 K)  was below the eutectic temperature of the perchlorate.
Liquid water is one of the necessary ingredients for the development of life as we know it. The behavior of various liquid states of H2O such as liquid brine, undercooled liquid interfacial water, subsurface melt water and ground water  needs to be understood in order to address the potential habitability of Mars for microbes and future human exploration. These results are relevant because they help in constraining the possible mechanisms of the formation of liquid water at Gale.
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