One of the biggest mysteries about Mars is where the oceans and atmosphere we know about ended up about 3.5 billion years ago. Just weeks after a study located a large amount of water in the planet’s interior that may have come from its ancient seas, a team of geologists from the Massachusetts Institute of Technology (MIT) is providing evidence that the remains of that ancient atmosphere may be hidden in plain sight, in the clay on its surface.
The study is published this Wednesday in the journal Scientific advances and its authors suggest that water could have seeped through certain types of rock and triggered a slow series of reactions that gradually pulled carbon dioxide out of the atmosphere and converted it into methane, a form of carbon that could be stored for eons in the planet’s surface. According to their calculations, and based on the amount of this material expected to cover Mars’ surface, the planet’s clay could contain up to 1.7 bars of carbon dioxide, which would be equivalent to about 80% of the planet’s initial, primitive atmosphere.
The researchers took their knowledge of similar interactions between rocks and gases that also occur on Earth and applied it to how they might develop on Mars. Based on these findings, it’s possible that this sequestered Martian carbon could one day be recovered and converted into propellant to power future Mars missions.
Large amounts of atmospheric CO2 could have been transformed into methane and sequestered in the clays
Olivier Jagoutz
— Study author and MIT geology professor
“Based on our findings on Earth, we show that similar processes likely occurred on Mars and that large amounts of atmospheric CO2 could have been transformed into methane and sequestered in clays,” says Oliver Jagoutz, author of the study and professor of geology in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS). “This methane could still be present and perhaps even used as an energy source on Mars in the future.”
In search of the lost atmosphere
Jagoutz’s group is looking to identify the geological processes and interactions that determine the evolution of Earth’s lithosphere, the hard, brittle outer layer that includes the crust and upper mantle, where tectonic plates meet. In 2023, he and the paper’s other co-author, Joshua Murray, focused on a type of shallow clay mineral called smectite, which is known to trap carbon very effectively. Within a single grain of smectite are a multitude of folds within which carbon can remain unchanged for billions of years.
The authors showed that smectite on Earth was likely a product of tectonic activity, and that once exposed on the surface, the clay minerals attracted and stored enough carbon dioxide from the atmosphere to cool the planet for millions of years. After reporting his results, Jagoutz looked at a map of the surface of Mars and realized that much of that planet’s surface was covered in the same smectite clays.
The question was clear: Could clays have had a similar carbon-fixing effect on Mars, and if so, how much carbon could the clays hold? “We know this process happens and is well documented on Earth. And these rocks and clays exist on Mars,” Jagoutz says. “So we wanted to try to connect the dots.”
Reaction with olivine
“At this point in Mars’ history, we think that CO2 is everywhere, in every nook and cranny, and that the water seeping through the rocks is also full of CO2,” Murray explains in an MIT press release. Over about a billion years, water seeping through the crust would have slowly reacted with olivine, an iron-rich mineral in reduced form. Oxygen molecules in the water would have bonded with the iron, releasing hydrogen and forming the rusty red iron that gives the planet its iconic color. This free hydrogen would then combine with carbon dioxide in the water to form methane. As this reaction progressed, the olivine would have slowly transformed into another type of iron-rich rock called serpentine, which would then have continued to react with the water to form smectite.
At this point in Mars’ history, we think CO2 is everywhere, in every nook and cranny, as well as in water seeping through rocks.
Joshua Murray
— Co-author of the study and researcher at MIT
“These smectite clays have a great capacity to store carbon,” Murray says. “So we took existing knowledge about how these minerals are stored in terrestrial clays and extrapolated to say, if the Martian surface has this much clay, how much methane can be stored in these clays?” He and Jagoutz found that if Mars were covered in a 1,100-meter-deep layer of smectite, that amount of clay could store a huge amount of methane, equivalent to most of the carbon dioxide in the atmosphere, which would have disappeared since the planet dried out.
“We found that estimates of global clay volumes on Mars are consistent with the idea that a significant fraction of Mars’ initial CO2 is sequestered as organic compounds in the clay-rich crust,” Murray says. “Somehow, Mars’ missing atmosphere could be hiding in plain sight.”
Energy for future missions
José Antonio Rodríguez Manfredi, a researcher at the Astrobiology Center (CAB-INTA-CSIC), believes that this article offers an interesting explanation for the loss of atmospheric carbon. “This was a key factor in the transition of Mars from a mild, warm and humid climate to the current arid, cold and dry one,” he says. “That is, the article suggests that the interactions between water and rocks played a fundamental role in the evolution of the Martian atmosphere.” On the other hand, he points out, this result seems to indicate that the methane produced and stored in clays could serve as a “local” energy source for future missions to Mars. “This would be very relevant for long-term exploration, for the propulsion of rockets and other surface vehicles, as well as for energy production systems.”
For geologist and popularizer Nahúm M. Chazarra, the paper’s proposal of a mechanism for capturing atmospheric CO2 through the chemical interaction between water and rocks would not be far-fetched and would help solve part of the mystery about the fate of Mars’ atmosphere. But it also allows us to raise useful questions for future exploration of the planet, he says. “The methane trapped in the rocks could be used as an energy source for future missions to Mars and perhaps to terraform Mars, because it is a powerful greenhouse gas.” And in turn, he adds, these technologies could help us capture atmospheric carbon dioxide here on Earth, which is the most urgent thing.
“This study not only sheds light on the climatic evolution of Mars, but also allows us to better understand the processes that could have affected other rocky planets in the early stages of their formation,” says Jorge Pla-García, a planetary science researcher at CAB-INTA-CSIC. On the other hand, he says, it is fascinating to think that the same reaction that contributed to the loss of carbon dioxide in Mars’ atmosphere could be an energy key for human exploration of the Red Planet. “Although on Earth, methane extraction requires high concentrations to be economically viable, on Mars, where resources are scarce, even small amounts could be of great value. “This scenario could change the viability of manned missions, making organic carbon extraction a crucial element of future space exploration.”
