NASA’s Curiosity rover, which landed on Mars in 2012, has made an important discovery that gives new clues about Mars' ancient environment. In 2022 and 2023, the rover drilled into rocks in Gale Crater—a large bowl-shaped area on Mars with a mountain in the centre—and found a mineral called siderite. This mineral is made of iron and carbon and is commonly found on Earth in places that were once underwater. Its presence on Mars is now helping scientists understand more about the planet’s wetter and possibly life-friendly past.

Siderite was found in rock samples from three different places inside Gale crater. The rover drilled about 3 to 4 centimetres deep into the rocks and studied their chemical makeup. The results showed that the rocks had up to 10.5% siderite by weight. This suggests that billions of years ago, Mars had a thick atmosphere full of carbon dioxide and also had water on its surface—like lakes, rivers, and maybe even oceans. These are conditions that could have supported simple life forms like microbes.

Why is this discovery so important?

Carbon dioxide (CO₂) is a gas that helps trap the sun’s heat, warming a planet’s surface. On Earth, this is called the greenhouse effect. Mars also had CO₂ in its atmosphere long ago. But today, the Martian atmosphere is very thin and has very little CO₂. Scientists have long wondered what happened to all that gas.

Credits to Curiosity’s findings, scientists believe that a large part of the CO₂ may have become locked inside rocks like siderite. On Earth, something similar happens when CO₂ from volcanoes mixes with water and minerals, forming carbonate rocks like limestone. Over time, this process stores carbon in the Earth’s crust. But Earth has plate tectonics—giant moving plates in its crust—that recycle the carbon back into the atmosphere through volcanic eruptions.

Mars, however, does not have plate tectonics. This means that once CO₂ was locked into carbonate rocks like siderite, it may have stayed there permanently. According to Benjamin Tutolo, a geochemist from the University of Calgary and lead author of the new study, “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 stabilise liquid water, why are there so few detections of carbonate minerals on the Martian surface?”

Tutolo explained that many scientists expected to find lots of carbonate minerals on Mars, based on models of the planet’s history. But earlier studies from both rovers and satellites found very little. So this new discovery of siderite is a big surprise and critical.

What does this say about Mars’ past environment?

The rocks containing siderite were formed about 3.5 billion years ago. At that time, Gale Crater is believed to have been a large lake. The climate was warmer, and the atmosphere was much thicker. Planetary scientist Edwin Kite, a co-author of the study, described the change since then as “the largest-known environmental catastrophe”.

He said, “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 discovery helps scientists better understand what happened to the carbon dioxide on Mars. Since the siderite is found in sedimentary rocks—rocks made from particles that settled in water—it also supports the idea that Mars had lakes or other bodies of water for a long time.

If similar rocks exist in other parts of Mars, then a large portion of the planet’s missing CO₂ may be hidden in the crust. This could explain how Mars went from being warm and wet to dry and cold over billions of years.

Tutolo said that this discovery changes the way scientists understand the Martian carbon cycle. Unlike Earth’s balanced carbon system, where carbon goes in and out of the atmosphere, Mars seems to have lost more carbon into rocks than it ever got back. “The important feature of the ancient Martian carbon cycle that we outline in this study is that it was imbalanced,” he explained.

This means that over time, carbon was locked into rocks and not released again, causing the planet to lose its warm climate. Tutolo added, “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.”

 

You might also be interested in: On his 70th birthday, NASA's oldest astronaut is set to come home from space