Mars is located at the extreme limits of brine stability; and only a combination of the most favorable environmental conditions and lowest eutectic temperature salts allows for brines to be at least temporarily stable on the Martian surface, according to a new study published in the Proceedings of the National Academy of Sciences.
This image of an impact crater in the Sirenum Fossae region of Mars was taken by NASA’s Mars Reconnaissance Orbiter on March 30, 2015. The crater is approximately 3,300 feet (1-km) wide and appears relatively recent as it has a sharp rim and well-preserved ejecta. The steep inner slopes are carved by gullies and include possible recurring slope lineae on the equator-facing slopes. Image credit: NASA / JPL / University of Arizona / Alfred McEwen.
Liquid water is a critical precondition for a habitable planet. Yet the combination of low temperature, atmospheric pressure and water vapor pressure on Mars means any liquid water found there would likely freeze, boil or evaporate immediately, making its presence unlikely.
Yet palnetary researchers continue to make the case for the presence of liquid water on the Red Planet.
Of particular interest has been the discovery of seasonal dark streaks called the recurring slope lineae.
These features appear in several locations on Mars when temperatures are above minus 23 degrees Celsius (minus 10 degrees Fahrenheit), and disappear at colder times.
They often have been described as possibly related to liquid water.
The new study throws cold water on the notion that we are likely find liquid water on Mars in recurring slope lineae, permafrost or brines anytime soon.
“A closer look at RSLs indicates their behavior is consistent with sand and dust flows with no water needed to create them,” said lead author Dr. Vincent Chevrier, a researcher at the University of Arkansas.
Other researchers think that brines, which are solutions with a high concentration of salts, such as Earth’s oceans, may hold the key to finding liquid water on Mars.
Brines can freeze at much lower temperatures, and there is an abundance of salts on Mars.
Of those salts, perchlorates would seem to be the most promising, since they have extremely low eutectic temperatures (which is when the melting point of a mixture is lower than any single ingredient).
For instance, a calcium perchlorate brine solidifies at minus 75 degrees Celsius (14 degrees Fahrenheit), while Mars has an average surface temperature of minus 50 degrees Celsius (minus 58 degrees Fahrenheit) at the equator, theoretically suggesting there could be a zone where calcium perchlorate brine could stay liquid, particularly in the subsurface.
Dr. Chevrier and his colleague, Dr. Rachel Slank from the Lunar and Planetary Institute, then examined all of the arguments for and against brines potentially forming stable liquids.
“The various limiting factors, including the relatively low amounts of the most promising salts, water vapor pressure and ice location strongly limit the abundances of brines on the surface or shallow subsurface,” they said.
“And even if brines did form, they would remain highly un-habitable by terrestrial standards.”
“Despite these drawbacks and limitations, there is always the possibility that Martian life adapted to those brines and some terrestrial organisms could survive in them, which is a consideration for planetary protection because life on Mars might exist today in that case.”
“Hence, detecting brines in situ remains a major objective of the exploration of the Red Planet.”
Moving forward, the authors suggest the next hurdles will be improving the instruments needed to detect small amounts of brines, doing a better job of identifying the best places to look for them, and being able to conduct more laboratory measurements under Martian conditions.
“Despite our best efforts to prove otherwise, Mars still remains a cold, dry and utterly unhabitable desert,” Dr. Chevrier said.
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Vincent F. Chevrier & Rachel A. Slank. 2024. The elusive nature of Martian liquid brines. PNAS 121 (52): e2321067121; doi: 10.1073/pnas.2321067121
