Evidence for deliquescence of perchlorate salts has been discovered in the Martian polar region while possible brine flows have been observed in the equatorial region. This appears to contradict the idea that bulk deliquescence is too slow to occur during the short periods of the Martian diurnal cycle during which conditions are favorable for it. We conduct laboratory experiments to study the formation of liquid brines at Mars environmental conditions. 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 during which the temperature is above the salts' eutectic value, and the humidity is above the salts' deliquescence value. However, when the salts are in contact with water ice, liquid brine forms in minutes, indicating that aqueous solutions could form temporarily where salts and ice coexist on the Martian surface and in the shallow subsurface.Key Points class="unordered" style="list-style-type:disc">The formation of brines at Martian conditions was studied experimentallyBulk deliquescence from water vapor is too slow to occur diurnally on MarsBrines form in minutes when salts are placed in direct contact with ice class="kwd-title">Keywords: brines, deliquescence, Mars, Raman spectroscopy, water on Mars class="head no_bottom_margin" id="__sec2title">1. IntroductionOur view of Mars has changed dramatically in the past two decades, from an inhospitable cold desert to a potentially habitable planet. Water ice was discovered in the shallow subsurface of areas ranging from polar latitudes to midlatitudes [; ; ; ; ]; salts such as Ca(ClO4)2, Mg(ClO4)2, and NaClO4, capable of deliquescing and forming aqueous solutions at Martian temperatures were discovered in the polar and equatorial regions [; ]. The idea that deliquescence (defined as the dissolution of a salt by the absorption of water vapor []) is a slow process not capable of producing bulk aqueous solutions during the few hours of the diurnal cycle in which conditions are favorable for it is well established [] because Mars is extremely cold and dry [; ]. This appears to contradict the discovery of observational evidence for deliquescence in Mars' polar region [] and of possible flows of liquid brines in the equatorial region [].In an effort to shed light on this issue, we use Raman scattering spectroscopy [] to study the formation of liquid brines from bulk amounts of salt at Mars environmental conditions. Here we define “bulk” as a macroscopic collection of salt grains. First, we study the formation of liquid brines when water vapor is the only source of water (bulk deliquescence). Then, we investigate the formation of liquid brines when the salts are placed in direct contact with water ice like that observed in Mars' polar region [; ; href="#b24" rid="b24" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_438279110">Whiteway et al., 2009]. Our results have important implications for the understanding of habitability because liquid water is essential for life as we know it, and halophilic terrestrial bacteria thrive in brines [href="#b15" rid="b15" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_438279109">Mikucki et al., 2009; href="#b1" rid="b1" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_438279103">Boetius and Joye, 2009].
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