BIOSIGNATURE GASES IN H₂-DOMINATED ATMOSPHERES ON ROCKY EXOPLANETS

Abstract

Super-Earth exoplanets are being discovered with increasing frequency and some will be able to retain stable H 2 -dominated atmospheres. We study biosignature gases on exoplanets with thin H 2 atmospheres and habitable surface temperatures, using a model atmosphere with photochemistry and a biomass estimate framework for evaluating the plausibility of a range of biosignature gas candidates. We find that photochemically produced H atoms are the most abundant reactive species in H 2 atmospheres. In atmospheres with high CO 2 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 accumulate detectable biosignature gases in an H 2 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 dimethylsulfide and CH 3 Cl, are therefore more favorable in low-UV, as compared with solar-like UV, environments. A few promising biosignature gas candidates, including NH 3 and N 2 O, 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 CH 4 and H 2 S, are not effective signs of life in an H 2 -rich atmosphere because the dominant atmospheric chemistry will generate such gases abiologically, through photochemistry or geochemistry. Suitable biosignature gases in H 2 -rich atmospheres for super-Earth exoplanets transiting M stars could potentially be detected in transmission spectra with the James Webb Space Telescope.

Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry

“…Planets orbiting well interior (Abe et al, 2011;Zsom et al, 2013) or exterior (Pierrehumbert and Gaidos, 2011) to an Earth-like planet's habitable zone boundaries (Kopparapu et al, 2013) must be considered. The planet diversity also could well extend to the surface sources and sinks of gases, especially the redox state of the planetary surfacea wide variety of habitable planets have been hypothesized in this regard, from water worlds (Kuchner, 2003;Léger et al, 2004), to planets with hydrogen-rich atmospheres (Pierrehumbert and Gaidos, 2011;Seager et al, 2013a), to Venuslike worlds (Schaefer and Fegley, 2011) and planets with increased volcanism (Kaltenegger and Sasselov, 2010;Hu et al, 2013). The extreme and far UV radiation that drives atmospheric photochemistry will vary depending on host star type and age (Guinan et al, 2003;Shkolnik and Barman, 2014).…”

“…Methyl chloride (CH 3 Cl) and dimethyl sulfide (CH 3 SCH 3 or DMS) have been studied as exoplanetary biosignature gases (Segura et al, 2003(Segura et al, , 2005Scalo et al, 2007;DomagalGoldman et al, 2011;Rugheimer et al, 2013;Seager et al, 2013aSeager et al, , 2013bRugheimer et al, 2015) and are additional examples of the diversity of gases produced by life whose production is not linked in any predictable way to the physical or chemical properties of Earth. Rather, they happen to be produced in relatively large amounts by organisms that are also relatively common (Bates et al, 1993;Kiene et al, 2000;Yoshida et al, 2004).…”

The Muscles Treasury Survey. I. Motivation and Overview*

“…The dominant energy input and chemistry driver for these atmospheres is the stellar spectral energy distribution (SED). The ultraviolet (UV) stellar spectrum which drives and regulates the upper atmospheric heating and chemistry on Earth-like planets is critical to the definition and interpretation of biosignature gases (e.g., Seager et al 2013), and may even produce false-positives in our search for biologic activity (Hu et al 2012;Domagal-Goldman et al 2014;Tian et al 2014).…”

The future of spectroscopic life detection on exoplanets

“…For Sun-Earth-like UV radiation environments, OH is created when H 2 O and/or CO 2 are photodissociated, and OH is a powerful radical that destroys many gases in a planet atmosphere (60). In Sun-Earthlike UV radiation environments, planets with H 2 -rich atmospheres, atomic H, produced from H 2 photodissociation (and in some cases, O, which is produced from CO 2 ), is the destructive molecule and will rapidly destroy nearly all biosignature gases of interest (76). A low-UV environment means that biosignature gases will be less likely to be destroyed than in a high-UV radiation environment, enabling biosignature gases to be more likely to accumulate to significant levels in the exoplanet atmosphere.…”