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Earth is the only habitable world we know of and it remains habitable because of natural cycles that maintain a balanced climate. Earth's carbon cycle plays a critical role in maintaining its temperate climate, and carbonate rocks are a big part of it. Carbonate rocks like limestone and dolomite are huge carbon sinks, and if their carbon was released into the atmosphere, Earth's temperature would spike catastrophically, rendering our planet uninhabitable. Conversely, if all of Earth's carbon were locked away in rock, Earth would likely become glaciated, photosynthesis would cease, and a mass extinction would leave extremophiles as the sole survivors of life's rich, living heritage.

As long as Earth's carbon keeps cycling between rock and atmosphere in a reasonable balance, the planet maintains its habitability.

With Earth's carbon cycle as an example, what can we learn about Mars? There's rock-solid evidence that Mars had habitable conditions in its past, though those conditions haven't persisted. The planet was once warm and wet and is now frigid and dry. What part did a carbon cycle play in Mars's habitability and uninhabitability?

New research in Nature says that Mars went through periods of habitability and uninhabitability due to carbon cycling. It's titled "Carbonate formation and fluctuating habitability on Mars," and the lead author is Edwin Kite. Kite is an associate professor of Planetary Science in the Department of Geophysical Sciences at the University of Chicago.

"The cause of Mars’s loss of surface habitability is unclear, with isotopic data suggesting a ‘missing sink’ of carbonate," the paper states. "Past climates with surface and shallow-subsurface liquid water are recorded by Mars’s sedimentary rocks, including strata in the approximately 4-km-thick record at Gale Crater." Gale Crater was chosen as MSL Curiosity's exploration site largely because Mt. Sharp rises more than 5 km and its layered slopes preserve a stratigraphic geological record of Mars's history. The layers of clays and sulphate-rich deposits show how the planet experienced periods of wetness. The researchers explain that the water was patchy and intermittent, and persisted late into the planet's history.

This figure from 2021 research shows some of the detail in Mt. Sharp's stratigraphic layers. It shows the ancient conditions in which each layer of the mountain formed. Research shows that Mars had alternating periods of wet and dry until it dried out completely about 3 billion years ago.

Image Credit: NASA/JPL-Caltech/MSSS/CNES/CNRS/LANL/IRAP/IAS/LPGN

The researchers draw a parallel between Earth's and Mars's carbon cycles. They write that Mars's patchy and intermittent surface water is best explained by a carbon cycle that locks carbon away into sedimentary carbonate rocks. The research is based on NASA's MSL Curiosity rover and its exploration of Gale Crater. It landed there almost 13 years ago to study the crater's geology. Among other findings, it measured carbonate materials in the crater and found that they make up 11% of the rocks in the region.

Mars once had a carbon-rich atmosphere, and the authors reference a paper by other researchers showing that stratigraphic layers and carbonate rocks in Gale Crater are clear evidence of that cycle. What drove the cycle?

"Here we show that a negative feedback among solar luminosity, liquid water and carbonate formation can explain the existence of intermittent Martian oases," Kite and his co-researchers write. They developed a model to illustrate and explain what happened to Mars.

The researchers say that as the Sun has brightened over billions of years, that increasing luminosity supported liquid surface water on Mars. Just like on Earth, available water combined with atmospheric carbon to form weak carbonic acid. That acid created carbonate weathering that acts as a natural thermostat by sequestering carbon into rock.

But things didn't end there. The atmosphere's loss of carbon reduced carbon dioxide's contribution to Mars's atmospheric pressure. The lower atmospheric pressure allowed water to more easily vaporize away into the atmosphere. The researchers say that Mars underwent cycles of wet periods and dry periods due to chaotic orbital forcing.

This figure shows histograms of the durations of wet events at Gale and globally in blue. Red shows durations of dry intervals within the time span of wet events. The three different lines of each type correspond to three different random orbital histories. Globally dry periods are sometimes very long and could have driven any surface life to extinction.

Image Credit: Kite et al. 2025. Nature.

Mars suffers from unpredictable and chaotic changes to its axial tilt that affects its climate, and this forcing drives Mars's carbon cycle and its ancient periods of wet habitability and dry uninhabitability. "The negative feedback restricted liquid water to oases and Mars self-regulated as a desert planet," the researchers explain. The researchers also explain that Gale Crater's stratigraphic record "...faithfully records the expected primary episodes of liquid water stability in the surface and near-surface environment."

