Paper Examines Role of Seasonal Frost in Brine Formation on Mars

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Vincent Chevrier

Due to extreme temperatures and the dryness of Mars, it’s thought to be impossible for liquid water to form on the planet’s surface, a critical precondition for habitability. The only hope of finding liquid water appears to be in the form of brines, which are liquids with high concentrations of salts that can freeze at much lower temperatures. But the question of whether brines can even form on Mars has yet to be answered.

Vincent Chevrier, an associate research professor at the University of Arkansas’ Center for Space and Planetary Sciences, has been studying that question for 20 years and now thinks he knows the answer: ‘yes they can.’

His case for the existence of liquid brines on Mars was recently published in Nature Communications Earth and Environment.

Chevrier used meteorological data taken from the Viking 2 landing site on Mars combined with computer modeling to determine that brines can develop for a brief period of time during late winter and early spring from melting frost. This challenges the assumption that Mars is entirely devoid of liquid water on the surface and suggests that similar processes may occur in other frost-bearing regions, particularly in the mid-to-high latitudes.

Data from Viking 2, which landed on Mars in 1976, was used because, Chevrier said, “It was the only mission that clearly observed, identified and characterized frost on Mars.” Melting frost presents the best chance to find liquid brines on Mars, but there’s a catch: frost on Mars tends to sublimate quickly, which means it transitions from a solid to a gas without spending time in a liquid state due to Mars’ unique atmospheric conditions.

But by sifting through the Viking 2 data, combined with data from the Mars Climate Database, Chevrier was able to determine that there was a brief window in late winter and early spring when the conditions were right for the formation of brines. Specifically, there is a period of one Martian month (roughly equivalent to two Earth months) where the conditions were ideal at two points during the day: roughly in the early morning and late afternoon.

There is an abundance of salts on Mars, and Chevrier has long speculated that perchlorates would be the most promising salts for brine formation since they have extremely low eutectic temperatures (which is the melting point of a salt–water mixture). Calcium perchlorate brine solidifies at minus 75 degrees Celsius, while Mars has an average surface temperature of minus 50C at the equator, suggesting there could be a zone where calcium perchlorate brine could stay liquid. 

Modeling based off known data confirmed that twice a day for a month in late winter and early spring there is a perfect window in which calcium perchlorate brines can form because the temperature hovers right around the sweet spot of minus 75C. At other times of day it is either too hot or too cold.

While Chevrier’s findings are not slam-dunk proof of brines, they make a strong case for their existence in small amounts on a recurring basis. Even if there were direct evidence of a calcium perchlorate brine detected by a past or future lander, it would not be in large amounts. Calcium perchlorate is only about 1% of the Martian regolith, and the frost that does form on Mars is extremely thin – far less than a millimeter thick. So it is unlikely to generate much water, certainly not enough to support human life.

But it doesn’t mean the planet couldn’t have supported life adapted to a much colder, drier planet.

Either way, Chevrier is encouraged to find that brines would form under established conditions and looks forward to further confirmation. He notes in the conclusion of his paper: “The strong correlation between brine formation and seasonal frost cycles highlights specific periods when transient water activity is most likely, which could guide the planning of future astrobiological investigations.

“Robotic landers equipped with in situ hygrometers [for measuring moisture content in air] and chemical sensors could target these seasonal windows to directly detect brine formation and constrain the timescales over which these liquids persist.”

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