A mineral called siderite found abundantly in rock drilled by a NASA rover on the surface of Mars is providing fresh evidence of the planet’s warmer and wetter ancient past when it boasted substantial bodies of water and potentially harboured life.
The Curiosity rover, which landed on Mars in 2012 to explore whether Earth’s planetary neighbour was ever able to support microbial life, found the mineral in rock samples drilled at three locations in 2022 and 2023 inside Gale crater, a large impact basin with a mountain in the middle.
Siderite is an iron carbonate mineral. Its presence in sedimentary rocks formed billions of years ago offers evidence that Mars once had a dense atmosphere rich in carbon dioxide, a gas that would have warmed the planet through the greenhouse effect to the point that it could sustain bodies of liquid water on its surface.
There are features on the Martian landscape that many scientists have interpreted as signs that liquid water once flowed across its surface, with potential oceans, lakes and rivers considered as possible habitats for past microbial life.
Carbon dioxide is the main climate-regulating greenhouse gas on Earth, as it is on Mars and Venus. Its presence in the atmosphere traps heat from the sun, warming the climate.
Until now, evidence indicating that the Martian atmosphere previously was rich in carbon dioxide has been sparse. The hypothesis is that when the atmosphere, for reasons not fully understood, evolved from thick and rich in carbon dioxide to thin and starved of this gas, the carbon through geochemical processes became entombed in rocks in the planet’s crust as a carbonate mineral.
The samples obtained by Curiosity, which drills 1.2 to 1.6 inches (3-4 centimetres) down into rock to study its chemical and mineral composition, lend weight to this notion. The samples contained up to 10.5% siderite by weight, as determined by an instrument onboard 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 lead author of the study published on Thursday in the journal Science.
“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,” Tutolo added.
Because rock similar to that sampled by the rover has been identified globally on Mars, the researchers suspect it too contains an abundance of carbonate minerals and may hold a substantial portion of the carbon dioxide that once warmed Mars.
The Gale crater sedimentary rocks – sandstones and mudstones – are thought to have been deposited around 3.5 billion years ago, when this was the site of a lake and before the Martian climate underwent a dramatic change.
“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.
“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,” Kite added.
The rover’s findings offer insight into the carbon cycle on ancient Mars.
On Earth, volcanoes spew carbon dioxide into the atmosphere, and the gas is absorbed by surface waters – mainly the ocean – and combines with elements such as calcium to form limestone rock. Through the geological process called plate tectonics, this rock is reheated and the carbon is ultimately released again into the atmosphere through volcanism. Mars, however, 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 said.
“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,” Tutolo added.
