Dark surface features called recurring slope lineae (RSL), polygonal formations in Martian permafrost, and the possibility of liquid brines have all fueled discussions about liquid water. However, a new study published in Proceedings of the National Academy of Sciences challenges these ideas, suggesting liquid water on Mars is far less likely than hoped.
Titled The Elusive Nature of Martian Liquid Brines, the paper is co-authored by Vincent Chevrier, 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. Slank completed her Ph.D. at the University of Arkansas under Chevrier's guidance.
Chevrier explained the motivation for the study, stating, "I wanted to write this paper for a very long time because I think there is a lot of confusion, a lot of misunderstanding, and a lot of erroneous interpretations of what the research papers are saying about the state of liquid water on Mars."
The paper reexamines RSL and concludes they are likely caused by sand and dust flows, with no need for liquid water to explain their behavior. Furthermore, data from Mars orbiters does not confirm any involvement of liquid water in their formation.
The authors also address the hypothesis that liquid brines, highly salty water solutions, could persist on Mars. Salts, such as perchlorates, lower the freezing point of water, theoretically allowing brines to remain liquid even in the frigid Martian environment. For example, calcium perchlorate brines can remain liquid at -75 degrees Celsius, while equatorial Mars averages about -50 C.
However, the paper concludes that conditions on Mars strongly limit the presence of brines. Factors such as insufficient salt concentrations, low water vapor pressures, and the distribution of ice reduce the likelihood of stable liquid brines forming on the surface or in the shallow subsurface. Moreover, even if such brines existed, they would "remain highly un-habitable by terrestrial standards."
The study does, however, acknowledge the planetary protection implications. The authors write: "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."
To advance the search for liquid water, the authors emphasize the need for improved detection instruments, better identification of promising locations, and enhanced laboratory simulations of Martian conditions.
"Despite our best efforts to prove otherwise," Chevrier concluded, "Mars still remains a cold, dry and utterly unhabitable desert."
Research Report:The elusive nature of Martian liquid brines
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