The application of Bayesian inversion to a range of lithologic parameters in the evaluation of seismic data hardly seems to be a topic of headlines. But when the seismic data you’re assessing comes from Mars, and when your logic suggests that the best explanation for them is liquid water in the pores between the rocks, you have a story. When it comes to planetary science, few people thirst for more than water on Mars. Why does the possibility generate so much excitement—and what does this new research add to the story?
Liquid water appears to be a necessity for life. And from the 19th century until the early days of space exploration, astronomers assumed that Mars had liquid water on its surface. Perhaps not much – the notion that the planet’s inhabitants had created a network of canals through which water could flow from the polar ice caps to the tropics was based on this idea, promulgated by the American astronomer Percival Lowell since the 1890s. That the planet was undergoing mass desertification. But enough to support life – if not the advanced life for which Lowell was so eager, then at least the primitive forms that astronomers continued to imagine even after he abandoned his visions.
The first spacecraft to Mars in the 1960s told a different story – in fact, crucially, two different stories. First, Mars had a much thinner atmosphere than previously thought, too thin for liquid water to persist on the surface. (Water boils under low pressure, even at very low temperatures.) Second, its ice layers appear to be composed largely of frozen carbon dioxide – dry ice – an idea that Lowell and his Many heirs had refused.
But spacecraft in the 1970s revealed that things were once different. Erosion had carved out river valleys in the planet’s southern highlands. There were plains which, in the eyes of experts, appeared to have been devastated by huge, devastating floods. There were what appeared to be coastlines, which led some to believe that the planet had lakes and perhaps even an ocean in its low northern plains. What now appears barren may once have been warmer, wetter, and more habitable. A disappointing absence became an enduring mystery: Where was that water now?
Much is frozen: the northern polar cap is mostly water ice beneath a thin seasonal blanket of carbon dioxide. There is also a lot of ice frozen in the pores of rocks near the surface, creating a “cryosphere” around the planet. And there are many more in minerals. Evidence of small droplets on the surface suggests that fragments sometimes left it in liquid form, at least for some time. But for the most part it just sits there. If you think that there wasn’t much water on Mars to begin with, it’s possible to imagine that it all froze or evaporated into space as the atmosphere thinned.
But what if there was once a lot of water – more that has been lost to space or frozen than can be accounted for. The only other place where it could be hidden is in the depths of the planet. That’s where Vashan Wright and his colleagues at the Scripps Institute of Oceanography think they’ve found it. In the Proceedings of the National Academy of Sciences they present an analysis of data provided by InSight, a US probe that monitored vibrations of the planet’s crust from Elysium Planitia, a plain near the Martian equator, between 2018 and 2022. The researchers looked at a range of possible crustal components to see which arrangement could best explain that data; The fit that seemed most sensible was one in which the rock’s pores were filled with liquid water from about 10 km deep to about 20 km deep. If they are correct, and the area InSight evaluated is typical of the entire planet, such aquifers could hold more than an ocean’s worth of water.
Reaching that water will require far more drilling than the current generation of Martian robots. Even on Earth it will be hard. But further geophysical research could do much to confirm or refute the findings. If aquifers are there, they could provide new insights into the dynamics of Mars’ aridity. And if Mars was once home to life, they may be home to its last survivors.
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