| dc.description.abstract | Finding life beyond the Earth remains one of the major drivers of science today. The search for extra-solar life has predominantly been based on Earth-like biosignatures from across Earth's history, such as oxygen and methane. Life on Earth has changed dramatically since the origin of life, with life on the early Earth potentially an important period to understand when looking for life on other planets. In this thesis, I investigate the interaction between life and the environment, with a general focus on life before the advent of oxygenic photosynthesis. 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. I used the Met Office Unified Model (UM), 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.
I then shift the focus to exoplanets. The majority of detected potentially habitable exoplanets orbit stars cooler than the Sun and are therefore irradiated by a stellar spectrum that peaks at longer wavelengths than the spectrum incident on Earth. I present results from a set of simulations of tidally locked terrestrial planets orbiting three different host stars to isolate the effect of the stellar spectra on the simulated climate. Specifically, we perform simulations based on TRAPPIST-1e, adopting an Earth-like atmosphere and using the UM. Whilst holding the planetary parameters constant, including the total stellar flux (900 W/m²) and orbital period (6.10 Earth days), we compare results between simulations where the stellar spectrum is that of a quiescent TRAPPIST-1, Proxima Centauri, and the Sun. In simulations with cooler host stars, an increased proportion of incident stellar radiation was absorbed directly by the troposphere compared to the surface. This in turn led to an increase in the stability against convection, that is, a reduction in overall cloud coverage on the dayside (reducing scattering), leading to warmer surface temperatures. The increased direct heating of the troposphere also led to more efficient heat transport from the dayside to the nightside and therefore to a reduced day-night temperature contrast. We inferred that planets with an Earth-like atmosphere orbiting cooler stars had lower dayside cloud coverage, potentially allowing habitable conditions at increased orbital radii, compared to similar planets orbiting hotter stars for a given planetary rotation rate.
Finally, after improving our understanding of the Archean and Earth-like planets orbiting M-dwarf planets, I focus on the interaction between the biosphere and the atmosphere for Earth and how this may differ on a planet orbiting an M-dwarf, TRAPPIST-1e, and the possible effect this may have on the detection of life on such a planet. I develop and apply a coupled 1D atmosphere-ocean-ecosystem model to understand how primitive biospheres that exploit free-energy gradients between possible abiotic sources of H₂, CO and O₂ could influence the atmospheric composition of rocky terrestrial exoplanets. I apply this to the Earth at 3.8 Ga and to TRAPPIST-1e. I focus on metabolisms that evolved before the evolution of oxygenic photosynthesis, which consume H₂ and CO and produce potentially detectable levels of CH₄. Oxygen-consuming metabolisms are also considered for TRAPPIST-1e as abiotic oxygen production is predicted. I show that these primitive biospheres can lead to high levels of abiotic oxygen (close to 10 %), which is a result of converting an H₂ flux into a biotic CH₄ flux which could stabilise high oxygen scenarios. The inclusion of oxygen-consuming metabolisms could lower oxygen levels to around 10 parts per million and support a productive biosphere at low reductant inputs. I find that CO metabolism is as productive as H₂ metabolisms for a planet orbiting an M-dwarf. Using predicted transmission spectral features from CH₄, CO, O₂/O₃ and CO₂ across the hypothesis space for tectonic reductant input, I 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 this species.
Finally, this thesis covers future work, and some proof of concept experiments that advance the discussed work further. | en_GB |
