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Sandy, Briny Water on Mars Has a Better Chance of Remaining Liquid

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image of the "dragon scale" texture on the surface of Mars
The "dragon scale" texture seen in this Mars Reconnaissance Orbiter image of Mars's surface is the result of water interacting with bedrock, forming clay-containing rock. [NASA/JPL-Caltech/UArizona]

New laboratory experiments suggest that salty water mixed with Martian surface material can remain a liquid under colder and drier conditions than water alone. This means that liquid water might be found over a larger area of Mars’s surface than previously thought, as well as throughout more of the Martian year, with important implications for habitability and exploration.

The Search for Water on Mars

dark streaks on sandy slopes on Mars

Warm temperatures on Mars are associated with the appearance of dark streaks on sloping terrain. On Earth, these streaks are caused by water, but on Mars they may be caused by shifting sand grains instead. [NASA/JPL-Caltech/UA/USGS]

Mars’s sinuous riverbeds and dry lake basins tell a tale of a planet once awash with water, but what about today? Proving the presence of liquid water on Mars’s surface has been tricky, and claims of evidence for modern-day liquid water often find themselves rebutted; for example, the dark streaks thought to indicate subsurface water seeping through the sand were reinterpreted as sand sliding down steep slopes (say that five times fast!).

But the search continues, with evidence mounting that liquid water might exist in the form of brine: a concentrated mixture of water and salt. Martian brine can form in several ways including by water vapor collecting on the surface of salt crystals. In the lab, researchers have tested the conditions under which brine remains a liquid, rather than freezing or evaporating in Mars’s cold, dry climate. But brine on Mars doesn’t exist in isolation. Instead, it’s muddled together with regolith: the loose mixture of rocks, sand, and dust that coats the planet’s surface. Could the mashup of these two materials help water remain a liquid on Mars’s surface?

photograph of Martian soil

An image of Martian soil scooped up by the Phoenix Mars Lander. For this study, the team used simulated Martian soil made from volcanic rocks in the Mojave Desert. [NASA/JPL-Caltech/University of Arizona/Max Planck Institute]

Throwing Regolith into the Mix

To explore this question, Andrew Shumway (University of Washington) and collaborators measured the properties of regolith–brine mixtures in a lab. Since we don’t yet have actual Martian regolith to experiment on, Shumway’s team used a simulated regolith that was originally developed to help NASA scientists test the navigation and sample-collecting skills of the Mars rovers. For their Martian brine, the team swirled together water and a salt called magnesium perchlorate (magnesium and perchlorate are common components of Mars’s surface material).

The team measured two key factors for each of their regolith–brine samples: 1) the freezing point, which partly determines where on the planet’s surface the mixture can remain a liquid, and 2) the amount of water that’s available to participate in chemical reactions and other processes important for life.

Briny Findings

Plot showing the melting points of samples with various concentrations

Melting temperature of frozen regolith–brine samples. Samples with a lower melting temperature also freeze at lower temperatures, making them remain liquid under colder conditions. Click to enlarge. [Shumway et al. 2023]

Shumway’s team found that mixtures of brine and regolith have more water available and freeze at a lower temperature than brine alone, and water can persist when the ambient air is drier, as well. This means that liquid water might be found across more of the Martian surface and during more of the Martian year than previously thought. While this is exciting news for the prospect of finding life on Mars, it also means that we’ll need to be even more careful not to spread earthly microbes to the Martian surface, as water helps to support Earth life as well!

Citation

“Regolith Inhibits Salt and Ice Crystallization in Mg(ClO4)2 Brine, Implying More Persistent and Potentially Habitable Brines on Mars,” Andrew O. Shumway et al 2023 Planet. Sci. J. 4 143. doi:10.3847/PSJ/ace891