Thick Clay Layers On Mars May Have Been A Stable Place For Ancient Life

The planet Mars is home to thick layers of clay that can span hundreds of feet. Since they need water to form, these outcrops have long been of interest to scientists looking for signs of past life on the Red Planet.

In a new study in Nature Astronomy, scientists from The University of Texas at Austin and collaborators took a closer look at these clay terrains and found that most formed near standing bodies of surface water, which were common on Mars billions of years ago. This environment would help foster the chemical weathering needed to create thick, mineral-rich layers of clay and could have provided the right mix of water, minerals and a calm environment for life to develop.

“These areas have a lot of water but not a lot of topographic uplift, so they’re very stable,” said the study’s lead author Rhianna Moore, who conducted the research as a postdoctoral fellow at the UT Jackson School of Geosciences. “If you have stable terrain, you’re not messing up your potentially habitable environments. Favorable conditions might be able to be sustained for longer periods of time.”

The study was conducted as part of UT’s Center for Planetary Systems Habitability, which investigates the origins and requirements for life on Earth and other planetary bodies. Moore is now with NASA as part of a team supporting the Artemis mission to Earth’s Moon.

The researchers noted that the thick clays could also be a sign of an imbalanced water and carbon cycle on ancient Mars, which could explain why Mars appears to be missing carbonate rocks in environments where they would be expected on Earth.

Billions of years ago, Mars was a wet world. It had lakes and rivers, which created geological formations that are carved on the surface of the planet today. The thick clay layers formed during this wet period. However, before this study, little was known about the environments in which they formed and how the surrounding terrain influenced their evolution.

Moore analyzed images and data from 150 clay deposits that had been previously identified in a global survey conducted by NASA’s Mars Reconnaissance Orbiter. She investigated trends in their topographical characteristics and how close they were to other geological features, such as former bodies of water.

A map from the study showing the location the clay deposits on Mars, along with other geological features that the researchers examined. Credit: Moore et al.

She found that the clays were mostly found at low elevations near lake deposits but away from valley networks, where water is thought to have flowed more vigorously across the terrain. This balance between chemical and physical weathering led to their preservation through time. Co-author Tim Goudge, an assistant professor at the Jackson School’s Department of Earth and Planetary Sciences, said that the Mars clay environment is similar to the tropical places where thick clay layers are found on Earth.

“On Earth, the places where we tend to see the thickest clay mineral sequences are in humid environments, and those with minimal physical erosion that can strip away newly created weathering products,” he said. “These results suggest that the latter element is true also on Mars, while there are hints at the former as well.”

However, the clays also reflect an ancient Martian world that was very different from the Earth of today.

On Earth, shifting tectonic plates are constantly exposing fresh rock that can readily react with water and CO₂ in the atmosphere, which helps regulate the climate. However, Mars lacks tectonic activity. When Martian volcanoes released CO₂ into the atmosphere, the lack of a source for new reactive rock would have led the greenhouse gas to linger — causing the planet to become warmer and wetter. The researchers suggest that these conditions may have contributed to the formation of the clays.

What’s more, the lack of new rock on the surface may have impeded the chemical reactions needed to form carbonate rock — which would normally form from volcanic rock that underlies most Martian geology given CO₂, water and time. Ongoing clay formation may have contributed to the dearth of carbonates by sucking up water and sequestering chemical byproducts in the clay, rather than having them leach out into the wider environment, where they could react with the surrounding geology.

“It’s probably one of many factors that’s contributing to this weird lack of predicted carbonates on Mars,” said Moore.

The research was funded by NASA and the Canadian Institute for Advanced Research.

Deep chemical weathering on ancient Mars landscapes driven by erosional and climatic patterns, Nature

Astrobiology, Astrogeology,