How Martian salts can make brine from air
A salt does not need a puddle to get wet. Some salts absorb water vapor from the surrounding air; if the air is humid enough, the salt can take up so much moisture that it dissolves into a thin film of liquid. This process is called deliquescence. On Mars, it offers a possible way to make brine from atmospheric moisture and salty soil, rather than from melting a layer of ice.
Why salt changes water’s freezing point
In pure water, molecules arrange themselves into an orderly ice crystal as they cool. Dissolved salt gets in the way of that arrangement, so the solution must become colder before it freezes. The effect depends on how concentrated the brine is: adding more dissolved salt generally lowers its freezing point further.
That does not mean every salty patch stays liquid whenever temperatures fall below 0°C (32°F). A brine’s behavior depends on its particular salts, its concentration, and the temperature and humidity around it. If the air is too dry, the water can escape back into the atmosphere; if the brine becomes too dilute or conditions change, it may freeze.
A narrow window on the Martian surface
Mars’s low air pressure creates another obstacle. Liquid water exposed at the surface can evaporate quickly, and in some conditions can boil, so a brine would have to form and persist within a limited range of local conditions. A salty grain might briefly acquire a damp coating without producing a visible pool.
That distinction matters when interpreting signs of water on Mars. The chemistry makes salty liquid possible under some conditions, but it does not show that a particular slope or patch of soil contains brine. Whether these salts actually form liquid water on the Martian surface remains unsettled—and any such water may be a fleeting film, not a stream.
