The Curiosity rover unearthed evidence pointing to an active carbon cycle on ancient Mars, suggesting the Red Planet was once habitable. The rover discovered carbon deposits, providing the first in situ evidence that an active carbon cycle occurred billions of years ago and indicating that carbon dioxide was once trapped in Martian rocks.
In Gale Crater, Curiosity found nearly pure crystalline siderite, an iron carbonate mineral, filling a gap in the previously low quantities of carbonates detected on Mars. The discovery suggests that carbon was sequestered in the planet's rocks, challenging previous satellite data and indicating that Mars may have a wealth of hidden carbonate deposits, particularly in regions rich in magnesium sulfate.
"The discovery of abundant siderite in Gale Crater represents both a surprising and important breakthrough in our understanding of the geologic and atmospheric evolution of Mars," said Benjamin Tutolo, a geochemist and lead author of the study published in Science. The presence of siderite in magnesium sulfate-rich layers supports the idea that Mars once had conditions suitable for liquid water.
The new findings reveal that the ancient Martian carbon cycle was unbalanced, with more carbon dioxide sequestered in rocks than released back into the atmosphere, likely due to volcanic activity. Researchers believe that reactions between water and rock, along with evaporation processes, led to the formation of siderite, indicating that CO₂ from the atmosphere was captured in the sedimentary rocks.
Scientists have long believed that Mars once had a thick, carbon dioxide-rich atmosphere and flowing liquid water on its surface, which would have required a much warmer climate than the planet has today. The discovery of carbonate minerals like siderite suggests that there was enough carbon dioxide in the atmosphere to support liquid water on the planet's surface, providing proof that Mars was habitable.
The Curiosity rover analyzed the mineralogy of these samples using its onboard X-ray diffractometer, CheMin, which uses X-ray diffraction to determine the mineral structure of the rocks. The samples obtained by the rover contained up to 10.5 percent siderite, providing additional weight to the idea that Mars once had a carbon dioxide-rich atmosphere.
"The broader implications are that the planet was habitable up until this time, but then, as the CO₂ that had been warming the planet started to precipitate as siderite, it likely impacted Mars' ability to stay warm," Tutolo added. This process possibly contributed to the loss of habitability in the long term.
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