Subsurface water on Mars subverts predictions

A new analysis of seismic data from NASA’s Mars InSight mission has led to several revelations about water on Mars.

Mars, now a dried-up desert, once ran with water, with the tracks of past streams and rivers still visible across the planet. A leading theory suggested that the water on Mars became part of the minerals that form the underground cement.  

However, a new analysis of seismic data from NASA’s Mars InSight mission has led to several revelations, contradicting this idea. The analysis, led by the Scripps Institution of Oceanography at the University of California, San Diego, discovered a lack of cemented sediments, suggesting a water scarcity. 

The analysis was published in Geophysical Research Letters.  

Revelations on Mars’ subsurface water

First, the upper 300 metres of the subsurface, beneath the landing site near the Martian equator, contains little or no ice. 

“We find that Mars’ crust is weak and porous. The sediments are not well-cemented. And there’s no ice or not much ice filling the pore spaces,” said geophysicist Vashan Wright of the Scripps Institution of Oceanography at the University of California, San Diego, and co-author of the analysis.  

“These findings don’t preclude that there could be grains of ice or small balls of ice that are not cementing other minerals together,” said Wright. “The question is, how likely is ice to be present in that form?”  

The second revelation contradicts a leading idea about what happened to the water on Mars. The red planet may have harboured oceans of water early in its history. Several experts suspected that much of the water became part of the minerals that comprise the underground cement. 

“If you put water in contact with rocks, you produce a brand-new set of minerals, like clay, so the water’s not a liquid. It’s part of the mineral structure,” said study co-author Michael Manga of the University of California, Berkeley. “There is some cement, but the rocks are not full of cement.” 

“Water may also go into minerals that do not act as cement. But the uncemented subsurface removes one way to preserve a record of life or biological activity,” Wright said. By their very nature, cement holds rocks and sediments together, which protects them from erosion.  

The lack of cemented sediments suggests a water scarcity in the 300 meters below InSight’s landing site, near the equator. With average temperatures below freezing, the conditions would be right for water to freeze if it were there.  

Manga, and many other planetary scientists, have long suspected that the Martian subsurface would be full of ice. With this new evidence, this does not appear to be the case. However, big ice sheets and frozen ground ice remain at the Martian poles. 

“As scientists, we’re now confronted with the best data, the best observations. And our models predicted that there should still be frozen ground at that latitude with aquifers underneath,” said Manga, professor and chair of Earth and planetary science at UC Berkeley. 

© iStock/namussi

About the InSight spacecraft

In 2018, the InSight spacecraft landed on Elysium Planitia, a flat, smooth, plain near the Martian equator. Its instruments included a seismometer that measures vibrations caused by marsquakes and crashing meteorites. 

Scientists can tie this information to a vast amount of knowledge about the surface, including images of Martian landforms and temperature data. The surface data suggested that the subsurface might consist of sedimentary rock and lava flows. Still, the team had to account for uncertainties about subsurface properties such as porosity and mineral content. 

Seismic waves from marsquakes indicate the nature of the materials they travel through. Possible cementing minerals, such as calcite, clay, kaolinite, and gypsum, affect seismic velocities. Wright’s team at Scripps Oceanography used rock physics computer modelling to interpret the velocities derived from the InSight data. 

“We ran our models 10,000 times each to get the uncertainties incorporated into our answers,” said co-author Richard Kilburn, a graduate student working in the Scripps Tectonorockphysics Lab led by Wright. The simulations that best fit the data show a subsurface consisting of mostly uncemented material. 

Scientists intend to probe the subsurface because it is where life on Mars would most likely exist. There is no liquid water on Mars’s surface, and subsurface life would be protected from radiation. 

NASA’s priorities for the future  

Following a sample-return mission, a NASA priority for the next decade is the Mars Life Explorer mission concept. The goal is to drill two metres into the Martian crust at high latitude to search for life. This is where ice, rock, and the atmosphere come together. 

The proposed international robotic Mars Ice Mapper Mission is already under consideration to help NASA identify potential science goals for the first human missions to Mars. Scripps Oceanography helps prepare young scientists to contribute to such missions. 

“All my life growing up, I’ve heard the Earth may become uninhabitable,” said study co-author Jhardel Dasent, another graduate student in the lab Wright leads. “I’m at the age now where I can contribute to producing the knowledge of another planet that may get us there.” 

This research was funded by the National Science Foundation, NASA, and the CIFAR Earth 4D program. 

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