Abstract
(Abridged) Super Earth exoplanets are being discovered with increasing
frequency and some will be able to retain stable H2-dominated atmospheres. We
study biosignature gases on exoplanets with thin H2 atmospheres and habitable
surface temperatures, by using a model atmosphere with photochemistry, and
biomass estimate framework for evaluating the plausibilty of a range of
biosignature gas candidates. We find that photochemically produced H atoms are
the most abundant reactive species in H2 atmospheres. In atmospheres with high
CO2 levels, atomic O is the major destructive species for some molecules. In
sun-Earth-like UV radiation environments, H (and in some cases O) will rapidly
destroy nearly all biosignature gases of interest. The lower UV fluxes from UV
quiet M stars would produce a lower concentration of H (or O) for the same
scenario, enabling some biosignature gases to accumulate. The favorability of
low-UV radiation environments to in an H2 atmosphere is closely analogous to
the case of oxidized atmospheres, where photochemically produced OH is the
major destructive species. Most potential biosignature gases, such as DMS and
CH3Cl are therefore more favorable in low UV, as compared to solar-like UV,
environments. A few promising biosignature gas candidates, including NH3 and
N2O, are favorable even in solar-like UV environments, as these gases are
destroyed directly by photolysis and not by H (or O). A more subtle finding is
that most gases produced by life that are fully hydrogenated forms of an
element, such as CH4, H2S, are not effective signs of life in an H2-rich
atmosphere, because the dominant atmospheric chemistry will generate such gases
abiologically, through photochemistry or geochemistry. Suitable biosignature
gases in H2-rich atmospheres for super Earth exoplanets transiting M stars
could potentially be detected in transmission spectra with the JWST.
Description
[1309.6016] Biosignature Gases in H2-Dominated Atmospheres on Rocky Exoplanets
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