•The deliquescence of salts from Don Juan Pond was studied with Raman microscopy.•The low T deliquescence behavior was similar to that of the major component, CaCl2.•Brines may form via deliquescence frequently during southern summer and autumn.•Recrystallization is hindered; salty soils may stay wet for months, day and night.•Observed water track darkening near DJP could be due to salts absorbing H2O vapor.The observed darkening of water tracks near Don Juan Pond (DJP) as well as the formation of wet patches elsewhere in the McMurdo Dry Valleys is attributed at least partially to deliquescence, a process by which salts absorb atmospheric water vapor and form brine, coupled with liquid-phase growth when the atmospheric relative humidity exceeds the water activity. Here we perform laboratory experiments to investigate the temperature and relative humidity conditions necessary for deliquescence to occur in calcium chloride-rich sediments collected from the DJP watershed. We use a Raman microscope equipped with an environmental cell to study both deliquescence and efflorescence (recrystallization) of the soluble salt component of DJP soils between −30 and +15 °C. In this temperature range, we find that the soluble salt component of the DJP sediments begins to deliquesce between 19 and 46% RH, slightly higher than the deliquescence relative humidity of the primary pure component, calcium chloride. We find a limited hysteresis between deliquescence and efflorescence, but much greater supersaturation of the salt brine can occur at temperatures above 0 °C. The relative humidity conditions were varied either slowly (over ∼8 h) to observe near-equilibrium phases or rapidly (over <1 h) to better mimic Antarctic conditions and no differences in deliquescence relative humidity or efflorescence relative humidity were noted. The results of this work can help predict when deliquescence could be actively occurring in the soils near Don Juan Pond and explain darkening of the salt pan after a high humidity period. In tandem with field data, our experimental results suggest that brines can be generated near Don Juan Pond via deliquescence frequently during the southern summer and autumn. Additionally, the soluble salts may persist in the aqueous phase continuously for several months during the southern summer. This work also suggests that salt deliquescence could be impacting the year-round hydrological cycle of the DJP watershed. Steep-sloped water tracks found near DJP have been suggested as a terrestrial analog for recurring slope lineae on Mars, for which salt deliquescence is a proposed formation mechanism. Therefore, understanding the formation of deliquescent brines in a hyper-arid region on Earth may have relevance to Mars.

•CaCl2 is very deliquescent (i.e., can absorb water vapor and form a liquid brine).•This salt can deliquesce at ~12 to 20% RH at temperatures between 273 and 223 K.•Single digit RH values are needed to recrystallize CaCl2 brine.•CaCl2 may help form liquid water that could cause slope streaks on Mars and Earth.There is significant interest in the potential existence of even small amounts of liquid brine on current Mars. It has been proposed that aqueous solutions could form on Mars via the deliquescence of hygroscopic salts in contact with atmospheric water vapor, and these hygroscopic salts have recently been detected in recurring slope linae (RSL). While past work has largely focused on perchlorate species, another Mars-relevant salt that has a low eutectic temperature and may be deliquescent is calcium chloride, CaCl2. This salt may be linked to RSL formation on Mars, and deliquescence of CaCl2 is also known to be responsible for the only terrestrial RSL analog features known thus far: water tracks in the McMurdo Dry Valleys. Here we use Raman microscopy to monitor the low-temperature (223−273 K) deliquescence (solid to aqueous phase transition) and efflorescence (aqueous to solid phase transition) of two hydration states of CaCl2, the dihydrate and the hexahydrate. We find the deliquescence relative humidity (DRH) decreases with decreasing hydration state and with increasing temperature. Average DRH values over the temperature range studied are 15.8±3.5% RH for the dihydrate and 63.3±12.5% RH for the hexahydrate, making this salt at least as deliquescent as many perchlorate salts. A remarkable property of CaCl2 is its ability to persist as metastable, supersaturated brine. Once an aqueous solution was formed, efflorescence (recrystallization) of the liquid did not occur until single-digit RH values were reached (3.9±2.4% RH on average). We show that temperature and relative humidity conditions in the martian subsurface are sufficient to allow deliquescence of CaCl2, and the resulting brines may persist for over half of a martian sol. Therefore, this salt could play a role in RSL formation, the martian water cycle, and have implications for the potential habitability of Mars.