Liquid water may be locked in rock just a few kilometres beneath Martian ground – much closer to the surface of the red planet than previously thought.
Using data from Nasa’s now-retired InSight lander, a China-led international team of researchers analysed seismic waves from “marsquakes” and meteorite impacts recorded between 2018 and 2022. They revealed a mysterious zone in the planet’s crust they said was best explained by a layer of water-saturated rock.
If confirmed, this underground layer – between 5.4km and 8km (3.3-5 miles) deep – could hold as much water as a global layer up to 780 metres (2,550 feet) thick, according to their paper published in National Science Review last month.
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The amount matched what scientists believe to be Mars’ missing water, after taking into account the water that has escaped into space, become locked into rocks or remains as ice and vapour, said the researchers from the Chinese Academy of Sciences, the Australian National University and the University of Milano-Bicocca.
“Our results provide the first seismic evidence of liquid water at the base of the Martian upper crust, shaping our understanding of Mars’ water cycle and the potential evolution of habitable environments on the planet,” they wrote in the paper.
Nasa’s InSight lander arrived on Mars in 2018 with a unique mission: not to roam the surface, but to listen to what was happening beneath it. For four years, it used a sensitive seismometer to detect subtle ground movements caused by crustal stress and meteorite impacts.
Just as doctors use ultrasounds to scan inside the human body, scientists pieced together information about Mars’ subsurface layers by studying how seismic waves travel through the Martian interior. Before shutting down in 2022, InSight recorded more than 1,000 seismic events, creating a trove of data that researchers are still analysing today.
A breakthrough came in 2024, when a team from the University of California San Diego and UC Berkeley found that Mars’ mid crust – around 11-20km deep – could contain fractured rock fully saturated with liquid water. Their study, based on theoretical models, made the first strong scientific case that Mars might still hold underground water.
The new study, led by Sun Weijia from the Chinese Academy of Sciences’ Institute of Geology and Geophysics, went a step further.
While most previous Mars seismic studies relied on low-frequency data, which could only detect large-scale structures, Sun’s team used higher-frequency signals, which gave them a much sharper view to about 500 metres in resolution, of the top 10km of Mars’ crust.
Looking at signals from three specific seismic events recorded by InSight – including the strongest quake ever recorded on another planet – the researchers identified a clear drop in wave speed at around 5.4km to 8km deep. This usually points to a layer of porous rock filled with liquid water.
At that depth, conditions on Mars are warm and pressurised enough for water to stay liquid. The researchers say this zone could represent a kind of “last refuge” for liquid water on modern Mars.
While the discovery is scientifically significant, the water is still far too deep to be accessed with existing drilling technology. It cannot be used for future missions – at least not any time soon.
The team also cautioned that their estimate was based only on data from the region directly beneath the InSight lander. Future missions with better seismometers could help confirm their findings, both at the InSight site and in other regions across Mars, they wrote.
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