Hygroscopic Salts and the Potential for Life on Mars

Abstract

Hygroscopic salts have been detected in soils in the northern latitudes of Mars, and widespread chloride-bearing evaporitic deposits have been detected in the southern highlands. The deliquescence of hygroscopic minerals such as chloride salts could provide a local and transient source of liquid water that would be available for microorganisms on the surface. This is known to occur in the Atacama Desert, where massive halite evaporites have become a habitat for photosynthetic and heterotrophic microorganisms that take advantage of the deliquescence of the salt at certain relative humidity (RH) levels. We modeled the climate conditions (RH and temperature) in a region on Mars with chloride-bearing evaporites, and modeled the evolution of the water activity (a(w)) of the deliquescence solutions of three possible chloride salts (sodium chloride, calcium chloride, and magnesium chloride) as a function of temperature. We also studied the water absorption properties of the same salts as a function of RH. Our climate model results show that the RH in the region with chloride-bearing deposits on Mars often reaches the deliquescence points of all three salts, and the temperature reaches levels above their eutectic points seasonally, in the course of a martian year. The a(w) of the deliquescence solutions increases with decreasing temperature due mainly to the precipitation of unstable phases, which removes ions from the solution. The deliquescence of sodium chloride results in transient solutions with a(w) compatible with growth of terrestrial microorganisms down to 252 K, whereas for calcium chloride and magnesium chloride it results in solutions with a(w) below the known limits for growth at all temperatures. However, taking the limits of a(w) used to define special regions on Mars, the deliquescence of calcium chloride deposits would allow for the propagation of terrestrial microorganisms at temperatures between 265 and 253 K, and for metabolic activity (no growth) at temperatures between 253 and 233 K.

The Modern Near-Surface Martian Climate: A Review of In-situ Meteorological Data from Viking to Curiosity

“…Two mechanisms have been suggested for brine formation on Mars: its formation by salts absorbing atmospheric water vapor (deliquescence) when the relative humidity exceeds a threshold value known as the deliquescence relative humidity and the temperature is above the salts' eutectic value (Clark 1978;Rennó et al 2009;Davila et al 2010;Gough et al 2011;Nuding et al 2015;Martín-Torres et al 2015;Nikolakakos and Whiteway 2015), and its formation by salts melting water ice when the temperature exceeds the eutectic value of salts in contact with water ice (Brass 1980;Clark and Van Hart 1981;Fairén et al 2009;. Perchlorate salts, in particular Ca(ClO 4 ) 2 , Mg(ClO 4 ) 2 and Na(ClO 4 ) salts, are extremely relevant for brine formation via deliquescence and melting because they have low eutectic temperatures and were found in polar and equatorial regions Glavin et al 2013), suggesting that they are distributed globally.…”

Carbon monoxide as a metabolic energy source for extremely halophilic microbes: Implications for microbial activity in Mars regolith

“…observed that intact saline soil cores with surface water potentials of about −41 MPa consumed atmospheric CO, and that surface soil (0-2 cm) from this site oxidized exogenous CO, as did saline soils, sediments, and crusts from three other systems with water potentials consistent with brines that might exist on Mars (33,34). These systems included salinized volcanic ash and sand from Hawai'i Island (water potentials from about −29 to −117 MPa), mixed gypsum and halite deposits from solar salterns in the Atacama Desert, Chile (water potentials of about −8 to −12 MPa), and salt-encrusted sediments from the Bonneville Salt Flats, Utah (BSF; water potentials of about −42 MPa).…”

A Microbial Oasis in the Hypersaline Atacama Subsurface Discovered by a Life Detector Chip: Implications for the Search for Life on Mars

“…Zorzano et al (2009) demonstrated that small amounts of sodium perchlorate spontaneously absorb moisture and melt into a liquid solution under martian atmospheric conditions. In addition, Davila et al (2010) reported the deliquescence capacity of hygroscopic salts and their hypothetical capacity to support microbial growth or metabolic activity under simulated martian conditions. Our AtacaMars field campaign demonstrates that subsurface hygroscopic salt environments may be true oasis for microbial life and further supports the possibility of a subsurface martian habitat based on the deliquescence capacity of similar hygroscopic salts.…”