The early Martian atmosphere was once flush with carbon dioxide, but over time this thick atmosphere thinned dramatically. Where did all that carbon go? The answer may lie in the very rocks and minerals that coat the Martian surface.

Geologists Joshua Murray and Oliver Jagoutz from the Massachusetts Institute of Technology have uncovered a potential solution to this longstanding mystery. Their new research, published in the journal Science Advances, suggests that the alteration of iron and magnesium-rich rocks on Mars could have produced substantial amounts of abiotic (non-biological) methane. This methane may have then become trapped and preserved within clay minerals across the Martian crust, representing a previously unrecognized carbon reservoir.

“On Earth, we know that the hydrothermal alteration of ultramafic rocks can produce abiotic methane,” explains Murray. “And we also know that clay minerals on Mars, like smectite, have a high capacity to adsorb and protect organic carbon compounds. So we wanted to explore whether this process could have played a major role in the loss of Mars’ early atmosphere.”

The early Martian atmosphere is estimated to have contained between 0.25 to 4 bars of carbon dioxide – much thicker than the wispy 0.054 bars that exist today. This dramatic thinning has puzzled planetary scientists for decades, as models of atmospheric escape to space cannot fully account for the missing carbon.

“The challenge is that the known rates of carbon loss to space are orders of magnitude too low to explain the disappearance of that much CO2,” says Jagoutz. “There has to be some other major carbon sink that we’re missing.”

Enter the clay minerals. As water-rock reactions alter ultramafic rocks rich in iron and magnesium on Mars, they can produce both serpentine (a greenish hydrous silicate mineral) and smectite clays. These clays have an incredibly high surface area, providing ample opportunity to adsorb and protect organic molecules.

“Essentially, the oxidation of iron in olivine during serpentinization releases hydrogen, which can then react with CO2 to form methane,” explains Murray. “And that methane can become trapped and preserved within the interlayer spaces of the clay minerals that form.”

Using a mass balance model, the researchers calculate that a global layer of serpentine just 2 kilometers thick could have reduced around 5 bars of atmospheric CO2 to methane. And the clay minerals that form from the alteration of these ultramafic rocks, especially smectite, have the capacity to store a staggering amount of this organic carbon.

“We estimate that between 0.07 to 1.7 bars worth of CO2 could have been sequestered as adsorbed methane in the Martian crust,” says Jagoutz. “That’s a huge potential reservoir, one that could go a long way towards explaining the missing carbon.”

But the implications of this work extend beyond just the carbon cycle. The researchers also show that the formation and preservation of this abiotic methane could have significantly impacted the isotopic composition of Mars’ atmosphere over time.

“Methane formation preferentially incorporates the lighter carbon-12 isotope,” explains Murray. “So as atmospheric CO2 gets converted to methane and buried in the crust, you’d expect to see the remaining CO2 become progressively enriched in the heavier carbon-13 isotope.”

Indeed, the team’s models indicate that for their “best estimate” of clay volumes, the atmospheric δ13C (the ratio of carbon-13 to carbon-12) could have been enriched by 1.9 to 14 per mil. This aligns remarkably well with measurements of the modern Martian atmosphere, which shows a δ13C value of 48 per mil – a far cry from the -30 to -20 per mil range expected for the planet’s primordial mantle composition.

“The fact that our abiotic methane model can account for a large fraction of this isotopic enrichment is really exciting,” says Jagoutz. “It provides a potential explanation for this long-standing mystery in Martian geochemistry.”

Importantly, the researchers note that their estimates of the organic carbon reservoir size are conservative. The clay minerals, especially smectite, have an even greater capacity to adsorb and protect polar organic molecules beyond just methane.

“Methane is a relatively simple, non-polar compound,” explains Murray. “But we know that Mars’ mudstones contain much more complex organic signatures. If those kinds of compounds were also being stabilized on clay surfaces, the total organic carbon reservoir could be even larger.”

This has intriguing implications, not just for our understanding of Mars’ past, but also for future exploration and potential resource utilization. If substantial amounts of organic carbon are indeed sequestered in the Martian crust, it could provide a valuable fuel source for future robotic and human missions.

“Methane is an incredibly useful propellant for spacecraft,” says Jagoutz. “And if we can tap into these buried organic carbon reserves, it could dramatically reduce the amount of material we’d need to launch from Earth to support long-term exploration of Mars.”

Beyond that, the team’s findings also shed new light on the fundamental processes that shape the habitability of rocky planets more broadly. On Earth, the cycling of carbon between the atmosphere, oceans, and crust is intimately tied to the operation of plate tectonics – a process that continuously renews the surface and recycles carbon.

But Mars, lacking active plate tectonics, appears to have developed a very different carbon cycle. Here, the alteration of ultramafic rocks and the subsequent trapping of organic carbon in clay minerals may have represented a quasi-permanent sink, with implications for the planet’s climate evolution.

“What we’re seeing on Mars is a snapshot of what can happen on a rocky planet without plate tectonics,” says Murray. “The adsorption of organic carbon onto clay surfaces may be a fundamental process in the atmospheric evolution of planets, one that’s intimately linked to the nature of a planet’s geology.”

This, in turn, has profound implications for the search for life elsewhere in the universe. After all, the preservation of organic compounds is a key prerequisite for the emergence and persistence of biology. And on a tectonically inactive world like Mars, mineral-protected organic carbon may represent one of the few avenues for such preservation.

“If we find evidence of complex organic molecules in Martian mudstones, it could be a tantalizing sign that life may have once taken hold there,” says Jagoutz. “But even in the absence of biology, these abiotic organic reservoirs are fascinating in their own right, providing a window into the early evolution of terrestrial planets.”

As the Perseverance rover continues to explore Jezero Crater and the Curiosity rover keeps uncovering new organic signatures, the hunt for Martian carbon is only just beginning. But with this new model of abiotic methane production and clay mineral adsorption, planetary scientists now have a powerful framework for understanding how the fate of that carbon may have played out over billions of years.

“Mars is this incredible natural laboratory for studying the co-evolution of a planet’s geology, climate, and potential for life,” concludes Murray. “And by unraveling the complex dance between water, rocks, and carbon, we’re getting closer to piecing together Mars’ remarkable story.”

Reference(s)

1. Murray and Jagoutz, Sci. Adv. 10, eadm8443 (2024) 25 September 2024

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