A new idea could explain how the climate of early Mars became warm enough to support oceans.
Scientists believe sulphur dioxide released from ancient volcanoes created a balance similar to Earth's carbon cycle, which controlled the climate.
The notion, outlined in the journal Science, could explain why Mars rovers have found sulphur minerals on the surface but no limestone like on Earth.
It may also provide clues to how life evolved on our own planet.
"Before the origin of life, our atmosphere may have looked much like early Mars," said Daniel Schrag, lead author of the Science paper. "Sulphur dioxide may have had an important role then as well."
Carbon cycle
The Earth's climate has been influenced for millions of years by the movement of carbon around the planet, and levels of carbon dioxide, an important greenhouse gas.
Silicate rocks remove carbon dioxide from the atmosphere and convert it into calcium carbonate, commonly known as limestone, in the presence of water.
"The presence of even a small amount of sulphur dioxide in the atmosphere would contribute to the warmer climate"
Scientists have been wondering why this is the case, given our current understanding of what happens on Earth.
"There is abundant evidence for a warmer climate, perhaps even a liquid water ocean, early in Martian history, between 3.5 and four billion years ago," explained Daniel Schrag, professor of Earth and planetary sciences at Harvard University, US.
"However, scientists have found it difficult to reconcile this evidence with our understanding of how the climate system is regulated on Earth."
Volcanic clues
The new idea proposes that sulphur dioxide released from giant volcanoes took the place of carbon dioxide in the early Martian atmosphere.
On Earth, sulphur dioxide is quickly oxidised to sulphate and removed from the atmosphere, but on early Mars, which lacked oxygen, it would have lingered in the atmosphere for much longer, acting as a greenhouse gas and warming the planet.
It would also have prevented limestone deposits from forming in any standing water on Mars; instead sulphate minerals would have prevailed.
"The sulphur dioxide would essentially pre-empt the role of carbon dioxide in surface weathering reactions," said Itay Halevy, also of Harvard.
"The presence of even a small amount of sulphur dioxide in the atmosphere would contribute to the warmer climate, and also prevent limestone deposits from forming."
The hypothesis also has implications for understanding how life evolved in water on Earth.
If confirmed, it would mean the oceans that supported early life were much more acidic than previously thought.
Other researchers are not entirely convinced by the idea, however.
"It was quite a surprise when the Mars rovers found some rocks that were 8-10% sulphur," said UK Mars expert Professor Colin Pillinger.
"I'm not 100% convinced that this is the answer but it is a good hypothesis for people to work with and prove or disprove."
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