Experimental Study of the Formation of Liquid Saline Water on Mars

Abstract

Liquid water is essential for life on Earth. To search for conditions that could support life on Mars, we need to understand if liquid water can be present, even temporarily, at its surface and shallow subsurface. Brine is a highly concentrated saline solution that can exist in the liquid state at temperatures well below the freezing point of pure water, such as those on Mars. Water ice and perchlorate salts capable of melting this ice and producing liquid solutions have been discovered in the Martian regolith from polar to equatorial regions. In addition to melting of ice, perchlorate salts may also form a brine on Mars by deliquescence, absorbing water vapor when the relative humidity is above a certain threshold. Evidence for brine at the surface and in the shallow subsurface of Mars has been reported in the last few years, such as in its polar region at the Phoenix landing site and at mid- and low-latitudes at surface features called Recurring Slope Lineae and Slope Streaks. We have designed and developed the Michigan Mars Environmental Chamber to perform experiments at Martian conditions. We study brine formation and assess whether deliquescence and/or melting are consistent its formation, and to calibrate a variety of in-situ sensors at Martian conditions. Our chamber is equipped with a Raman spectrometer, which we use to provide reference spectra of various mixing states of liquid water, ice and perchlorate salt during brine formation. We show that perchlorate brines can be identified by analyzing the decomposed Raman spectra of the investigated samples. This serves as an important reference for future in-situ Raman spectrometers on Mars and can aid in the detection of brine formation. We find that when water vapor is the only source of water, bulk deliquescence of perchlorates is not rapid enough to occur during the short periods of the day when the environmental conditions are favorable. However, when the salts are in contact with water ice, liquid brine forms more quickly, indicating that aqueous solutions could form temporarily where salts and ice coexist in the Martian regolith. We further simulate full diurnal cycles of temperature and atmospheric water vapor at the Phoenix landing site of mixtures of ice and salt such as encountered at the Phoenix landing site and show that brine can form and stay liquid for most of the diurnal cycle. This is predicted to occur seasonally in areas of the polar region where the temperature exceeds the eutectic value and frost or snow is deposited on saline soils, or where water ice and salts coexist in the shallow subsurface. Finally, we show results of the recalibration of the relative humidity sensor of the Phoenix lander. We have use a spare engineering unit to recalibrate the sensor in the full range of Phoenix landing site conditions. This provides processed relative humidity data at Martian polar conditions to enhance our understanding of the hydrological cycle at the Phoenix landing site, which is important for potential brine formation. We conclude that deliquescence is unlikely to form temporarily habitable conditions on Mars. The most likely places to do so are regions where subsurface water ice and perchlorate salts coexists. This is most likely at the north and south polar region and parts of the mid- to low-latitudes bearing water ice.