THE APPLICATION of Bayesian inversion to a range of lithologic parameters in an assessment of seismic data hardly seems the stuff of headlines. But when the seismic data you are assessing come from Mars, and when your reasoning suggests that the best explanation for them is liquid water in the pores between rocks, you have a story. There is little people thirst for more, when it comes to planetary science, than water on Mars. Why does the prospect provoke such excitement—and what does this new research add to the story?
Liquid water seems to be a necessity for life. And from the 19th century to the early days of the exploration of space, astronomers assumed that there was liquid water on the surface of Mars. Perhaps not much—the notion that the planet’s inhabitants had built a network of canals through which water could flow from the polar ice caps to the tropics, promulgated by the American astronomer Percival Lowell from the 1890s onwards, was based on the idea that the planet was undergoing widespread desertification. But enough to support life—if not the advanced life that Lowell was so keen on, at least the primitive forms that astronomers continued to imagine there after his fancies had been discarded.
In the 1960s the first spacecraft to get to Mars told a different story—indeed, crucially, two different stories. First, Mars had a far thinner atmosphere than had been thought, much too thin for liquid water to persist on the surface. (In low pressure water boils away, even at very low temperatures.) Second, its icecaps seemed to be composed largely of frozen carbon dioxide—dry ice—an idea that Lowell and many of his successors had pooh-poohed.
But the spacecraft that followed in the 1970s showed that things had once been different. Erosion had carved river valleys into the planet’s southern highlands. There were plains which, to expert eyes, appeared to have been scoured by vast, catastrophic floods. There were what looked like shorelines, suggesting to some that the planet had had lakes and perhaps even an ocean in its low-lying northern plains. What now looked barren might once have been warmer, wetter and much more habitable. A disappointing absence became an enduring mystery: where was that water now?
A lot turned out to be frozen: beneath a thin seasonal veneer of carbon dioxide the northern polar cap is mostly water ice. There is also a lot of ice frozen into the pores within rocks near the surface, forming a “cryosphere” around the planet. And there’s more bound up in minerals. Evidence of little trickles on the surface suggest that sometimes bits of it escape into liquid form, at least briefly. But for the most part it just sits there. If you think that Mars did not have much water to begin with, it is possible to imagine that it has all been frozen or blown away into space as the atmosphere thinned.
But what if there was once a lot of water—more than can be accounted for by that which has been lost to space or frozen. The only other place it could be hiding is deep in the planet’s crust. That is where Vashan Wright of the Scripps Institute of Oceanography and his colleagues think they have found it. In Proceedings of the National Academy of Sciences they present an analysis of data provided by InSight, an American probe that monitored the tremblings of the planet’s crust from Elysium Planitia, a plain near the Martian equator, between 2018 to 2022. The researchers looked at a range of possible crustal constituents to see what arrangement would best explain that data; the fit that seemed most sensible was one in which the pores in the rock were full of liquid water from about 10km deep to around 20km. If they are right, and the area that InSight assessed is typical of the planet as a whole, such aquifers could contain more than an ocean’s worth of water.
To reach that water would require drills far beyond anything that the current generation of Mars robots could carry. Even on Earth it would be hard. But further geophysical research could do a lot to confirm or deny the findings. If the aquifers are there, they may provide new insights into the dynamics of Mars’s desiccation. And if Mars ever was the abode of life, they may be the habitat of its last survivors.
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