Biological, Equilibrium and Photochemical Signatures of C, N and S Isotopes in the Early Earth and Exoplanet Atmospheres

The unambiguous detection of biosignatures in exoplanet atmospheres is a primary objective for astrobiologists and exoplanet astronomers. The primary methodology is the observation of combinations of gases considered unlikely to coexist in an atmosphere or individual gases considered to be highly biogenic. Earth-like examples of the former include CH₄ and O₃, and the latter includes dimethyl sulfide (DMS). To improve the plausibility of the detection of life, I argue that the isotope ratios of key atmospheric species are needed. The C isotope ratios of CO₂ and CH₄ are especially valuable. On Earth, thermogenesis and volcanism result in a substantial difference in δ¹³C between atmospheric CH₄ and CO₂ of ~−25‰. This difference could have changed significantly, perhaps as large as −95‰ after the evolution of hydrogenotrophic methanogens. In contrast, nitrogen fixation by nitrogenase results in a relatively small difference in δ¹⁵N between N₂ and NH₃. Isotopic biosignatures on ancient Earth and rocky exoplanets likely coexist with much larger photochemical signatures. Extreme δ¹⁵N enrichment in HCN may be due to photochemical self-shielding in N₂, a purely abiotic process. Spin-forbidden photolysis of CO₂ produces CO with δ¹³C < −200‰, as has been observed in the Venus mesosphere. Self-shielding in SO₂ may generate detectable ³⁴S enrichment in SO in atmospheres similar to that of WASP-39b. Sufficiently precise isotope ratio measurements of these and related gases in terrestrial-type exoplanet atmospheres will require instruments with significantly higher spectral resolutions and light-collecting areas than those currently available.