NASA’s Curiosity rover captured this photo as it ascended the Martian mountain Mt. Sharp. A study proposes a new explanation for why Mars is a barren desert today, despite having many similarities to Earth. Credit: Photo by NASA/ JPL-Caltech/ MSSS. Public domain.

UChicago-led analysis of Mars Science Laboratory Curiosity rover data may explain why the planet was likely harsh desert for most of recent past.

One of the great unsolved problems in modern planetary science is written on the surface of Mars.

Mars has canyons that were carved by rivers, so it was once warm enough for liquid water. How — and why — did it become it a barren desert today?

A study led by University of Chicago planetary scientist Edwin Kite, an associate professor of geophysical sciences who is a participating scientist for the Curiosity mission, puts forth a new explanation for why Mars never seems to stay balmy for long.

Their model suggests that the periods of liquid water we see in the past were initiated by the Sun brightening, and that conditions on Mars mean it trends towards desert over time — in contrast to Earth, which has stayed habitable over time.

The study builds on findings from NASA’s Mars Science Laboratory Curiosity mission that were announced in April: the rover finally found rocks rich in carbonate materials, which could explain where Mars’ atmosphere went.

Kite said: “For years, we’ve had this huge unanswered question for why Earth has managed to keep its habitability while Mars lost it.

“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.”

A ‘golden age’ of Mars science

Mars has almost the same makeup as Earth — it’s a rocky planet, with plenty of carbon and water, near enough to the Sun to be warmed but not cooked by it — and yet today it’s a frozen desert, while Earth teems with life.

For years, scientists have been searching for an answer to why you are reading this from Earth and not Mars.

The mystery deepened when we were able to see river-carved valleys and old lakebeds on the surface of Mars, showing the planet at some point did have a climate that was warm enough for liquid water.

Kite said: “Fortunately, Mars preserves a trace of that environmental catastrophe in the rocks on its surface, 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.”

When it comes to keeping a planet balmy and mild, it isn’t enough just to start out that way — there need to be mechanisms for stability over time that can respond to changes on and around the planet.

Scientists think that Earth does this through a finely balanced system that moves carbon from sky to rock and back again.

Carbon dioxide in the atmosphere warms the planet, but warmer temperatures also speed up reactions that lock up carbon dioxide into rock, which eventually counteracts the temperature rise.

Eventually, carbon leaks back out into the atmosphere via volcanic eruptions. Over millions of years, this cycle appears to have kept Earth relatively stable and hospitable for life.

Self-limiting cycle

On Mars, the researchers suggested, a similar cycle could also take place — but a self-limiting one. 

It hinges on the fact that our Sun’s brightness rises very, very slowly over time — about 8% per billion years.

As the Sun brightens, the scientists hypothesise, liquid water begins running on Mars. But then this water starts causing carbon dioxide to get locked into rocks, as it does on Earth, which swings the planet back to cold and barren desert.

Kite explained: “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.

“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 group built extensive models showing how these swings could happen. They suggest that Mars experiences short periods of liquid water, followed by 100-million-year-long periods of desert.

Needless to say, a 100-million-year-long gap in habitability is bad for life.

A Martian mystery

The explanation was made possible by Curiosity’s discovery, announced earlier this year, of carbonate-rich rocks on Mars’ surface.

This had been a missing piece of the puzzle for years, the scientists explained; to have had liquid water, Mars had to have a thicker atmosphere made up of a greenhouse gas like carbon dioxide.

But today there’s very little atmosphere, leaving a puzzle of where the carbon went.

Kite said: “People have been looking for a tomb for the atmosphere for years.”

The simplest explanation would be that it was drawn down into rocks, as it is on Earth, but the first rover tests hadn’t turned up any evidence of carbonate-rich rocks.

It took Curiosity’s journey up a Martian mountain named Mt Sharp to finally find these carbonate rocks. As it continues, further tests will show whether the carbonate is as widespread as researchers suspect.

Study co-author Benjamin Tutolo, a professor at the University of Calgary, said: “It really is something you cannot know until you have a rover on the surface.

“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.”

The study is published in Nature.