New Clues May Explain Why Mars’ Atmosphere Disappeared

Mars has the solar system’s most dramatic before-and-after photo. Billions of years ago, the Red Planet had rivers, long-lived lakes, mineral-rich groundwater and environments that may have been suitable for microbial life. Today, it is a frozen desert wrapped in an atmosphere so thin that an unprotected visitor would have considerably more urgent problems than getting red dust in their shoes.

Scientists have long agreed that ancient Mars possessed a thicker atmosphere, probably rich in carbon dioxide. That atmospheric blanket would have provided the pressure and greenhouse warming needed to keep at least some water liquid. The mystery was what happened next. Did the Sun sweep the atmosphere into space? Did chemical reactions bury it underground? Or did several planetary processes work together like an exceptionally efficient demolition crew?

Recent discoveries from NASA’s Curiosity rover, the MAVEN orbiter and laboratory-based geological models suggest the answer is not a single cinematic catastrophe. Mars may have lost its atmosphere through a combination of carbon being locked inside rocks and gases escaping into space after the planet’s magnetic protection weakened. In other words, part of the atmosphere may be gone, while another part is still on Mars—just hiding under the floorboards.

Mars Was Not Always the Cold Desert We See Today

Modern Mars has a thin atmosphere composed mostly of carbon dioxide, along with smaller amounts of nitrogen and argon. Surface pressure averages less than one percent of Earth’s sea-level pressure. Under such conditions, exposed liquid water is unstable: it tends to freeze, evaporate or boil away rather than collect in persistent lakes.

The ancient landscape tells a radically different story. Orbital photographs and rover observations show dried river valleys, lake sediments, deltas, flood channels and wave-shaped deposits. Gale Crater, explored by Curiosity, once contained a lake system. Jezero Crater, where Perseverance is working, preserves an ancient river delta. These formations required moving water and, in many cases, conditions lasting long enough to sort, transport and deposit enormous quantities of sediment.

A thicker atmosphere does not automatically guarantee tropical beaches and umbrella drinks. Early Mars was farther from a younger, dimmer Sun, and researchers still debate whether it remained continuously warm or experienced shorter wet periods triggered by volcanism, impacts, orbital changes or greenhouse gases. Nevertheless, the geological evidence makes one point difficult to avoid: ancient Mars maintained liquid surface water far more successfully than it does now.

The Missing Carbonate Problem

If early Mars had a dense carbon dioxide atmosphere and abundant water, scientists expected the planet to contain large deposits of carbonate minerals. Carbonates form when carbon dioxide, water and rock interact. On Earth, familiar examples include limestone and minerals that store carbon for millions of years.

The chemistry seemed straightforward. Atmospheric carbon dioxide dissolves in water, producing carbon-bearing compounds that react with elements such as calcium, magnesium or iron. The resulting carbon becomes incorporated into solid minerals. If Mars once carried a substantial carbon dioxide atmosphere, its rocks should contain the mineral equivalent of a very large receipt.

For decades, however, orbiting spacecraft and surface missions found less carbonate than many models predicted. Mars certainly had some carbonates, but not enough obvious deposits to account for the thick ancient atmosphere implied by its watery landscapes. This became known as the missing carbonate problem: the atmosphere appeared to have vanished without leaving the expected geological forwarding address.

Curiosity Finds Siderite Hidden in Sulfate-Rich Rocks

A major clue emerged from samples drilled by NASA’s Curiosity rover in Gale Crater. While climbing Mount Sharp, the layered mountain at the crater’s center, Curiosity investigated sulfate-rich rocks deposited during a period when the region was becoming increasingly dry.

Using its Chemistry and Mineralogy instrument, commonly called CheMin, the rover analyzed powdered material collected from several drill sites. The instrument identified substantial amounts of siderite, an iron carbonate mineral. Some samples contained unexpectedly high carbonate concentrations, including levels of roughly 5 to 10 percent by weight.

That discovery matters because siderite stores carbon that may once have circulated through the Martian atmosphere. The carbonate was mixed into sulfate-bearing rocks, where its spectral signature could be difficult for orbiting instruments to detect. Researchers had not necessarily been looking in the wrong places; the carbonates may have been wearing geological camouflage.

Why Satellites May Have Missed the Carbonates

Orbiters frequently identify minerals by measuring how sunlight reflects from the surface at different wavelengths. Each mineral produces a characteristic spectral pattern, rather like a chemical barcode. The method is powerful, but it is most effective when the target mineral is exposed, abundant and not masked by dust or neighboring compounds.

