Stability against freezing of aqueous solutions on early Mars

Abstract

Many features of the Martian landscape are thought to have been formed by liquid water flow and water-related mineralogies on the surface of Mars are widespread and abundant. Several lines of evidence, however, suggest that Mars has been cold with mean global temperatures well below the freezing point of pure water. Martian climate modellers considering a combination of greenhouse gases at a range of partial pressures find it challenging to simulate global mean Martian surface temperatures above 273 K, and local thermal sources cannot account for the widespread distribution of hydrated and evaporitic minerals throughout the Martian landscape. Solutes could depress the melting point of water in a frozen Martian environment, providing a plausible solution to the early Mars climate paradox. Here we model the freezing and evaporation processes of Martian fluids with a composition resulting from the weathering of basalts, as reflected in the chemical compositions at Mars landing sites. Our results show that a significant fraction of weathering fluids loaded with Si, Fe, S, Mg, Ca, Cl, Na, K and Al remain in the liquid state at temperatures well below 273 K. We tested our model by analysing the mineralogies yielded by the evolution of the solutions: the resulting mineral assemblages are analogous to those actually identified on the Martian surface. This stability against freezing of Martian fluids can explain saline liquid water activity on the surface of Mars at mean global temperatures well below 273 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.…”

Mineral paragenesis on Mars: The roles of reactive surface area and diffusion

“…Our work, which considers relatively diluted solutions, is also a departure from previous studies that focused on evaporite mineralogy and brine evolution [e.g., Tosca et al ., 2005; Fairén et al ., 2009]. In the same vein, some previous investigations of Mars evaporitic precipitation using laboratory analogues had different starting and boundary conditions; for example, Moore et al .…”

“…In our models the system is open to the atmosphere, and therefore, the loss of water through evaporation is regarded as the main factor inducing supersaturation (as multiple indicators for evaporation have been reported on Mars) [see e.g., Squyres et al ., 2004; Sears and Moore , 2005; Tosca et al ., 2005; Osterloo et al ., 2008; Fairén et al ., 2009; Arvidson et al ., 2014; Toner et al ., 2015; Schwenzer et al ., 2016; Hurowitz et al ., 2017]; and because the system is likewise open at the water‐sediment interface, reactive transport was also considered (Figure 1). In addition, we considered that dissolved volcanic gases (H 2 O, CO 2 , Cl 2 , and SO 2 ) [e.g., Halevy and Head , 2014] are the main anion source to the aqueous solutions, as well as drivers of redox processes and alkalinity, modifying the dissolution kinetics of primary minerals, which, in turn, are largely dependent of pH, temperature, and the reactive surface.…”

“…Indeed, if the initial concentration at the surface and/or the selected time interval is greater, the length of the diffusion gradient will be also greater, but the main rationale and conclusions will remain the same. That is the actual case for the surface of Mars, where the final diffusion processes seem to have been acting on very concentrated ponds generated after vigorous processes of evaporation and for extended time periods [e.g., Tosca et al ., 2005; Tosca and McLennan , 2006; Fairén et al ., 2009; Fairén , 2010]. But even a short duration of the diffusion gradient would have not precluded the existence of sediment advection or erosion.…”

“…In the case of evaporation, supersaturation is induced by removing the solvent at specific rates. Cooling selectively affects the supersaturation of secondary phases as a function of their solubility dependence on temperature and, upon ice formation, by removing reactive water from the system [ Fairén et al ., 2009]. From the point of view of a liquid‐vapor phase transition, the rate of evaporation in mass terms can be expressed as $\frac{d{M}_w}{\mathit{dt}}=\frac{S}{V}\bullet E_v\left(1-\frac{p}{p_0}\right)$where M w (mol/kg water ) stands for the moles of H 2 O evaporated; S / V is the surface‐to‐volume ratio (m 2 /dm 3 ), which depends on the particular geometry and size of the basin; E v is the specific rate constant (mol/m 2  s); and the saturation term (1 −  p / p 0 ) represents the distance from the equilibrium, where p is the atmospheric pressure and p 0 the saturation pressure of H 2 O (g) at that temperature [ Gibbs , 1878; Becker and Doring , 1935; Appelo and Postma , 1993].…”