Simulating biosignatures from pre-oxygen photosynthesising life on TRAPPIST-1e

This is the author accepted manuscript. The final version is available on open access from Oxford University Press via the DOI in this recordData availability: The model output used for this study will be made available following this work’s acceptance for publicationIn 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.Science and Technology Facilities Council (STFC)UK Research and InnovationJohn Templeton FoundationLeverhulme TrustHill Family ScholarshipInstitute of Physic

Predicting biosignatures for nutrient limited biospheres

This is the author accepted manuscript. The final version is available on open access from Oxford University Press via the DOI in this recordData availability statement: https://github.com/nicholsonae/archean_worldWith the characterisations of potentially habitable planetary atmospheres on the horizon, the search for biosignatures is set to become a major area of research in the coming decades. To understand the atmospheric characteristics that might indicate alien life we must understand the abiotic characteristics of a planet and how life interacts with its environment. In the field of biogeochemistry, sophisticated models of life-environment coupled systems demonstrate that many assumptions specific to Earth-based life, e.g. specific ATP maintenance costs, are unnecessary to accurately model a biosphere. We explore a simple model of a single-species microbial biosphere that produces 4 as a byproduct of the microbes’ energy extraction - known as a type I biosignature. We demonstrate that although significantly changing the biological parameters has a large impact on the biosphere’s total population, such changes have only a minimal impact on the strength of the resulting biosignature, while the biosphere is limited by 2 availability. We extend the model to include more accurate microbial energy harvesting and show that adjusting microbe parameters can lead to a regime change where the biosphere becomes limited by energy availability and no longer fully exploits the available 2, impacting the strength of the resulting biosignature. We demonstrate that, for a nutrient limited biosphere, identifying the limiting nutrient, understanding the abiotic processes that control its abundance, and determining the biospheres ability to exploit it, are more fundamental for making type I biosignature predictions than the details of the population dynamics of the biosphere.Leverhulme TrustScience and Technology Facilities Council (STFC)UKRIJohn Templeton Foundatio

3D simulations of the Archean Earth including photochemical haze profiles

This is the author accepted manuscript.Data availability: The research data supporting this publication are openly available with CC BY 4.0 at https://doi.org/10.5281/zenodo.8178651We present results from 3D simulations of the Archean Earth including a prescribed (non-interactive) spherical haze generated through a 1D photochemical model. Our sim ulations suggest that a thin haze layer, formed when CH4/CO2 = 0.1, leads to global warming of ∼10.6K due to the change of water vapour and cloud feedback, compared to the simulation without any haze. However, a thicker haze layer, formed when CH4/CO2 > 0.1, leads to global cooling of up to ∼65K as the scattering and absorption of short wave radiation from the haze reduces the radiation from reaching the planetary surface. A thermal inversion is formed with a lower tropopause as the CH4/CO2 ratio increases. The haze reaches an optical threshold thickness when CH4/CO2 ∼ 0.175 beyond which the atmospheric structure and the global surface temperature do not vary much.Bell Burnell Graduate Scholarship FundUKRIScience and Technology Facilities Council (STFC)Leverhulme TrustNASAHill Family Scholarshi

3D climate simulations of the Archean find that Methane has a strong cooling effect at high concentrations

This is the author accepted manuscriptOpen Research: The research data supporting this publication are openly available from the University of Exeter’s institutional repository at: https://doi.org/10.24378/exe.4347 with CC BY 4.0 (Eager-Nash et al., 2022)Methane is thought to have been an important greenhouse gas during the Archean, although its potential warming has been found to be limited at high concentrations due to its high shortwave absorption. We use the Met Office Unified Model, a general circulation model, to further explore the climatic effect of different Archean methane concentrations. Surface warming peaks at a pressure ratio pCH4:pCO2 of approximately 0.1, reaching a maximum of up to 7 K before significant cooling above this ratio. Equator-to-pole temperature differences also tend to increase up to pCH4 ≤ 300 Pa, which is driven by a difference in radiative forcing at the equator and poles by methane and a reduction in the latitudinal extend of the Hadley circulation. 3D models are important to fully capture the cooling effect of methane, due to these impacts of the circulation.UK Research and InnovationLeverhulme TrustScience and Technology Facilities Counci