Water Uptake and Release of Nitrate Salt Mixtures of Relevance to the Atacama Desert and Mars

Abstract

Understanding the behavior of water in extreme environments is crucial in assessing its habitability potential. This study investigates the water uptake and release of nitrate and nitrate salt mixtures under Martian and Atacama Desert-like conditions. The Atacama serves as a Mars analogue due to its hyper-arid climate and shared salt composition. This study determines deliquescence and efflorescence relative humidities (DRH and ERH, respectively) for pure magnesium nitrate hexahydrate (Mg(NO 3 ) 2 •6H 2 O) as a function of temperature and perchlorate/nitrate mixtures at various ratios at 248 K. Finally, the effects of magnesium sulfate anhydrous (MgSO 4 ) on Mg(NO 3 ) 2 •6H 2 O are also investigated. Pure Mg(NO 3 ) 2 DRH varied with temperature (80% RH at 223 K to 63% at 268 K), while ERH remained constant at 24% RH across temperatures. When mixed with perchlorate, the DRH values were lowered, reaching values close to 40% RH. The less deliquescent MgSO 4 had minimal impact on Mg(NO 3 ) 2 water uptake when mixed in equimolar ratios. The laboratory DRH conditions did not align with conditions found on Mars, indicating that the salt mixtures are unlikely to deliquesce under Martian conditions found at Gale Crater. However, the warmer temperatures of the Atacama may favor water uptake. Therefore, the DRH and ERH data were applied to two sites in the Atacama. Conditions in the Atacama support water uptake by nitrates in the fall/winter seasons, allowing for the possibility of metastable brines persisting for extended periods. Thus, although nitrates may enhance habitability in the Atacama, they may play less of a role in habitability on Mars.