In a study published in the journal Nature Geoscience, the researchers said the presence of molecular hydrogen, along with carbon dioxide and water, could have created a greenhouse effect that pushed temperatures high enough to allow for liquid water.
Previous climate models for Mars that were based on carbon dioxide and water alone were unsuccessful in showing possible conditions warm enough, the researchers said.
However, introducing molecular hydrogen into the computer models showed the martian atmosphere could have made surface temperatures warm to above freezing.
That would allow liquid water to flow across the martian surface and form the ancient valley networks seen on the planet today, the researchers said.
"This is exciting because explaining how early Mars could have been warm and wet enough to form the ancient valleys had scientists scratching their heads for the past 30 years," Penn State doctoral student Ramses M. Ramirez said. "We think we may have a credible solution to this great mystery.
"The hydrogen molecule is symmetric and appears to be quite boring by itself. However, other background gases, such as carbon dioxide, can perturb it and get it to function as a powerful greenhouse gas at wavelengths where carbon dioxide and water don't absorb too strongly. So hydrogen fills in the gaps left by the other two greenhouse gases."