Doomed Desert

Canyons carved by ancient rivers on Mars indicate that the planet was once warm enough for liquid water, and even possibly life. 

However, the Red Planet today is a barren desert, and a new study has now provided a possible explanation for why Mars, unlike Earth, never remains warm for long. 

“We’ve had this huge unanswered question for why Earth has managed to keep its habitability while Mars lost it,” lead study author Edwin Kite said in a statement. “People have been looking for a tomb for the atmosphere for years.”  

In April, as part of the Mars Science Laboratory mission, NASA’s Curiosity rover found rocks rich in carbonate minerals on Mount Sharp.  

The rover’s discovery provided the crucial, missing piece, showing evidence of carbon dioxide locked into rock, providing insights into what happened to Mars’ thick atmosphere. 

Mars has a similar makeup to Earth: It is rocky, it has plenty of carbon and water, and is close enough to the Sun to feel its warmth without being burned by it. Yet, Mars today is a frozen desert while Earth is not. 

A planet cannot remain warm and mild just because it started out that way, researchers say. It needs built-in processes that can maintain stable conditions over long periods, even as the environment changes. 

Scientists believe that Earth achieves this through a precisely balanced system that moves carbon from sky to rock and back again. 

Carbon dioxide in the atmosphere warms the planet, but higher temperatures also accelerate chemical reactions that trap carbon dioxide in rocks, which cools things down again. Eventually, volcanic eruptions release some of that carbon back into the air. This natural cycle appears to have kept Earth’s climate relatively stable and suitable for life over millions of years. 

“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,” explained Kite. “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 researchers suggested that on Mars, a similar, but self-limiting, cycle could take place. 

According to the team’s model, periods of liquid water on Mars in the past were sparked by the Sun slowly growing brighter, by about 8 percent every billion years. As temperatures rose, however, chemical reactions caused carbon dioxide to be trapped in rocks, reducing the greenhouse effect.  

This process pushed the planet to revert to a frozen desert over time, unlike Earth, which remained habitable through a more balanced carbon cycle. 

The team developed detailed models to show how these climate swings could happen. They found that Mars experiences brief periods of liquid water, followed by 100-million-year periods of desert.  

“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,” said Kite.