The unique combination of low temperatures, atmospheric pressure, and water vapor pressure on Mars indicates that any liquid water present would likely freeze, boil, or evaporate almost instantly, making it highly improbable.
Over a century ago, astronomer Percival Lowell advocated for the existence of canals on Mars, which he believed were built to transport water from the ice caps to the dryer areas. This premise implied that there must be Martians to construct these canals.
Although better telescopes later disproved Lowell’s theory, the question of whether liquid water exists on Mars continues to intrigue scientists. Liquid water is essential for a planet to be considered habitable. However, due to the low temperature, atmospheric pressure, and water vapor pressure on Mars, the existence of liquid water is unlikely since it would likely freeze, boil, or evaporate quickly.
Nonetheless, researchers are still advocating for the potential presence of liquid water on Mars.
One significant discovery is the “recurring slope lineae” (RSL), which are dark, streaky formations observed on steep slopes in certain Mars regions. RSLs exhibit seasonal behavior; they appear in warmer months and fade when it’s cooler, which could suggest the activity of liquid water. Additionally, distinct striped and polygonal patterns in Martian permafrost lend credence to this idea, possibly indicating thermal cycles. Researchers have also proposed various types of possible liquid brines contributing to this discussion.
However, a recent paper published in the Proceedings of the National Academy of Sciences of the United States of America questions the likelihood of discovering liquid water in RSLs, permafrost, or brines on Mars in the near future.
The paper, titled “The Elusive Nature of Martian Liquid Brines,” was co-authored by Vincent Chevrier, an associate research professor at the University of Arkansas’ Center for Space and Planetary Sciences, and Rachel Slank, a postdoctoral fellow at The Lunar and Planetary Institute in Houston, Texas. Slank completed her Ph.D. at the University of Arkansas under Chevrier’s guidance. He has dedicated two decades to searching for signs of liquid water on Mars, and while he is enthusiastic about the possibility, he believes the evidence to support it is insufficient at this time.
This paper aims to inform the public about our current understanding regarding the presence of liquid water on Mars.
“I’ve wanted to write this paper for quite some time,” Chevrier stated, “because there’s a lot of confusion, misunderstanding, and misinterpretation of what research really says about the state of liquid water on Mars.”
The authors argue that a detailed analysis of RSLs suggests their characteristics fit the patterns of sand and dust flows, negating the need for water in their formation. Moreover, data from Martian orbiters have not verified any role of liquid water in the creation of RSLs.
Some researchers believe that brines, which are high-salt solutions similar to Earth’s oceans, might be key to locating liquid water on Mars. Brines can remain liquid at significantly lower temperatures, and Mars has a plentiful supply of salts. Among these, perchlorates seem particularly promising, given their extremely low melting points. For example, a calcium perchlorate brine freezes at -75 degrees Celsius, whereas Mars averages -50 degrees Celsius at the equator, indicating potential environments in which this brine could remain liquid, particularly below the surface.
The authors analyze the arguments for and against the stable formation of brines. They conclude that factors such as limited availability of significant salts, water vapor pressure, and ice distribution “strongly limit the abundance of brines on the surface or shallow subsurface.” Even if brines were to form, they would likely be “highly uninhabitable by Earthly standards.”
In the conclusion, the authors state: “Despite these drawbacks and limitations, there’s always a chance that Martian life has adapted to these brines, and some Earth organisms might endure there. This raises considerations for planetary protection because that might mean life could exist on Mars today. Therefore, detecting brines in situ is a key goal for exploration of the red planet.”
The authors suggest that future challenges will involve enhancing the tools needed to find small amounts of brines, accurately selecting the best locations for exploration, and conducting additional lab tests under conditions similar to those on Mars.
“Despite our best efforts to prove otherwise,” Chevrier concludes, “Mars remains a cold, dry, and completely uninhabitable desert.”
