NASA's InSight lander ended its mission in 2022, but the data it collected revealed evidence of liquid water deep below the landing site. Credit: NASA/JPL-Caltech
Deep beneath the surface of Mars lies a vast reservoir of liquid water, according to seismometer data from NASA's retired InSight lander. The findings, published Aug. 12 in Proceedings of the National Academy of Sciencessuggest there is enough water to fill the oceans and globally cover Mars to a depth of more than a mile (1 to 2 kilometers).
But the reservoir will be of little help. The MartianAccording to Mark Watney and other human explorers: It is found in tiny cracks and pores in the deep rock of the planet's crust, between 7.1 miles (11.5 km) and 12 miles (20 km) beneath the surface.
That means this underground reservoir is inaccessible, at least for now. But this study is the best evidence yet that vast reserves of Martian water — most of which was thought to have evaporated into space billions of years ago — may have ended up on Earth, raising hopes for the search for life.
It also opens a window into the planet's past, adding to a rich body of evidence that water flowed abundantly on Earth's neighbour billions of years ago. That ancient world was warmer, wetter and surrounded by a thicker atmosphere than the desiccated, sub-zero, radiation-ravaged desert of today's Mars.
To understand how that world disappeared, it’s crucial to better understand the water cycle on Mars, said 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 identify where there is water and how much there is.”
Wright and his colleagues Michael Manga of the University of California, Berkeley, and Matthias Morzfeld of Scripps used a mathematical rock physics model identical to methodologies used on Earth to map underground aquifers and oil fields. Their results suggest that the best explanation for InSight’s seismic data is the existence of a layer of fractured igneous rock saturated with liquid water embedded in the core of the planet’s crust.
“Establishing that there is a large reservoir of liquid water provides insight into what the climate was or could be like,” Manga says.
A world that was once habitable
During the geological time of Noah on Mars (between 4.1 and 3.7 billion years ago, roughly contemporary with the first hint of life on Earth), an ocean perhaps filled one-third of the planet's northern hemisphere, and conditions may have been habitable.
“Mars may have had a hydrologic cycle similar to Earth’s, where groundwater was connected to rivers, lakes and possibly oceans,” Wright said.
Today, traces of that watery past are everywhere: in drainage channels, ravines, dry river beds, and minerals and rocks that could only have formed in the presence of water.
However, due to its low gravity, lack of a magnetic field, and strong solar wind, much of Mars's early atmosphere is thought to have escaped into space, along with most of its water. For decades, the only water found on Mars was in the form of ice, in the polar ice caps, and buried underground.
However, recent research has cast doubt on whether Mars has lost as much water as scientists thought. An analysis of data acquired on several missions to Mars published in Science in 2021 Scientists have suggested that more than 30 percent of Martian water may remain trapped in minerals in the planet’s crust. “There is a lot of uncertainty about how much water was lost to space,” Wright says. “But the amount we infer[that has been lost]is much larger than the volume of ice on the surface.”
At the same time, observational evidence has been accumulating that Mars hosts a substantial amount of groundwater, in liquid form. In 2018, researchers analyzed data from The European orbiter Mars Express What appeared to be a 20km-wide salt lake was reported located 1.5km beneath the South Polar ice cap. A follow-up study in 2020 using more advanced techniques strengthened the case, finding additional briny areas nearby. And geological evidence from Mars Express hinted at a Groundwater system at a planetary level which flourished 3.5 billion years ago.
Going deeper
Launched in May 2018, InSight traveled 484 million kilometers (300 million miles) to Mars and landed in November of the following year in Elysium Planitia, a 3,000-kilometer-wide plain that straddles the equator and is made up of river valleys, crustal faults and ancient volcanoes. The probe was equipped with an array of instruments and sensors designed to probe the interior of Mars.
InSight’s ultra-sensitive seismometer, the French-made Seismic Experiment for Interior Structures (SEIS), provided the data that led to the new results. The sensor, encased in a dome and capable of detecting vibrations as gentle as a breeze, was placed on the ochre surface of Mars 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 levels were depleted.
The findings are based on InSight's measurements of the density of rock layers beneath the spacecraft and the speed of seismic waves — produced by earthquakes and impacts — as they travel through that rock. But if crustal dynamics are similar all over Mars, there should be more than enough water in this mid-crustal reservoir to fill the Red Planet's ancient oceans, the authors say.
“Based on the data we have elsewhere on Mars, there’s no clear reason to think that the middle crust elsewhere would be very different from what’s beneath InSight,” Wright said. “If there’s groundwater beneath InSight, we would expect there to be groundwater elsewhere as well.”
It’s not clear how the water got to this depth in the crust, but its presence is another line of evidence for Mars’ watery past. “The interior of Mars has been cooling over time. If water is liquid now, it would have been liquid in the past,” Wright said. “To get from the surface to these depths, the surface crust would have to be warmer, too.”
It also raises the possibility that deep below the surface, the porous, saturated rock is teeming with microbes. “Water is necessary for life as we know it. This is true on Earth: very deep mines harbor life, the ocean floor harbors life,” Manga says. “We have identified a place that, in principle, should be able to support life.”
