Molecular hydrogen as an early greenhouse gas on Mars

Molecular hydrogen is being discussed as a possible greenhouse gas to explain an early warm mars. Other models using just CO2 and H2O do not result in a warm Mars with a dim Sun but adding molecular hydrogen might do the trick.

http://www.sciencedaily.com/releases/2013/11/131124200548.htm

Previous efforts to produce temperatures warm enough to allow for liquid water used climate models that include only carbon dioxide and water and were unsuccessful. The researchers used a model to show that an atmosphere with sufficient carbon dioxide, water and hydrogen could have made the surface temperatures of Mars warm to above freezing. Those above-freezing temperatures would allow liquid water to flow across the Martian surface over 3.8 billion years ago and form the ancient valley networks, such as Nanedi Valles, much the way sections of the Grand Canyon snake across the western United States today.