Warmer and wetter Mars needed a different atmosphere, not just higher temperatures, to have been a warmer, wetter, more Earth like world in the past, according to a new study.
A report in the journal Nature Geoscience, suggests the Martian atmosphere would have included significant levels of hydrogen if it was to have once had large amounts of liquid water on its surface.
Ancient geological features on Mars, which were carved out about four billion years ago by flowing water, indicate the planet was once covered with oceans, rivers and lakes.
However, the Sun would have been less bright at that time than compared to today.
Previous Martian climate models show a cooler dimmer Sun, combined with the greenhouse effect from carbon dioxide and water vapour alone, would have been insufficient to create the warm surface conditions needed for liquid water on the red planet.
"Either ancient Mars was not as warm as we think, or else there were other greenhouse gasses that were helping to keep it warm," says Professor Jim Kasting of Pennsylvania State University, who is one of the study's authors.
Kasting and colleagues used climate models to show that simply pumping more carbon dioxide into the Martian atmosphere wouldn't work.
Once the amount of carbon dioxide in the atmosphere reached a critical level, it began condensing out in the upper troposphere, forming dry ice clouds.
"We tried sulfur dioxide, methane, and even putting nitrogen dioxide into the atmosphere, but none of these things worked," says Kasting.
The researchers also confronted another problem. They found the planet's albedo, or reflectivity, would have increased as the surface pressure increased, resulting in less energy reaching the planet's surface.
The researchers eventually realised that they had been overlooking molecular hydrogen.
Kasting and colleagues developed a climate model of a primordial Martian atmosphere containing between 1300 and 14000 hectopascals of carbon dioxide and water, in addition to between five and twenty per cent molecular hydrogen.
"This could have raised surface temperatures on early Mars above the freezing point of water," says Kasting.
"Molecular hydrogen is very common on giant planets; it's the major constituent of the atmospheres of both Jupiter and Saturn."
On terrestrial planets, hydrogen is released into the atmosphere through volcanic out gassing. However it also escapes into space from terrestrial planetary atmospheres.
"That doesn't mean you have zero hydrogen, you have a balance between volcanic out gassing of hydrogen and its escape into space," says Kasting.
"So you have to make the case that there's enough volcanic out gassing and a slow enough escape rate on early Mars, that you can retain significant hydrogen."