A new study reveals that Mars’ missing atmosphere—once thick and supportive of water, but significantly diminished about 3.5 billion years ago—might be hidden in the planet’s clay-rich crust. Water on ancient Mars could have triggered a series of chemical reactions that pulled CO2 from the atmosphere and transformed it into methane, which became trapped in the planet’s clay minerals.
Mars wasn’t always the cold, barren desert we see today. Evidence suggests that billions of years ago, rivers and lakes once flowed across the surface of the Red Planet. If there was liquid water, there must have been a dense atmosphere to prevent it from freezing. Yet, around 3.5 billion years ago, the water disappeared, and the carbon dioxide-heavy atmosphere thinned dramatically, leaving behind only a faint trace of what was once there.
So, where did Mars’ thick atmosphere go? This has been one of the most puzzling mysteries of Mars’ 4.6-billion-year history.
According to two MIT geologists, the answer might lie beneath the planet's surface, hidden in the clay that covers large parts of Mars. In a paper published in Science Advances, they propose that much of the missing atmosphere could be locked up in Mars’ clay-rich crust.
Mars’ Clay as a Carbon Storehouse
The researchers argue that when water was present on Mars, it likely seeped through the rocks, sparking a slow chain reaction that pulled carbon dioxide from the atmosphere and converted it into methane. This methane, in turn, was stored in the planet’s clay minerals for billions of years.
This process isn’t unique to Mars. Similar reactions occur on Earth, and the researchers used their understanding of Earth’s rock-gas interactions to model what might have happened on Mars. Their findings suggest that Mars’ clay could hold up to 1.7 bar of carbon dioxide, which would account for around 80% of the planet’s early atmosphere.
This trapped carbon could one day be used as a resource. The researchers speculate that future Mars missions might be able to extract and convert this carbon into methane fuel for return trips to Earth.
“We’ve shown that similar processes, which sequester CO2 into clay minerals on Earth, likely occurred on Mars,” says Oliver Jagoutz, professor of geology at MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS) and co-author of the study. “This methane could still be present and may even be used as a future energy source on Mars.”
The study's lead author, Joshua Murray, recently graduated with a Ph.D. from MIT’s EAPS department.
The Role of Clay in Mars' Geology
Jagoutz’s group at MIT specializes in studying the geological processes that shape Earth’s lithosphere—the hard outer shell that includes the crust and upper mantle. In 2023, he and Murray focused on smectite, a type of clay mineral known for being highly effective at trapping carbon. These clays form intricate folds, which can store carbon for billions of years.
Their previous research on Earth showed that smectite is created through tectonic activity and that once these minerals are exposed at the surface, they can absorb enough carbon dioxide to cool the planet over millions of years.
Jagoutz then noticed that large portions of Mars are covered in smectite clays. This led him to wonder: Could Mars' clays have stored carbon in a similar way? And if so, how much?
“We know these processes happen on Earth, and these clays exist on Mars,” Jagoutz said. “So, we wanted to connect the dots.”
A Different Geology Than Earth’s
Unlike Earth, where tectonic plate activity is responsible for creating smectite, Mars lacks such activity. However, the team considered other ways these clays might have formed, based on what scientists know about Mars’ history and composition.
For instance, remote measurements of Mars’ surface suggest that parts of its crust contain ultramafic igneous rocks, similar to those that form smectites on Earth through weathering. Additionally, geological features resembling ancient river systems suggest that water once flowed and reacted with Mars’ rock.
Jagoutz and Murray hypothesized that water interacting with Mars' deep ultramafic rocks could have led to the formation of the smectite clays now covering the surface. Using a simple model based on the interactions between igneous rocks and water on Earth, they estimated how olivine-rich rock might have reacted with water on early Mars, which was thought to be rich in carbon dioxide.
“At this point in Mars' history, CO2 was likely everywhere, permeating both the atmosphere and the water that flowed through the rocks,” Murray explains.
Over about a billion years, this water would have slowly reacted with olivine—a mineral rich in reduced iron. As oxygen from the water bonded with the iron, it released hydrogen and formed oxidized iron, giving Mars its iconic red color. This free hydrogen then combined with CO2 in the water, creating methane, while the olivine gradually transformed into another iron-rich mineral known as serpentine. Serpentine continued to react with water, eventually forming smectite.
“These smectite clays have a huge capacity to store carbon,” says Murray. “We used our knowledge of how these clays store carbon on Earth to estimate how much methane Mars’ clays could hold.”
Their findings indicate that if Mars’ surface is covered with smectite clay up to 1,100 meters deep, it could store an enormous amount of methane—equivalent to most of the carbon dioxide thought to have disappeared from Mars' early atmosphere.
“In some ways, Mars’ missing atmosphere might be hiding in plain sight,” Murray says.
Implications for Future Mars Missions
This research not only solves the mystery of Mars' missing atmosphere but also provides a potential resource for future explorers. The stored methane could be tapped into as a source of energy for human missions to Mars, offering a sustainable way to fuel future interplanetary travel.
Supported in part by the National Science Foundation, this groundbreaking work opens up new possibilities for understanding Mars’ past and utilizing its resources in the future.