Time was, Earth may not have been the solar system’s only garden planet. For its first billion or so years, Mars was partly covered in water, as dry ocean basins and riverbeds on its surface now attest. But three billion years ago the planet lost its magnetic field, possibly due to a cooling of its core, allowing the solar wind to strip away its atmosphere. This, so the thinking went, caused the water to evaporate into space.
However, according to a new study in the Proceedings of the National Academy of Sciences, it appears that the water may still be there, trapped in the pores of volcanic rock 11.5 to 20 km (7 to 13 mi.) underground. If that much water were transported back to the surface, it would cover the entirety of Mars in a globe-girdling ocean about a mile deep. Even below ground, the presence of so much water could have significant implications for the possibility of Martian life in the rocky pores or slurry.
“Water is necessary for life as we know it,” said Michael Manga, study co-author and professor of geophysics and planetary sciences at the University of California, Berkeley, in a statement. “I don’t see why [the underground reservoir] is not a habitable environment. It’s certainly true on Earth—deep, deep mines host life, the bottom of the ocean hosts life.”
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The new findings are the result of studies conducted by the Mars InSight spacecraft, which landed on the Red Planet in 2018 and functioned until 2022. InSight’s suite of instruments included a seismometer, which detected marsquakes up to a magnitude of 5—a moderate quake on the Richter scale. The tremors may have been caused by volcanic stirrings, meteor impacts, or the contraction of the crust, but what mattered more than the source was the speed at which the energy was transmitted through the subsurface. Computer models of those measurements were consistent with the tremors passing through a water-logged region at the 11.5 to 20 km depth.
“The available data are best explained by a water-saturated mid-crust,” the authors wrote.
It’s not just the seismic data that point to that conclusion. The amount of surface water locked in Mars’s polar caps is not remotely enough to account for what was once there—judging by the depth of the river and ocean depressions. That means that the water either sank or vanished into space, and the new findings indicate it was the former.
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“Understanding the Martian water cycle is critical for understanding the evolution of the climate, surface and interior,” said Vashan Wright, another co-author of the paper and an assistant professor of geophysics at the University of California San Diego’s Scripps Institution of Oceanography, in a statement. “A useful starting point is to identify where water is and how much is there.”
The presence of the water may mean big things for Martian biology as well as for our understanding of the planet’s history, but we are no closer to one day being able to live off the Martian land. The deepest hole ever drilled on Earth is the Kola superdeep borehole in northwest Russia, which extends down about 12 km. That may be deep enough to touch the Red Planet water, but nobody pretends it would be possible to transport the same kind of massive drilling equipment that dug the Kola hole to Mars.
“Drilling a hole 10 kilometers deep on Mars—even for [Elon] Musk—would be difficult,” Manga told BBC News.
