Title: Distribution and habitability of (meta)stable brines on present-day Mars
Authors: Edgard G. Rivera-Valentín, Vincent F. Chevrier, Alejandro Soto, and Germán Martínez (Rivera-Valentín and Chevrier contributed equally)
Contributing Authors’ Institutions: Lunar and Planetary Institute, Universities Space Research Association, Houston, TX; Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, AR.
Status: Published in Nature Astronomy, closed access
A fresh snowfall can be a beautiful sight. A week-old, dirty, salty slush puddle, not so much. Beauty is relative, they tell me, and so is a brine, because that same dirty, salty slush puddle on Mars could be the key to life on the Red Planet.
An exceptional recent Astrobite outlined the latest in the saga of water discovery on Mars: the 2018 detection of liquid water deep under the South Pole was confirmed by the MARSIS instrument aboard the Mars Express spacecraft. Armed with new, high-resolution maps of the water, the authors of that paper concluded brines were responsible for liquid water formation, not recent magma flows. The authors of today’s paper step things up a level—literally—to see if stable brines on the surface of Mars could form and support life.
Quick brine recipe for a weeknight
After a long day in grad school (that’s all of them) there’s nothing like a quick recipe to make dinner prep easy. That’s why I make my famous Martian surface brine the old fashioned way, the way Mars has for millions of years. The only ingredients you need are ice and salt, which you can procure in abundance from the Martian surface. Specifically, magnesium perchlorate and calcium perchlorate will produce the best results. Preheat the ice to the eutectic temperature of the salt, which is the lowest possible temperature any solution of salt and ice will melt at. For magnesium perchlorate, this is about 206 K, which is not hard to achieve on the surface of Mars. The hard part is getting the salt to deliquesce, which is when salt absorbs humidity from the air and spontaneously dissolves into water. You’ll need a large pan.
Unfortunately for brines on Mars, their rapid formation is also their doom. If the temperature drops below the eutectic temperature, the brine will freeze out and the salt will crystalize. If the temperature rises to about 270 K, the brine will evaporate. If the surface pressure drops considerably, the brine will actually boil, which is what happens to liquids in the vacuum of space before they freeze.
High resolution brine solution
The authors of today’s paper use a general circulation model (GCM), which is a complex simulation of atmospheric conditions. GCMs on Mars are checked against the direct measurements from rovers and landers to ensure accuracy. For each surface grid point in the model, the temperature and relative humidity are calculated. If the temperature is above the eutectic temperature and the humidity is above the deliquescence threshold for each salt, that area forms a brine. The model is run for six Martian years, recording conditions and stability of each brine.
The authors find surface brines typically survive for only two hours at a time, but in the high northern latitudes, calcium perchlorate brines can stay metastable for up to six hours a day, for a few Martian weeks per year. A brine is considered metastable if it’s supersaturated, meaning it can freeze or evaporate at any moment. Figure 1 shows the locations on Mars and percentages of the Martian year where calcium perchlorate brines are stable or metastable.
Life of brine
These authors show that despite the odds, brines can persist with some regularity on large portions of the surface of Mars. The next natural question is: can those brines harbor life? NASA cares deeply about this question because planetary protection efforts do not allow landings in so-called “Special Regions” where contamination from Earth microbes might be possible. To be a Special Region, a brine must pass two conditions:
- Temperature of at least 255 K.
- Water activity of at least 0.6.
Water activity is not the moisture of a substance but rather the amount of “free” water available. Water is considered available if it’s loosely bound, in a microscopic sense, to the substance. A greater water activity means more moisture is available for microorganisms.
Figure 2 shows the limits of temperature, partial pressure of water vapor (amount of water in the air), and water activity on the surface of Mars. These results were obtained from the GCM. Mars can sustain temperatures above 255 K or water activity above 0.6 near the freezing point of the brine, but not both. Any brine that had both a high water activity and high temperature would evaporate quickly because of the low water vapor pressure on Mars.
On the bright side, the results of this paper indicate that future missions to Mars would be very unlikely to contaminate the surface with microbes from Earth.
There may not be life in a Martian brine but there can still be more brine in your life. The Lunar Planetary Institute recently launched its Brines Across the Solar System initiative to connect brine research on Mars, Ceres, Enceladus, meteorites, and other bodies. BrinesCon is a new conference to be held for the next three years. 2021 will feature modern brines, 2022 ancient brines, and 2023 future brines.
Astrobite edited by Sunayana Bhargava.
Cover image credit: NASA/JPL