Volcanic sulphur spewed into the atmosphere on Mars could have trapped heat on the planet, allowing oceans of liquid water to exist on the surface of the planet, scientists said Thursday.

Sulphur-based greenhouse gases could also have effectively scrubbed away carbon trapped in the ground, hiding evidence of a carbon cycle similar to the one seen on Earth, the authors said.

Scientists have found ample evidence that Mars, though dry and icy today, once had liquid water on its surface, with recent studies pointing to ancient coastlines as signs that the planet once held vast oceans.

But strong evidence of the planet's watery past has always been tempered by a mystery: If Mars had oceans, why has so little evidence of carbonates been found on the planet's surface?

Carbonates are minerals — such as limestone — that form as part of the natural carbon cycle here on Earth: liquid water reacts with carbon dioxide gas in the atmosphere, cooling the carbon and trapping it in the ground. Carbon dioxide is then released back into the atmosphere during volcanic activity.

Writing in the journal Science, Itay Halevy and Daniel Schrag of Harvard University and Maria Zuber from the Massachusetts Institute of Technology suggest that if Mars once had a carbon cycle similar to Earth's, it should have manifested itself in a carbonate layer approximately 600 metres thick.

Puzzling lack of limestone

"The virtual absence of carbonate minerals above the detection limit … coincident with evidence for an early climate warm enough for liquid water, is therefore puzzling," they wrote.

To resolve the problem, the three scientists suggest a Martian atmosphere with sulphur dioxide could have also trapped heat on the planet enough to support liquid water, and point to the abundance of sulphur in soil and rock samples.

The sulphur dioxide could have been released into the atmosphere through volcanic activity, since Mars was at one time more geologically active than it is now.

An atmosphere heavy in compounds such as sulphur dioxide and hydrogen sulphide would account for both why Mars had so little evidence of carbonates and how it might have supported liquid water on its surface, the authors said.

Water, which absorbed the sulphur during the planet's atmospheric process would have become more acidic, the authors contend, and this acidic water would in turn break down the limestone on the ocean floor.

Potent greenhouse gases

Likewise, the common gases containing sulphur are potent greenhouse gases.

"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," said Halevy in a statement.

"We think we now understand why there is so little carbonate on Mars, and so much sulphur."

The researchers hope understanding how Mars lost its oceans and atmosphere could provide clues to Earth's atmospheric processes. They also said it could provide clues as to the conditions on Mars today.

The subzero surface temperatures and low atmospheric pressure of Mars preclude water from remaining a liquid for long in Mars' current climate. Once water reaches the surface, it would either freeze into ice or disperse in the atmosphere as gas.

But a number of missions to Mars are investigating the possibility that liquid water might lie beneath the surface of the planet. Liquid water along with a stable heat source are considered important conditions for the potential to support life.