Wordsworth, RobinKalugina, Y.Lokshtanov, S.Vigasin, A.Ehlmann, B.Head, J.Sanders, C.Wang, H.2018-02-212017Wordsworth, R., Y. Kalugina, S. Lokshtanov, A. Vigasin, B. Ehlmann, J. Head, C. Sanders, and H. Wang. 2017. “Transient Reducing Greenhouse Warming on Early Mars.” Geophysical Research Letters 44 (2) (January 21): 665–671. doi:10.1002/2016gl071766.0094-8276http://nrs.harvard.edu/urn-3:HUL.InstRepos:34858096The evidence for abundant liquid water on early Mars despite the faint young Sun is a long-standing problem in planetary research. Here we present new ab initio spectroscopic and line-by-line climate calculations of the warming potential of reduced atmospheres on early Mars. We show that the strength of both CO2-H2 and CO2-CH4 collision-induced absorption (CIA) has previously been significantly underestimated. Contrary to previous expectations, methane could have acted as a powerful greenhouse gas on early Mars due to CO2-CH4 CIA in the critical 250-500 cm^-1 spectral window region. In atmospheres of 0.5 bar CO2 or more, percent levels of H2 or CH4 raise annual mean surface temperatures by tens of degrees, with temperatures reaching 273 K for pressures of 1.25-2~bar and 2-10% of H2 and CH4. Methane and hydrogen produced following aqueous alteration of Mars' crust could have combined with volcanically outgassed CO2 to form transient atmospheres of this composition 4.5-3.5 Ga. Our results also suggest that inhabited exoplanets could retain surface liquid water at significant distances from their host stars.en-USJournal Article2018-02-0720172018-02-2110.1002/2016GL071766