Scientists are confident Mars was once abundant with water, as seen in massive flood-carved channels, ancient river valleys, and minerals that form only in liquid water. But how the Red Planet lost its water, leaving behind the arid world we see today, is still up for debate.
Now, a new challenge to a recent theory surrounding vast amounts of water stored beneath the Martian surface suggests the Red Planet may not be hiding liquid water beneath its crust after all.
In a letter to Proceedings of the National Academy of Sciences (PNAS) published on March 6, Bruce Jakosky, a senior research scientist at the University of Colorado Boulder and former principal investigator of NASA’s Mars Atmosphere and Volatile EvolutioN mission (MAVEN), argues that last year’s theory has an alternative explanation.
“We expect there to be water or ice in [Mars’] crust,” Jakosky said in a statement. “Actually detecting it and possibly determining its abundance is challenging, but extremely important for understanding how much water there is on Mars and what its history has been.”
InSight and hidden water on Mars
In 2024, a team led by Vaughan Wright at the Scripps Institution of Oceanography, the University of California San Diego, determined that a mid-crust made up of fractured, water-saturated igneous rock best explains data collected by NASA’s now-retired Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) mission.
While previous missions focused on surface features, understanding Mars’ formation requires studying its deep interior. The InSight lander, launched in 2018, was designed to measure the planet’s internal activity, including its temperature, seismic waves, and core dynamics.
Wright and his colleagues analyzed InSight’s data to model the types of rocks and water saturation levels that might explain the seismic activity detected about 10 to 12 kilometers below the Martian surface.
Based on their findings, Wright and colleagues estimated that Mars’ crust could hold the equivalent of 0.62 to 1.24 miles (1 to 2 kilometers) of water if it were spread evenly across the planet’s surface—this is known as the global equivalent layer.
For comparison, Earth’s global equivalent layer is about 3.6 kilometers, which is mostly made up of water in the oceans, with only a small amount in the crust.
“While the approach and analysis are reasonable and appropriate, the results of their modeling suggest an alternative conclusion,” Jakosky commented.
Both Wright’s and Jakosky’s examinations of the modeling data evaluate a metric known as liquid water saturation, which is the fraction of pore spaces in the rock filled with liquid water.
Wright’s modeling suggested that the liquid water saturation in the mid-crust beneath InSight is near 1, meaning almost all of the pore spaces in the rocks are filled with liquid water. This conclusion, they said, helped explain InSight’s seismic data, as water-saturated rocks would behave differently under seismic waves.
However, Jakosky’s reexamination of the data brought in other possibilities.
Jakosky and colleagues considered that the pore spaces in the crust could also contain solid ice or even be empty rather than being filled entirely with liquid water. They think this could still explain the seismic and gravity data InSight collected.
Jakosky pointed out that while the InSight data doesn’t confirm that liquid water is present in the mid-crust, it also doesn’t completely rule it out. After factoring in the distribution of pore space and the potential presence of ice or empty spaces, Jakosky proposed that the amount of water could range from zero to 1.24 miles (0 to 2 kilometers) if spread evenly across the planet’s surface.
This adjusted the lower estimate from Wright’s team, suggesting a broader range of possibilities.
“It may be possible with future spacecraft measurements to constrain the abundance of water in the crust through more precise determination of the crustal properties,” Jakosky concluded.
So, at least for now, the Mars water debate is set to rage on.