Scientists Suspected That Mars Was Once a Habitable Planet. NASA’s Curiosity Rover Has Just Cleared Up Any Doubts

NASA’s Curiosity rover recently provided some of the strongest evidence to date of Mars’ past habitability, highlighting the fragility of this once-habitable ecosystem.

The discovery. Curiosity landed on Mars on Aug. 6, 2012. Despite being covered in dust and wearing worn-out wheels, it continues to explore the Gale Crater methodically. It recently made a significant discovery in a 290-foot stretch of rock strata on the slopes of Mount Sharp.

A study published in Science revealed that Curiosity’s X-ray crystallography and gas analysis instruments detected substantial amounts of siderite, an iron carbonate mineral, within sulfate-rich rocks.

Why it matters. Although scientists have found carbonates on Mars before, this discovery is remarkable for several reasons. First, NASA noted the abundance of siderite, with samples showing it comprises between 4.8% and 10.5% of their weight. These samples have been named Tapo Caparo, Ubajara, and Sequoia.

Second, the siderite is almost pure ferrous carbonate, containing very little magnesium or calcium. This contrasts with other Martian carbonates. Third, the siderite’s presence alongside highly soluble salts such as calcium and magnesium sulfates suggests that an ancient evaporation process occurred.

Insights into ancient Mars. The formation of siderite suggests that Mars’ atmosphere had enough carbon dioxide to dissolve in water and react with rocks. Carbon dioxide became “trapped” within the rocks. Gale Crater was once a lake that gradually dried up, leaving behind layers of different salts and minerals.

The study estimates that the recent samples could contain between 2.6 and 36 millibars of atmospheric carbon dioxide. This is up to six times the current carbon dioxide pressure on Mars. The recent discovery confirms that a significant carbon reservoir once interacted with surface water, a critical factor for habitability.

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An incomplete cycle. However, the team also discovered evidence that some of the formed siderite was subsequently destroyed. A nearby sample, nicknamed Canaima, lacked siderite but contained abundant iron oxyhydroxides. Samples with siderite also contained these oxides in varying amounts.

Researchers believe this is due to a diagenesis process. Subsequent fluids interacted with the rocks, partially dissolving the siderite. This destruction oxidized the iron, forming the oxyhydroxides and releasing some of the previously sequestered carbon dioxide into the atmosphere. This cycle of formation (carbon sequestration) and partial destruction (carbon release) provides the best evidence for an ancient Mars carbon cycle.

However, the presence of siderite indicates that this process was partial and incomplete. More carbon was sequestered than was subsequently released. This contrasts with the carbon cycle on Earth, which has maintained a better balance over geological time.

Fragile habitability. The recent discovery reinforces the idea that early Mars was habitable. Liquid water interacted with a carbon dioxide-rich atmosphere until those warm, wet conditions ended. It shows that while the planet was habitable, that habitability is a very fragile state.

This may offer a lesson about Earth’s climatic stability in the face of climate change. Once again, Mars serves as a planetary laboratory to improve the scientists’ understanding of the evolution and fragility of habitable worlds.

Image | NASA

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