Mars wasn’t always the cold desert we see today. Evidence suggests that water once flowed on the Red Planet’s surface, billions of years ago. Such a scenario necessitates a thick atmosphere to prevent the water from freezing. However, around 3.5 billion years ago, the water vanished, and the carbon dioxide-rich atmosphere thinned drastically, leaving behind the faint trace we observe today. The question of where Mars’ atmosphere went has puzzled scientists for years.
Two geologists from MIT believe the answer might lie in the planet’s clay. Their paper, published in Science Advances, proposes that a significant portion of Mars’ missing atmosphere could be locked within its clay-covered crust.
The team suggests that liquid water, present on ancient Mars, interacted with specific rock types, triggering a chain reaction. This process pulled carbon dioxide from the atmosphere and converted it into methane, a form of carbon that could be stored within the planet’s clay surface for billions of years.
Similar processes occur on Earth, and the researchers applied this knowledge to Mars. They determined that, based on the estimated amount of clay covering Mars, the planet’s crust could hold up to 1.7 bar of carbon dioxide. This amount is equivalent to roughly 80 percent of Mars’ presumed early atmosphere.
The researchers even propose that this sequestered Martian carbon could potentially be extracted and converted into fuel for future missions between Mars and Earth.
“Based on our findings on Earth, we show that similar processes likely operated on Mars and that copious amounts of atmospheric CO2 could have transformed to methane and been sequestered in clays,” says study author Oliver Jagoutz, professor of geology at MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS). “This methane could still be present and maybe even used as an energy source on Mars in the future.”
Jagoutz’s research group at MIT focuses on understanding the geological processes that shape Earth’s lithosphere. In 2023, their work highlighted smectite, a type of clay mineral, as a highly effective carbon trap. Each smectite grain contains numerous folds where carbon can remain undisturbed for billions of years. They demonstrated that smectite on Earth likely formed through tectonic activity, and once on the surface, these clays drew down atmospheric carbon dioxide, contributing to global cooling over millions of years.
Upon observing a map of Mars, Jagoutz noticed that a large portion of the planet’s surface was covered in smectite clays. This observation led them to investigate whether these clays could have played a similar carbon-trapping role on Mars.
While Earth’s smectite formation is linked to tectonic activity, Mars lacks such processes. The team explored alternative formation pathways based on existing knowledge of Mars’ history and composition. Remote measurements suggest that parts of Mars’ crust contain ultramafic igneous rocks, similar to those that produce smectites on Earth through weathering. Additionally, geological patterns resembling ancient rivers and tributaries indicate potential areas where water flow could have interacted with underlying rock.
Using a model based on the interaction between igneous rocks and their environment on Earth, Jagoutz and Murray simulated how water might have reacted with Mars’ deep ultramafic rocks. They focused on olivine, a mineral abundant in Mars’ crust, and estimated the changes it might undergo assuming the presence of surface water and a carbon dioxide-rich atmosphere for at least a billion years.
“At this time in Mars’ history, we think CO2 is everywhere, in every nook and cranny, and water percolating through the rocks is full of CO2 too,” Murray says.
Their model suggests that over a billion years, water filtering through the crust would have reacted with olivine. Oxygen molecules from the water would have bonded with iron in the olivine, releasing hydrogen and forming the oxidized iron responsible for Mars’ red hue. This free hydrogen would then combine with carbon dioxide in the water, forming methane. Over time, olivine would transform into serpentine, another iron-rich rock, which would further react with water to produce smectite.
“These smectite clays have so much capacity to store carbon,” Murray says. “So then we used existing knowledge of how these minerals are stored in clays on Earth, and extrapolate to say, if the Martian surface has this much clay in it, how much methane can you store in those clays?”
Their calculations showed that a 1,100-meter-deep layer of smectite covering Mars could store a vast amount of methane, potentially equivalent to most of the carbon dioxide thought to have been lost from the planet’s atmosphere.
“We find that estimates of global clay volumes on Mars are consistent with a significant fraction of Mars’ initial CO2 being sequestered as organic compounds within the clay-rich crust,” Murray says.
In some ways, Mars’ missing atmosphere could be hiding in plain sight.
Bruce Jakosky, professor emeritus of geology at the University of Colorado and principal investigator on the Mars Atmosphere and Volatile Evolution (MAVEN) mission, commented on the study’s significance: “Where the CO2 went from an early, thicker atmosphere is a fundamental question in the history of the Mars atmosphere, its climate, and the habitability by microbes. Murray and Jagoutz examine the chemical interaction of rocks with the atmosphere as a means of removing CO2. At the high end of our estimates of how much weathering has occurred, this could be a major process in removing CO2 from Mars’ early atmosphere.”
This research was supported, in part, by the National Science Foundation.
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