Curiosity drilled several centimeters into the rock, reaching material hidden from ordinary orbital observations. Its findings suggest that siderite may exist beneath weathered surfaces or alongside sulfates that obscure carbonate signatures in near-infrared data. If similar sulfate-rich layers across Mars also conceal siderite, the planet’s crust could contain far more carbon than surface maps have indicated.

The discovery does not prove that every missing molecule of ancient carbon dioxide is trapped as siderite. Calculations based on the Gale Crater deposits suggest that hidden carbonates could account for a meaningful fraction of the former atmosphere, but probably not all of it. The remainder may be stored in other minerals, buried more deeply or lost to space. Science has supplied a strong clue, not a tidy confession signed by Mars.

How Carbonate Formation Could Have Cooled the Planet

Siderite may explain more than where atmospheric carbon went. Its formation could have contributed directly to Mars’ climate collapse.

Carbon dioxide is a greenhouse gas. A sufficiently thick carbon dioxide-rich atmosphere can trap heat and increase surface pressure, making liquid water more stable. When water reacts with rock and converts atmospheric carbon into carbonate minerals, that carbon dioxide is removed from the air. If the gas is not replaced quickly enough by volcanic activity, atmospheric pressure falls and greenhouse warming weakens.

Earth avoids permanent atmospheric depletion partly because it has an active long-term carbon cycle. Carbon is stored in ocean sediments and rocks, carried into the interior by plate tectonics and eventually released again through volcanoes. Mars has no modern system of moving tectonic plates comparable to Earth’s. Once carbon became locked in the crust, the planet may have had no effective recycling service.

The result may have been a self-reinforcing transformation. Water encouraged chemical weathering. Weathering created carbonates. Carbonate formation removed carbon dioxide. Less carbon dioxide produced colder, lower-pressure conditions. Surface water then became harder to maintain, reducing the processes that had once supported a potentially habitable climate. Mars did not simply misplace its thermostat; parts of the thermostat may have turned into rock.

Another Possibility: Carbon Trapped in Clay Minerals

Siderite is not the only candidate reservoir. MIT researchers have proposed that a large quantity of early atmospheric carbon may have been captured through reactions involving olivine-rich rocks and water.

Olivine is an iron- and magnesium-rich mineral found in volcanic rocks. When water moved through the ancient Martian crust, it could have altered olivine into clay minerals such as smectite. During this process, iron released from the rock may have interacted with carbon dioxide and hydrogen-bearing compounds, potentially producing methane.

That methane could then have become trapped within the layered structure of clay minerals. Modeling suggested that suitable clay-rich regions might theoretically store an amount of carbon equivalent to a large portion of Mars’ early atmosphere—possibly as much as about 80 percent under favorable assumptions. The estimate is not a direct measurement of a planet-wide methane warehouse, but it demonstrates that crustal chemistry could hide a surprisingly large carbon reservoir.

This clay-storage hypothesis and the siderite discovery are not necessarily competitors. Mars has had billions of years, vast volcanic provinces and many water-rock environments. Some atmospheric carbon may have become iron carbonate, some may have entered clay-associated methane, and some may remain in deposits that no rover has sampled.

The Sun Was Also Stealing Mars’ Air

Rock chemistry cannot tell the entire story because spacecraft have directly observed gases escaping from Mars. The planet’s atmosphere is still leaking today.

Early Mars generated a global magnetic field through motion in its interior. That magnetic shield helped deflect charged particles streaming from the Sun. As the small planet cooled, its internal dynamo weakened and the global field largely disappeared. The upper atmosphere was left much more exposed to solar wind, ultraviolet radiation and energetic solar storms.

NASA’s MAVEN mission was designed to study this interaction. Its measurements indicate that solar activity drives several escape processes capable of removing atmospheric particles. Isotope ratios, especially those involving argon, carbon and hydrogen, preserve evidence that lighter atoms escaped more readily, leaving heavier versions behind. This is the atmospheric equivalent of finding only the slow runners after everyone else has fled the building.

MAVEN Directly Observes Atmospheric Sputtering

In 2025, MAVEN researchers announced the first direct observation at Mars of atmospheric sputtering, a process scientists had predicted for years.

Sputtering begins when solar wind particles interact with the Martian upper atmosphere. Charged atmospheric particles can be accelerated by electric and magnetic fields and then slam back into neutral atoms. Those collisions transfer enough energy to knock atoms into space, much as a cue ball can scatter other balls across a pool table—except the table is a planet and the balls never come back.

Researchers used MAVEN measurements to map the distribution and velocity of argon atoms high above Mars. The observed patterns matched the distinctive signature expected from sputtering. The process was especially intense during periods of strong solar activity, suggesting that the young Sun, which was more active than it is today, could have removed atmospheric gas much faster in the distant past.

