Some of the biggest questions in planetary science are written across the dusty surface of Mars. Why did a planet that once had flowing rivers and lakes end up as a frozen desert? How did Mars lose its grip on liquid water – and what does that say about a planet’s ability to stay habitable?
A new study may bring us closer to an answer. Scientists from the University of Chicago have proposed a new model that explains why Mars couldn’t hold on to its early climate.
The mystery of Mars’ climate
The team suggests that Mars had short bursts of warmth triggered by gradual changes in sunlight. But each time conditions improved, the planet pushed itself right back into a deep freeze. That’s very different from Earth, which has stayed habitable for billions of years.
“For years, we’ve had this huge unanswered question for why Earth has managed to keep its habitability while Mars lost it,” said Edwin Kite, a participating scientist on NASA’s Curiosity rover and associate professor at the University of Chicago.
“Our models suggest that periods of habitability on Mars have been the exception, rather than the rule, and that Mars generally self-regulates as a desert planet.”
The case of the missing carbon
Curiosity’s recent discovery of carbonate-rich rocks on Mars helped make this new theory possible. These minerals were the missing link in a puzzle scientists have been trying to solve for years.
If Mars once had a thick, carbon dioxide-rich atmosphere – enough to warm the planet and allow water to flow – where did all that carbon go? “People have been looking for a tomb for the atmosphere for years,” Kite said.
Scientists had suspected that, like Earth, Mars might lock away carbon dioxide in its rocks through chemical reactions with water. But early tests by Mars rovers failed to find those carbonate deposits.
That changed when Curiosity reached higher elevations on Mt. Sharp and finally hit the carbonate-rich layers they’d been hoping to find.
“It really is something you cannot know until you have a rover on the surface,” said study co-author Benjamin Tutolo, a professor at the University of Calgary.
“The chemistry and mineralogy measurements they provide really are essential in our continuing quest to understand how and why planets stay habitable, in order to search for other hospitable worlds out in the universe.”
Mars’ climate cycle worked against life
Mars and Earth started out with a lot in common. Both are rocky planets. Both have water and carbon. Both are at a decent distance from the Sun. Yet only one has remained friendly to life.
The new study explains how small differences added up over time. Earth has a built-in thermostat: carbon cycles from the atmosphere into rocks and back again through volcanic activity.
When Earth heats up, reactions pull carbon dioxide out of the air, cooling things down. Then volcanoes push it back out again, preventing a deep freeze. On Mars, that cycle doesn’t work the same way.
“In contrast to Earth, where there are always some volcanoes erupting, Mars right now is volcanically dormant, and the average rate of volcanic outgassing on Mars is slow,” Kite said.
“So in that situation, you don’t really have a balance between carbon dioxide in and carbon dioxide out, because if you have even a little bit of liquid water, you’re going to draw down carbon dioxide through carbonate formation.”
The role of carbon in Mars’ climate
The carbon imbalance means any brief warming on Mars – such as from a slowly brightening sun – triggers its own undoing. Water reappears. It helps form carbonates. That pulls carbon out of the air. Then the greenhouse effect fades, and Mars cools off again.
The models show Mars could have had warm, wet periods lasting a few million years, followed by dry spells that lasted 100 million years or more. That kind of stop-start habitability, with massive gaps in between, isn’t great for sustaining life.
Traces of environmental catastrophe
Between Curiosity, Perseverance, and the fleet of orbiters circling the planet, we’re finally piecing together a real history of what happened to Mars’ climate.
“Fortunately, Mars preserves a trace of that environmental catastrophe in the rocks on its surface,” said Kite. “And today we’re in a golden age of Mars science, with two plutonium-powered rovers on the surface and an international fleet of spacecraft in orbit that allow us to deeply explore the planet for these traces.”
The more we understand about how Mars lost its atmosphere, the more we can learn about what keeps a planet stable – and what makes it fragile.
The full study was published in the journal Nature.
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