A mineral called siderite, found abundantly in rock samples drilled by a National Aeronautics and Space Administration (Nasa) rover on Mars, is providing compelling new evidence of the planet’s warmer and wetter ancient history, a time when it possessed significant bodies of water and potentially supported life.
The Curiosity rover, which landed on Mars in 2012 with the mission to determine if Earth’s neighboring planet ever had the conditions suitable for microbial life, discovered the mineral in rock samples obtained from three different locations within Gale crater in 2022 and 2023. Gale crater is a large impact basin featuring a central mountain.
Siderite is an iron carbonate mineral. Its presence within sedimentary rocks formed billions of years ago suggests that Mars once had a dense atmosphere rich in carbon dioxide, a greenhouse gas that would have warmed the planet sufficiently to sustain liquid water on its surface.
The Martian landscape exhibits features that many scientists interpret as signs of past liquid water flow, with potential oceans, lakes, and rivers considered as possible habitats for ancient microbial life.
Carbon dioxide is the primary climate-regulating greenhouse gas on Earth, as it is on Mars and Venus. Its presence in the atmosphere traps solar heat, leading to a warmer climate.
Until now, direct evidence indicating a carbon dioxide-rich early Martian atmosphere has been limited.
The prevailing hypothesis suggests that the atmosphere evolved from a thick, carbon dioxide-rich state to a thin, carbon dioxide-depleted state – for reasons not fully understood – and during this transition, carbon became trapped within the planet’s crust as carbonate minerals through geochemical processes.
The samples collected by Curiosity, which drills 1.2 to 1.6 inches (3-4 centimeters) into rock to analyze its chemical and mineral composition, lend strong support to this idea. These samples contained up to 10.5% siderite by weight, as measured by an instrument aboard the car-sized, six-wheeled rover.
“One of the longstanding mysteries in the study of Martian planetary evolution and habitability is: if large amounts of carbon dioxide were required to warm the planet and stabilize liquid water, why are there so few detections of carbonate minerals on the Martian surface?” said University of Calgary geochemist Benjamin Tutolo, a participating scientist on Nasa’s Mars Science Laboratory Curiosity rover team and the lead author of the study published on Thursday in the journal Science.
Tutolo added, “Models predict that carbonate minerals should be widespread. But, to date, rover-based investigations and satellite-based orbital surveys of the Martian surface had found little evidence of their presence.”
Given that rock similar to the rover’s samples has been identified globally on Mars, the researchers suspect that these regions also contain significant amounts of carbonate minerals and may hold a substantial portion of the carbon dioxide that once warmed the planet.
The sedimentary rocks within Gale crater – sandstones and mudstones – are believed to have been deposited approximately 3.5 billion years ago, when the crater was the site of a lake and before the Martian climate underwent a dramatic transformation.
“The shift of Mars’ surface from more habitable in the past, to apparently sterile today, is the largest-known environmental catastrophe,” said planetary scientist and study co-author Edwin Kite of the University of Chicago and Astera Institute.
Kite further stated, “We do not know the cause of this change, but Mars has a very thin carbon dioxide atmosphere today, and there is evidence that the atmosphere was thicker in the past. This puts a premium on understanding where the carbon went, so discovering a major unsuspected deposit of carbon-rich materials is an important new clue.”
The rover’s findings provide valuable insights into the carbon cycle on ancient Mars.
On Earth, volcanoes release carbon dioxide into the atmosphere, and this gas is absorbed by surface waters – primarily the ocean – where it combines with elements like calcium to form limestone rock. Through the geological process of plate tectonics, this rock is reheated, and the carbon is eventually released back into the atmosphere through volcanism. However, Mars lacks plate tectonics.
“The important feature of the ancient Martian carbon cycle that we outline in this study is that it was imbalanced. In other words, substantially more carbon dioxide seems to have been sequestered into the rocks than was subsequently released back into the atmosphere,” Tutolo explained.
Tutolo concluded, “Models of Martian climate evolution can now incorporate our new analyses, and in turn, help to refine the role of this imbalanced carbon cycle in maintaining, and ultimately losing, habitability over Mars’ planetary history.”
