Beneath Mars’ barren surface, researchers have discovered a hidden ocean of water. The water is locked away deep inside the planet’s crust and stretches across its entire landscape. To make things even more exciting, this layer of water could very well be habitable.

Marsquakes and water

The discovery was made using seismic data from NASA’s InSight lander, which was a groundbreaking mission for our understanding of Mars. The lander measured seismic waves inside Mars from 2018 to 2022 from marsquakes and meteorite impacts. With this data, InSight has been able to map out the layers beneath the Martian surface — this is also the core of this new study.

Seismic waves behave differently depending on the materials they pass through. Different types of rock, sediment, or liquid change how these waves propagate. This allows scientists to infer the composition and structure of the Martian crust, mantle, and core. In particular, variations in wave speed and attenuation give clues about the density and composition of subsurface layers. In this case, they provide valuable information about what’s on the inside of Mars.

This approach is not new; it’s been used on Earth for decades. If you’ve ever wondered how oil and gas reservoirs are discovered, it’s often with seismic analysis (although the methodology can be different the principle is the same).

In this instance, researchers looked at the data and then created a mathematical model of the subsurface rocks. The goal is to create a model that best explains the observed data. In this case, the model that fared best was a deep layer of fractured igneous rock saturated with liquid water.

In other words, it seems that there’s a deep layer rich in water deep in the Martian crust. This layer lies 11.5 to 20 kilometers below the surface and is about 1-2 km thick. This water is not in large, open bodies like lakes or oceans but rather dispersed within tiny cracks and pores in the rock.

So that’s where Mars’ water went

It’s essentially certain that Mars used to have rivers, lakes, and oceans. However, when the Martian atmosphere started disappearing some 3 billion years ago the water also dissipated. The new research suggests the water (or some of it) didn’t escape into outer space, but seeped deeper underground.

“Understanding the Martian water cycle is critical for understanding the evolution of the climate, surface and interior,” said Vashan Wright, a former UC Berkeley postdoctoral fellow who is now an assistant professor at UC San Diego’s Scripps Institution of Oceanography. “A useful starting point is to identify where water is and how much is there.”

However, there is still some uncertainty regarding the conclusions of this study. Researchers say that more information regarding the Martian geology and the rocks in its subsurface could help to finesse the model even more.

“While available data are best explained by a water-saturated mid-crust, our results highlight the value of geophysical measurements and better constraints on the mineralogy and composition of Mars’ crust,” the authors wrote. 

Does this mean that Mars is habitable (underground)?

This deep reservoir also opens up new possibilities for the search for life on the Red Planet. The more we know about Mars’ geology, the better we can assess the planet’s current habitability. Mars’ surface is not currently habitable, but there’s no reason why this deep water wouldn’t be, the researchers say.

“Establishing that there is a big reservoir of liquid water provides some window into what the climate was like or could be like,” said Manga, a UC Berkeley professor of earth and planetary science. “And water is necessary for life as we know it. 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. We haven’t found any evidence for life on Mars, but at least we have identified a place that should, in principle, be able to sustain life.”

For a practical long-term mission on Mars, this doesn’t mean that much. Digging a hole that deep is extremely difficult on Earth, let alone on Mars. While this water is extremely intriguing, we can’t tap into it anytime soon.

A human mission on Mars would likely obtain water from ice. Data from orbiters and landers, like InSight and Mars Odyssey, have found vast fields of perennial water ice around the Red Planet’s polar areas — that type of source is much more usable than something that’s kilometers deep underground.

Meanwhile, researchers continue to praise the InSight mission and all the information it’s given us — not just about the potential for water on Mars, but also regarding the crust, mantle, and even core of the Red Planet.

“The mission greatly exceeded my expectations,” Manga said. From looking at all the seismic data that Insight collected, they’ve figured out the thickness of the crust, the depth of the core, the composition of the core, even a little bit about the temperature within the mantle.

The study has been published in Proceedings of the National Academy of Sciences. DOI 10.1073/pnas.2409983121