Water Escaped Along With the Atmosphere

The loss of atmospheric protection also affected Martian water. Water vapor reaching high altitudes can be split apart by ultraviolet radiation. Lightweight hydrogen escapes relatively easily, while some oxygen is removed through other chemical and physical mechanisms.

Observations from MAVEN and the Hubble Space Telescope show that the rate of hydrogen escape changes with Mars’ distance from the Sun, seasons, dust activity and atmospheric heating. Global dust storms can carry water vapor to unusually high altitudes, giving sunlight more opportunities to break the molecules apart. Over immense stretches of time, this gradual leakage helped turn rivers and lakes into dry channels, buried ice and atmospheric traces.

So Where Did Mars’ Atmosphere Actually Go?

The strongest current answer is: in more than one direction.

A portion of the ancient carbon dioxide appears to have reacted with water and rock, becoming trapped in carbonate minerals such as siderite. Another portion may be associated with clay-forming reactions or stored in deeper crustal deposits. Meanwhile, solar wind, ultraviolet radiation and sputtering removed gases from the top of the atmosphere, particularly after Mars lost its global magnetic shield.

The proportions remain uncertain. Gale Crater represents one location on a diverse planet, and Curiosity’s drill holes are tiny compared with the continent-scale deposits researchers must evaluate. Siderite abundance may vary widely from region to region. Some deposits could be buried too deeply for orbital instruments, while others may have been altered or destroyed by later chemistry.

Atmospheric escape estimates also depend on how conditions changed through time. The young Sun was more active, Mars’ magnetic field did not vanish instantaneously, volcanic eruptions periodically replenished gases and large impacts could both remove and release atmospheric material. Reconstructing the process is less like solving a simple equation and more like balancing a four-billion-year bank account after someone misplaced most of the statements.

Why the Discovery Matters for the Search for Life

Understanding atmospheric loss helps scientists estimate how long potentially habitable environments survived. A lake that existed for a few centuries presents different biological possibilities from one sustained or repeatedly renewed over millions of years.

Carbonate minerals are particularly valuable because they can preserve information about the water, temperature, atmospheric composition and chemical conditions present when they formed. Their isotopes may reveal whether ancient environments underwent extreme evaporation, alternating wet and dry periods or interactions with cold, salty water. Some carbonate-bearing rocks may also preserve organic compounds or microscopic textures relevant to the search for ancient life.

The new results also improve future exploration strategies. Scientists can reconsider sulfate-rich terrain that once seemed unpromising for carbonates. Rovers may target buried layers, fractures and altered volcanic rocks where atmospheric carbon could be concealed. Returned Martian samples would allow laboratories on Earth to measure mineral structures and isotopic compositions with far greater precision than any compact rover instrument.

What Mars Can Teach Us About Habitable Planets

Mars demonstrates that habitability is not a permanent status awarded to a planet at birth. It is a balance among atmosphere, geology, water, solar radiation, planetary size and internal activity.

Earth has retained a thick atmosphere, a strong magnetic field, liquid oceans and an active geological carbon cycle. Mars began with some similar ingredients but followed a different path. Its smaller size allowed its interior to cool more quickly. Its global magnetic dynamo faded. Carbon became buried without being efficiently recycled, and the upper atmosphere was increasingly exposed to the Sun.

This history influences how researchers evaluate rocky planets around other stars. Detecting a planet in a star’s habitable zone does not guarantee that the world has kept its atmosphere. Scientists must also consider stellar activity, magnetic shielding, geological recycling and whether gases are being replenished as quickly as they are removed.

Conclusion: Mars’ Missing Atmosphere May Be Both Buried and Lost

The discovery of siderite beneath sulfate-rich rocks offers one of the clearest explanations yet for Mars’ missing carbonates. It indicates that part of the planet’s carbon dioxide-rich atmosphere was converted into solid minerals and hidden from orbital instruments. Clay alteration may have stored additional carbon, while MAVEN’s observations confirm that solar-driven processes carried atmospheric particles into space.

Mars therefore may not have suffered one sudden atmospheric disappearance. Its climate probably unraveled through a long sequence of chemical reactions, internal cooling, weakening magnetic protection and relentless solar erosion. The new clues do not close the investigation, but they connect pieces that once appeared contradictory.

The Red Planet’s lost atmosphere was apparently not careless enough to leave all its evidence in one place. Fortunately, scientists have rovers with drills, orbiters with particle detectors and the patience to investigate a mystery measured in billions of years.