December 11, 2020 | David F. Coppedge

Water Quickly Evaporates on Mars

Scientists overestimate how much water could exist on Mars. New studies show it can’t last even a day.

Many planetary scientists and astrobiologists have assumed that water is plentiful on Mars. That’s where they like to imagine life evolving. But if liquid water existed on Mars, it would most likely take the form of brines (salty water), because brines are not as quick to evaporate as pure water would in the thin atmosphere. How long, actually, could water last? Nothing like a little empiricism to find out.

Water on Mars Not as Widespread as Previously Thought, Study Finds (University of Arkansas News). At UA’s Center for Space and Planetary Sciences, scientists used their Mars simulation chamber to measure the survivability of Martian brines. They concluded that “Water on Mars, in the form of brines, may not be as widespread as previously thought.

Researchers combined data on brine evaporation rates, collected through experiments at the center’s Mars simulation chamber, with a global weather circulation model of the planet to create planetwide maps of where brines are most likely to be found.

Previous studies looked at one phase change at a time. In this study, the researchers considered all the phase changes at the same time: freezing, boiling and evaporation. Lead author of the paper in The Planetary Science Journal, Vincent Chevrier, explains what they found and why their method was more accurate:

“It is looking at all the properties at the same time, instead of one at a time,” said Chevrier. “Then we build maps taking into account all those processes simultaneously.”

Doing so indicates that previous studies may have overestimated how long brines remain on the surface in the cold, thin and arid Martian atmosphere, Chevrier said. “The most important conclusion is that if you do not take all these processes together, you always overestimate the stability of brines. That is the reality of the situation.”

How long, then, can a brine survive on the surface? Just half a day! (A Martian day, called a sol, is just a bit longer than an Earth day: 24 hours 39 minutes 35.244 seconds).

In the best-case scenario, brines could be present for up to 12 hours per day. “Nowhere is any brine stable for an entire day on Mars,” he said.

The source paper gives the details:

We find that equatorial regions typically have temperatures too high for stable brines, while high latitudes are susceptible to permanent freezing. In the subsurface, this trend is reversed, and equatorial regions are more favorable to brine stability, but only for the lowest water activities (and lowest eutectic temperatures). At locations where brines may be stable, we find that their lifetimes can be characterized by two regimes. Above a water activity of ~0.6, brine duration is dominated by evaporation, lasting at most a few minutes per sol. Below a water activity of 0.6, brine duration is bound by freezing or boiling; such brines are potentially stable for up to several consecutive hours per sol. Our work suggests that brines should not be expected near or on the Martian surface, except for low eutectic water activity salts such as calcium or magnesium perchlorate or chlorate, and their (meta)stability on the surface would require contact with atmospheric water vapor or local ice deposits.

Eutectic refers to the ingredient in the mixture with the lowest melting point. Perchlorate solids were found to be pervasive on the Martian surface, initially by the Phoenix lander (4 August 2008), a fact widely acknowledged a decade later (26 July 2018). These highly active compounds would likely have been very toxic to any life trying to survive, including astronauts.

Finding them [brines] has implications for where scientists will look for past or present life on Mars and where humans who eventually travel to the planet could look for water.

Salty water is also disastrous for cell membranes (18 Feb 2008). In vain, therefore, do scientists hope that finding “water on Mars” is going to help the search for life.

Anyone wish to predict what the new Mars Perseverance rover will find on Feb 18, 2021? One of its mission goals is to search for “signs of ancient microbial life.” For hints, you can research how habitable previous missions found the surface to be. Start here: 5 May 2018, and notice the Update near the end.





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