During these cycles, the atmosphere eventually thickens and approaches water's triple point. The triple point is a specific combination of pressure and temperature wherein water can exist in equilibrium in all three phases: vapour, liquid, and solid. Water's triple point is 0.01 °C (273.16 K) and 611.73 pascals (0.006 atm). The researchers explain that this restricted the sustained stability of liquid water and the planet's surface habitability.

This figure from the research illustrates the researchers' model. a shows the distribution of carbonate detections in sedimentary rocks and soil on Mars, with yellow showing abundant detections, red showing no detections, and brown representing unexplored areas. b shows the fluxes and feedbacks for geologic carbon and climate regulation on Mars and Earth. On Earth, volcanic CO2 output is regulated by rapid carbonate formation. On Mars, solar brightening increased the temperature slowly and is balanced by slow carbonate formation. However, Mars' chaotic orbital forcing means that water is only available for carbonate formation intermittently during orbital optima, leading to intermittent periods of warmth and surface water.

Image Credit: Kite et al. 2025. Nature.

The researchers' model has limitations just as all models do. For example, it assumes that the carbonate content at Gale Crater is representative of the whole of Mars. For this reason, they present their research as a testable idea rather than a definitive conclusion.

"Carbonate formation and surface liquid-water availability are linked by a negative feedback that can explain fluctuating habitability on Mars," the authors write in their conclusion. They write that this cycle can potentially explain the intermittent and patchy nature of oases on Mars, and the sedimentary rocks that entomb those oases. They also say their model can explain how Mars's surface habitability came to an end, a question that has motivated scientists for a long time, and speaks to our wider questions about life elsewhere in the Universe.

How did Earth, alone among the Solar System's rocky planets, become the home for life? How, among all this frigid lifelessness, did our planet become warm, hospitable, and life-sustaining? The answer to these questions is complex and multi-faceted, and part of the answer comes from cosmochemistry, an interdisciplinary field that examines how chemical elements are distributed.

The Solar System is a busy place where everything is in motion. It was even more chaotic 4.5 billion years ago, with planets still forming and planetesimals and planetary embryos whizzing around and crashing into one another. Somehow, in all that chaos, Earth received more than its share of carbonaceous chondrites and the amino acids and other life-enabling chemicals that came with them.

Cosmochemistry studies have shown that between 5% and 10% of Earth's mass came from carbonaceous chondrites that crashed into the young planet. Studies also show that a large chunk of that came from the Theia impactor that created the Moon. To test these ideas more rigorously, a trio of researchers used dynamical simulations of the Solar System's formation to see if they could replicate it.

The research is titled "Dynamical origin of Theia, the last giant impactor on Earth." The lead author is Duarte Branco from the Institute of Astrophysics and Space Sciences at the Lisbon Astronomical Observatory in Portugal. The research will be published in the journal Icarus.

One of the critical distinctions in cosmochemistry is the difference between carbonaceous chondrites (CCs) and non-carbonaceous meteorites (NCs). It divides the Solar System's meteor population into two groups and suggests that the Solar System contains two distinct reservoirs of material. CCs formed further from the Sun, likely beyond Jupiter, and carry more volatiles like water and organic compounds with them. NCs include things like iron meteorites, and contain fewer volatiles.

In order to test the idea that Theia delivered CCs and volatiles to Earth, the researchers ran detailed simulations of the Solar System. These were N-body simulations of the later stages of the growth of terrestrial planets.

The simulations began in the late stages of planetary growth after the Solar System's gaseous disk was dispersed. The available solid mass was divided into planetesimals and planetary embryos. The simulation included CCs that were scattered inward as Jupiter and Saturn were still growing and accreting matter. Because of the size distinction between planetesimal and planetary embryos, embryos have a higher possibility of interacting with the terrestrial planets and delivering CC material.

The researchers ran three types of simulations. The first they call small only and includes only small CC objects, or planetesimals. The second they call large only and includes only large CC objects, planetary embryos. The third includes both CC planetesimals and embryos and is called the mixed scenario.

For a subset of 10 simulations from each of those scenarios, they included the effect of the giant planet dynamical instability. This is known as the "Nice model" in astronomy and describes how the giant planets shifted their orbits from where they initially formed.

