How did wet and warm Mars become a cold, barren desert?

Mars might have always been doomed to be a lifeless desert planet, according to a new study based on samples retrieved by NASA’s Curiosity rover.

Ancient lakebeds, rivers and chemical signals all point to Mars having been a warm, wet world in the distant past. But, while Earth remained a lush, watery planet, Mars has become cold and desolate.

“For years, we’ve had this huge unanswered question for why Earth has managed to keep its habitability while Mars lost it,” says Edwin Kite, an associate professor at the University of Chicago in the US and first author of the study published in Nature.

“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.”

Kite says that planetary scientists are in a good position to finally reveal some of the mysteries of the Martian past.

“Today we’re in a golden age of Mars science, with 2 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.”

It’s not enough for a planet to start off warm and wet.

Stabilising these conditions over long periods of time requires additional mechanisms that can respond to changes. On Earth, this is done through a system of carbon cycling between the rocks on the surface and the atmosphere.

Carbon dioxide (CO₂) in the atmosphere leads to temperature rise. This, in turn, speeds up the chemical reactions which lock CO₂ up in rock, preventing unfettered temperature rise. The carbon then leaks back into the atmosphere through volcanic eruptions and the cycle resumes.

CO₂ locked up in Martian rocks back when the planet was still wet and warm hasn’t leaked back into the atmosphere due to lack of volcanic activity.

“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 explains.

“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.”

Kite’s team used computer models based on data from NASA’s Curiosity rover. Kite was involved in a paper published in April that revealed the rover had found siderate, an iron carbonate material.

Simulations suggest that Mars had short warm, wet periods followed by periods lasting 100 million years of desert. Less than ideal conditions for life.