The mission of NASA’s Mars probe InSight ended in 2022, but the data it collected has revealed evidence of liquid water deep beneath the landing site. Image credit: NASA/JPL-Caltech

Deep beneath the surface of Mars lies a large reservoir of liquid water, as seismometer data from NASA’s retired InSight lander show. The results were released on August 12 in Proceedings of the National Academy of Sciencesindicate that there is enough water to fill the oceans and cover Mars globally to a depth of over one to two kilometers.

But the reservoir will not help much, The Martianby Mark Watney or other human explorers: It lies in tiny cracks and pores in the rock deep in the Earth’s crust, between 11.5 and 20 kilometers below the surface.

That means that this underground reservoir is out of reach – at least for now. But this study is the best evidence yet that huge reservoirs of water on Mars – thought to have mostly evaporated into space billions of years ago – may have ended up in Earth instead, raising hopes for the search for life.

It also opens a window into the planet’s past, adding to a wealth of evidence that water was plentiful on our neighbor billions of years ago. That past world was warmer, wetter, and surrounded by a thicker atmosphere than the parched, subfreezing, radiation-ravaged desert of today’s Mars.

To understand how this world disappeared, a more comprehensive understanding of the Martian water cycle is crucial, says Vashan Wright of the Scripps Institution of Oceanography at the University of San Diego, one of the report’s authors. “A useful starting point is to figure out where water is and how much of it there is.”

Wright and colleagues Michael Manga of the University of California, Berkeley, and Matthias Morzfeld of Scripps used a mathematical model of rock physics that is identical to methods used on Earth to map underground aquifers and oil fields. Their results suggest that InSight’s seismic data are best explained by a layer of fractured igneous rock saturated with liquid water embedded in the center of the Earth’s crust.

“The finding that there is a large reservoir of liquid water provides insight into what the climate was or could have been like at that time,” says Manga.

A once habitable world

During Mars’ Noachian geologic period—4.1 to 3.7 billion years ago, roughly the same time as the first signs of life on Earth—an ocean may have covered a third of the planet’s northern hemisphere, and conditions may have been habitable.

“Mars may have had a water cycle similar to Earth, with groundwater connected to rivers, lakes and possibly oceans,” Wright said.

Today, traces of this water-rich past can be found everywhere: in drainage channels, ravines, dried-up riverbeds, and in minerals and rocks that could only have been formed in the presence of water.

Because of Mars’ low gravity, lack of a magnetic field, and strong solar wind, much of its early atmosphere is thought to have escaped into space – along with most of its water. For decades, the only water that existed on Mars was in the form of ice at the polar caps and buried underground.

However, recent research questions whether Mars has really lost as much water as scientists have suspected. An analysis of data from several Mars missions conducted in Science in 2021 suggested that over 30 percent of Mars’ water may remain trapped in minerals in the Earth’s crust. “There is great uncertainty about how much water has been lost to space,” Wright says. “But the amount we think (has been lost) is much larger than the volume of ice on the surface.”

At the same time, observational evidence is mounting that Mars hosts a significant amount of subsurface water in liquid form. In 2018, researchers analyzing data from Europe’s Mars Express spacecraft reported what appeared to be a 20-kilometer-wide salt lake lying 1.5 kilometers beneath the South Pole ice cap. A follow-up study in 2020 using more advanced techniques backed this up, finding more saline areas nearby. And geological evidence from Mars Express pointed to a planet-wide groundwater system that flourished 3.5 billion years ago.

Dig deeper

InSight launched in May 2018 and traveled 484 million kilometers to Mars. The following November, it landed on the 3,000-kilometer-wide Elysium Planitia, an equatorial plain with river valleys, crustal faults and ancient volcanism. The probe was equipped with a range of instruments and sensors designed to explore the interior of Mars.

InSight’s highly sensitive seismometer – the French-built Seismic Experiment for Interior Structure (SEIS) – provided the data that led to the new results. The dome-shaped sensor, which can detect even slight vibrations like a breeze, was placed on Mars’ ochre surface by the lander’s robotic arm. The data was collected from December 2018 until the end of the mission in December 2022, when InSight’s power reserves ran out.

The results are based on Insight measurements of the density of rock layers beneath the probe and the speed of seismic waves generated by marsquakes and impacts as they move through the rock. But if crustal dynamics are similar across Mars, there should be more than enough water in this mid-crust reservoir to fill the Red Planet’s ancient oceans, the authors argue.

“Based on the data we have from other places on Mars, there is no reason to believe that the middle crust is much different elsewhere than it is beneath InSight,” Wright says. “If there is groundwater beneath InSight, we would expect groundwater elsewhere as well.”

How the water found its way so deep into the crust is still unclear, but its presence is further evidence of Mars’ watery past. “The interior of Mars cooled over time. If the water is liquid now, it must have been liquid in the past,” Wright said. “To get from the surface to these depths, the shallow crust would also have to be warmer.”

There’s also the possibility that deep beneath the surface, the porous, saturated rock is teeming with microbes. “Water is necessary for life as we know it. That’s certainly true on Earth – deep, deep mines harbor life, the seafloor harbors life,” says Manga. “We’ve identified a place that could, in principle, sustain life.”