In order to assess observational evidence for potential atmospheric biosignatures on exoplanets, it will be essential to test whether
spectral fingerprints from multiple gases can be explained by abiotic or biotic-only processes. Here, we develop and apply a
coupled 1D atmosphere-ocean-ecosystem model to understand how primitive ...
In order to assess observational evidence for potential atmospheric biosignatures on exoplanets, it will be essential to test whether
spectral fingerprints from multiple gases can be explained by abiotic or biotic-only processes. Here, we develop and apply a
coupled 1D atmosphere-ocean-ecosystem model to understand how primitive biospheres, which exploit abiotic sources of H2
,
CO and O2
, could influence the atmospheric composition of rocky terrestrial exoplanets. We apply this to the Earth at 3.8 Ga
and to TRAPPIST-1e. We focus on metabolisms that evolved before the evolution of oxygenic photosynthesis, which consume
H2 and CO and produce potentially detectable levels of CH4
. O2
-consuming metabolisms are also considered for TRAPPIST-1e,
as abiotic O2 production is predicted on M-dwarf orbiting planets. We show that these biospheres can lead to high levels of
surface O2
(approximately 1–5 %) as a result of CO consumption, which could allow high O2
scenarios, by removing the main
loss mechanisms of atomic oxygen. Increasing stratospheric temperatures, which increases atmospheric OH can reduce the
likelihood of such a state forming. O2
-consuming metabolisms could also lower O2
levels to around 10 ppm and support a
productive biosphere at low reductant inputs. Using predicted transmission spectral features from CH4
, CO, O2
/O3 and CO2
across the hypothesis space for tectonic reductant input, we show that biotically-produced CH4 may only be detectable at high
reductant inputs. CO is also likely to be a dominant feature in transmission spectra for planets orbiting M-dwarfs, which could
reduce the confidence in any potential biosignature observations linked to these biospheres.