The goal was to determine how CCs and NCs were distributed in the Solar System and to understand how Earth ended up with more CCs than the other rocky planets, especially Mars. The researchers also wanted to understand if the Theia impact could be responsible for delivering a large amount of Earth's CC material.

One clear result is that the role of giant planet instability, especially Jupiter's shift to a different orbit, had a pronounced effect on Earth's accretion of CC material.

This figure shows snapshots from the mixed simulation scenario without giant planet dynamical instability. In early times, CC objects and NC bodies mix together where the terrestrial planets are forming. Some CCs remained orbiting between planets or were still too far to collide. By the simulation's end, four terrestrial planets existed, including good analogues for Earth and Mars.

Image Credit: Branco et al. 2025. Icarus

When the researchers added giant planet dynamical instability, things looked even more interesting. "The giant planet instability dramatically changed the evolution of the system causing a strong pulse of eccentricity excitement, which lead to a wave of collisions and ejections," the authors write. However, the final state of the system didn't change much.

This figure shows eccentricity and position snapshots over the time of the simulation, including giant planet dynamical instability. The final snapshot is the real Solar System.

Image Credit: Branco et al. 2025. Icarus

A critical part of the simulations concerns the Theia impactor. Previous research suggests that Theia may have been a carbonaceous object. If that's true, much of Earth's life-giving habitability may have resulted from that collision.

"In the mixed scenario with no giant planet instability, Earth’s final impactor included a CC component in more than half of all simulations. In 38.5% of simulations, the final impactor was a pure CC embryo, and in another 13.5%, the impactor was an NC embryo that had previously accreted a CC embryo," the researchers write.

Overall, the simulations paint a picture of the early Solar System where two distinct rings of planetesimals. An inner ring consisting of rocky planetesimals and an outer ring of carbonaceous chondrites. Later, as the ice giants migrated inward, they propelled CC material into the inner Solar System. Some of these were trapped in the asteroid belt, while more massive ones were preferentially scattered into the orbits of the rocky planets. "The late-stage accretion of the terrestrial planets involved a series of giant impacts between NC embryos and planetesimals, with occasional impacts of CC objects," the authors explain.

This scenario explains several things about the Solar System. It explains the masses and orbits of the terrestrial planets, and the orbital distribution of asteroids. It also matches the CC mass fraction of Earth and Mars, where Mars lacks the same concentrations of CC material as Earth. If the small only simulation were correct, where CC material was only in the form of planetesimals, the CC mass fraction of Mars and Earth would be roughly the same.

This figure compares the timing of the last giant impacts in 10 mixed simulations that were run both with and without the giant planet instability. The black line represents the point where both values are equal. Each point has two halves with the left half representing the impactor type in the simulation without the giant planet instability and the right half representing the simulation with the giant planet instability. Dry NC impactors are black, CC embryos are blue and CC+NC mixed embryos are green.

Image Credit: Branco et al. 2025. Icarus

The researchers set out to show that, in line with other research, Theia could've been Earth's final large impactor and that it contained ample CC material. They appear to have succeeded.

In the simulations, Earth's final giant impact was with Theia, and that object had higher concentrations of CC material which helped make Earth habitable. That result is in line with scientific thinking. The work shows that the last impact was after between 5 to 150 million years after gas dispersal. A large fraction of those were within 20 to 70 million years. There are uncertainties in the timing of the Theia impact and these results work within those.

The simulations also support other conclusions showing that CC embryos and planetesimals could've been accreted throughout Earth's growth, but were concentrated in later phases of growth.

"Within the context of this scenario, the last giant impactor on Earth contained a CC component in roughly half of all of the mixed simulations," the authors write. "In the majority of these (38% of simulations), Theia was a pristine CC embryo, and in the remainder of cases Theia was an NC embryo that had previously accreted a CC embryo."

The research also shows that Jupiter played an important role in the Solar System's architecture. It not only truncates the asteroid belt, but played an important role in determining the final composition of the terrestrial planets by scattering CC material from the outer Solar System into the path of the rocky planets, especially Earth.

A million things had to be just right for Earth to become the life-sustaining world it is today. How likely it is that there are other worlds out there like it is unknown. It may take more than being in a habitable zone for an exoplanet to support life. There may be a bewildering number of variables that have to go right, including outer giant planets that migrate and deliver carbon to rocky worlds in habitable zones.

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