Tracing the chemistry of high-energy processes in planetary atmospheres

Abstract

Lightning, X-ray & UV radiation, stellar energetic particles, and galactic cosmic rays provide important sources of disequilibrium chemistry in planetary atmospheres and might be responsible for the production of important precursors for the origin of life. A growing number of known exoplanets with a large, diverse range of atmospheres provides new opportunities to explore the effects of these high-energy processes on the chemistry of planetary atmospheres. By combining X-ray & UV observations of the host star with 3D climate simulations, we study the effect of different types of high-energy radiation on the production of organic and prebiotic molecules in the atmosphere of the hot Jupiter HD 189733 b. We identify ‘fingerprint’ ions for the ionization of the atmosphere by both galactic cosmic rays and stellar energetic particles and find an enhancing effect on the abundance of key organic molecules that are potentially detectable by JWST. In addition to these types of external high-energy radiation, lightning provides another energy source from within a cloudy atmosphere when charged particles are separated to build up an electric field strong enough for a discharge to develop. Lightning has been suggested to play a role in triggering the occurrence of bio-ready chemical species and nutrients for Earth’s earliest biosphere. We present results from spark discharge experiments in gas mixtures resembling the atmospheres of modern and early Earth. They suggest that lightning-driven nitrogen fixation may have been efficient on early Earth, but measurements of the isotopic composition of the discharge products do not agree with isotope ratios from the sedimentary rock record, which supports the early development of biological nitrogen fixation. By combining our experimental results with photochemical simulations, we can investigate the effect of lightning on the atmospheric chemistry of terrestrial exoplanets. We find that lightning may be able to mask the ozone features of an oxygen-rich, biotic atmosphere, making it harder to detect the biosphere of such a planet. Similarly, lightning can reduce the concentration of ozone in the anoxic, abiotic atmosphere of a planet orbiting a late M dwarf, thereby reducing the potential for a false-positive life-detection. In summary, the work presented in this thesis provides new constraints for the full characterisation of atmospheric and surface processes on exoplanets and for the interpretation of observations of their atmospheres.