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(Abridged) Super Earth exoplanets are being discovered with increasingfrequency and some will be able to retain stable H2-dominated atmospheres. Westudy biosignature gases on exoplanets with thin H2 atmospheres and habitablesurface temperatures, by using a model atmosphere with photochemistry, andbiomass estimate framework for evaluating the plausibilty of a range ofbiosignature gas candidates. We find that photochemically produced H atoms arethe most abundant reactive species in H2 atmospheres. In atmospheres with highCO2 levels, atomic O is the major destructive species for some molecules. Insun-Earth-like UV radiation environments, H (and in some cases O) will rapidlydestroy nearly all biosignature gases of interest. The lower UV fluxes from UVquiet M stars would produce a lower concentration of H (or O) for the samescenario, enabling some biosignature gases to accumulate. The favorability oflow-UV radiation environments to in an H2 atmosphere is closely analogous tothe case of oxidized atmospheres, where photochemically produced OH is themajor destructive species. Most potential biosignature gases, such as DMS andCH3Cl are therefore more favorable in low UV, as compared to solar-like UV,environments. A few promising biosignature gas candidates, including NH3 andN2O, are favorable even in solar-like UV environments, as these gases aredestroyed directly by photolysis and not by H (or O). A more subtle finding isthat most gases produced by life that are fully hydrogenated forms of anelement, such as CH4, H2S, are not effective signs of life in an H2-richatmosphere, because the dominant atmospheric chemistry will generate such gasesabiologically, through photochemistry or geochemistry. Suitable biosignaturegases in H2-rich atmospheres for super Earth exoplanets transiting M starscould potentially be detected in transmission spectra with the JWST.

Biosignature Gases in H2-Dominated Atmospheres on Rocky Exoplanets