Distribution And Habitability Of (meta)stable Brines On Present-day Mars

Water activity of stable brines over Mars-relevant temperatures and water vapor pressures. Over the range of possible global surface temperature and water vapor pressure on Mars, the resulting water activity of an arbitrary stable solution is shown here in color. The Mars-relevant pairings of surface temperature and water vapor are contained between the cyan lines (see Supplementary Figure 1 for details). The solid blue line is where saturation with respect to ice is achieved (i.e., where relative humidity with respect to ice is RHice = 100%). However, depending on the eutectic temperature of the brine, a metastable liquid and ice solution may be present. Here we show the eutectic temperature of a calcium perchlorate brine (dashed blue line) as an example. Only ice will exist below the eutectic temperature of the brine. Additionally, the Special Region lower limit temperature2 (T = 255 K) is noted by the green dashed line.

Special Regions on Mars are defined as environments able to host liquid water that meets certain temperature and water activity requirements that allow known terrestrial organisms to replicate, and therefore could be habitable.

Such regions would be a concern for planetary protection policies owing to the potential for forward contamination (biological contamination from Earth). Pure liquid water is unstable on the Martian surface, but brines may be present. Experimental work has shown that brines persist beyond their predicted stability region, leading to metastable liquids. Here we show that (meta)stable brines can form and persist from the equator to high latitudes on the surface of Mars for a few percent of the year for up to six consecutive hours, a broader range than previously thought.

However, only the lowest eutectic solutions can form, leading to brines with temperatures of less than 225 K. Our results indicate that (meta)stable brines on the Martian surface and shallow subsurface (a few centimeters deep) are not habitable because their water activities and temperatures fall outside the known tolerances for terrestrial life. Furthermore, (meta)stable brines do not meet the Special Regions requirements, reducing the risk for forward contamination and easing threats related to the exploration of the Martian surface.

Edgard G. Rivera-Valentín, Vincent F. Chevrier, Alejandro Soto, Germán Martínez

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Atmospheric and Oceanic Physics (physics.ao-ph)
Journal reference: Rivera-Valentin, E. G., Chevrier, V. F., Soto, A., Martinez, G. (2020) Distribution and habitability of (meta)stable brines on present-day Mars. Nat Astron 4, 756-761
DOI: 10.1038/s41550-020-1080-9
Cite as: arXiv:2012.00100 [astro-ph.EP] (or arXiv:2012.00100v1 [astro-ph.EP] for this version)
Submission history
From: Edgard Rivera-Valentín
[v1] Mon, 30 Nov 2020 21:23:21 UTC (21,286 KB)

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