Water that used to exist on Mars slowly leaked out into space, or at least that’s the going theory. A new paper challenges this assumption, offering an alternative scenario in which the Red Planet has clung to much of its ancient water—we just can’t see it.
An unexpectedly large amount of water is tucked away inside minerals buried below the Martian crust, according to new research published in Science. Data presented in the new paper, co-authored by Caltech graduate student Eva Scheller, suggests anywhere from 30% to 99% of the original water on Mars has been retained.
At the same time, the authors say the prevailing theory about Martian water leaking out into space—a consequence of the planet’s low gravity—isn’t quite up to snuff and that their new theory resolves a key shortcoming rather nicely. These results were presented on March 15 at the 52nd Lunar and Planetary Science Conference.
We know the Red Planet was once covered in flowing water, as evidenced by the remnants of deep ocean basins, lakes, rushing rivers, and even stupendously huge tsunamis. The total volume of water that used to exist on the ancient Martian surface is estimated at half the total volume of the Atlantic Ocean, which is hardly a trivial amount. Such was the case billions of years ago, but most of this water appears to be gone, and the small amount that’s left has retreated to the polar ice caps and (possibly) the odd subterranean reservoir.
But as Scheller explained in a NASA statement, the atmospheric escape of Martian water “doesn’t fully explain the data that we have for how much water actually once existed on Mars.”
Key to any study about the history of water on Mars is the observed ratio of deuterium to hydrogen (D/H), which is typically used to bolster the atmospheric escape theory. Water is composed of hydrogen and oxygen, but a very tiny number of hydrogen atoms exist as deuterium, also known as “heavy hydrogen” owing to an extra neutron within the atomic nucleus, in addition to the standard proton. Normal hydrogen, which accounts for 99.98% of all hydrogen, can easily escape the low Martian gravity and leach out into space, but such is not the case for deuterium. Accordingly, Mars should exhibit a surplus of deuterium, which it does.
Trouble is, the currently observed rate of atmospheric water leakage is too low, according to the study authors, and this process cannot exclusively account for all that historic water loss through the atmosphere. Instead, Scheller and her colleagues argue that, in addition to some slight leakage through the atmosphere, Mars’s ancient water became trapped in minerals within the planet’s crust. Together, these two mechanisms can explain the observed D/H ratio and the missing water, according to the paper.
Evidence for this hypothesis was pulled from NASA’s Planetary Data System, which serves as a general data repository for past missions. In this case, the authors analyzed Mars-specific data gathered by telescopes, satellites, and rovers in order to reconstruct historical water volumes—whether in liquid, vapor, or ice form—on Mars, and to study the chemical composition of the Martian atmosphere and crust.
By running simulations under various conditions, the authors showed that Mars lost much of its water during its Noachian period, around 4.1 billion to 3.7 billion years ago, and that 30% to 99% of this ancient water became buried beneath the crust, with the remainder getting lost to space, in a finding that respects the currently observed D/H ratio.
The process responsible for the disappearance of Mars’ water is known as crustal hydration, and it’s not as exotic as it sounds. Chemical weathering caused by the mixture of rocks with water produces clays and other soggy minerals. This happens on Earth as well as on Mars, as evidenced by ground observations made by NASA’s Curiosity rover. The fate of these materials, however, played out differently on the two planets.
“The hydrated materials on our own planet are being continually recycled through plate tectonics,” Michael Meyer, lead scientist for NASA’s Mars Exploration Program, said in the NASA release. “Because we have measurements from multiple spacecraft, we can see that Mars doesn’t recycle, and so water is now locked up in the crust or been lost to space,” said Meyer, who’s not directly involved with the new research.
Kevin Olsen, a fellow at the University of Oxford and an expert on the Martian atmosphere, said the new paper makes “bold” but “new and intriguing” assumptions.
“Our basis for [making inferences] about the ancient climate of Mars comes from comparison with Earth, and one aspect of the evolution of Mars that differs from Earth is the silencing of its volcanoes, the largest in the solar system,” wrote Olsen, who’s not affiliated with the new study, in an email. “By modelling how large the exchange is between water reservoirs near the surface and those in the rocky crust, they have opened up many plausible scenarios where Mars was once much wetter but turned out the way we see it today.”
“This is a very interesting paper, in which many different mechanisms and models are combined to explore the fate of water on Mars,” Geronimo Villanueva, a planetary scientist at NASA-Goddard Space Flight Center, also not involved with the new study, said in an email. “Considering the high degree of uncertainty that exist on some of the model parameters—the range of possible scenarios is great—yet they importantly pose testable predictions that can be pursued in the future.”
Villanueva said the new paper will be of assistance to future investigations about the history of water on the Red Planet.
Thankfully, the Perseverance rover could soon contribute to this line of research. NASA’s new Martian explorer will soon begin its scientific work in Jezero crater, the site of a former lake and river delta. Evidence to bolster this new theory could exist within this ancient expanse, which Perseverance will explore for the next two years.
For future Martian colonists, this is both good and bad news. It’s good news because, hey, Mars still has lots of water, at least in theory. The bad news is that this water, should it exist, is locked in hydrated materials like clay. Living on Mars would be tough enough, but developing the infrastructure to mine, extract, and clean the water pulled from these minerals sounds wildly complicated and costly.
To quote from the “Rime of the Ancient Mariner,” it could be a classic case of, “Water, water, every where, And all the boards did shrink; Water, water, every where, Nor any drop to drink.”