Lightning-induced chemistry on tidally-locked Earth-like exoplanets

Abstract

Determining the habitability and interpreting atmospheric spectra of exoplanets requires understanding their atmospheric physics and chemistry. We use a 3-D Coupled Climate-Chemistry Model, the Met Office Unified Model with the UK Chemistry and Aerosols framework, to study the emergence of lightning and its chemical impact on tidally-locked Earth-like exoplanets. We simulate the atmosphere of Proxima Centauri b orbiting in the Habitable Zone of its M-dwarf star, but the results apply to similar M-dwarf orbiting planets. Our chemical network includes the Chapman ozone reactions and hydrogen oxide (HOx=H+OH+HO2) and nitrogen oxide (NOx=NO+NO2) catalytic cycles. We find that photochemistry driven by stellar radiation (177–850 nm) supports a global ozone layer between 20–50 km. We parameterise lightning flashes as a function of cloud-top height and the resulting production of nitric oxide (NO) from the thermal decomposition of N2 and O2. Rapid dayside convection over and around the substellar point results in lightning flash rates of up to 0.16 flashes km−2yr−1, enriching the dayside atmosphere below altitudes of 20 km in NOx. Changes in dayside ozone are determined mainly by UV irradiance and the HOx catalytic cycle. ∼45 per cent of the planetary dayside surface remains at habitable temperatures (Tsurf > 273.15 K) and the ozone layer reduces surface UV radiation levels to 15 per cent. Dayside-nightside thermal gradients result in strong winds that subsequently advect NOx towards the nightside, where the absence of photochemistry allows NOx chemistry to involve reservoir species. Our study also emphasizes the need for accurate UV stellar spectra to understand the atmospheric chemistry of exoplanets.

3D modelling of the impact of stellar activity on tidally locked terrestrial exoplanets: atmospheric composition and habitability

“…Our implementation, and this study are focused on the stratospheric ozone distribution on an 'Earth-like' planet with an initial atmospheric composition similar to modern Earth in a tidally locked orbit of a host M dwarf star, based on ProxCen b. This work has been performed alongside and in close collaboration with that of Braam et al ( 2022 ) who further adapted the UKCA framework, building on the work of Yates et al ( 2020 ), to study the impact of lightning on the tropospheric ozone composition of ProxCen b also using the UM. As we have been using the same GCM and the same planetary configuration, but different chemistry schemes, this has provided a useful environment for testing and developing both of our models.…”

“…The increased rate of photolysis leads to significantly higher abundances of atomic oxygen, which leads to significantly faster growth in ozone through the three-body reaction O 2 + O( 3 P) + M −→ O 3 + M , where M denotes a third body. Braam et al ( 2022 ) used the same spectrum as this work to simulate a similar 'Earth-like' planet as Yates et al ( 2020 ), and found that when compared to Yates et al ( 2020 ) (who used the BT-Settl spectrum) they had significantly higher amounts of ozone. This was due to two factors, the change in spectrum to one which has higher UV increases the amount of ozone significantly, and an impro v ed calculation of photolysis rates as compared to the work done by Yates et al ( 2020 ).…”

“…In reality, to make progress we need a range of approaches. In this work we introduce a new model, complementary in its approach to that of Chen et al ( 2021 ) and Braam et al ( 2022 ) in examining ozone chemistry in 3D, but are aware that this first step moti v ates extensi ve follow-up both in terms of more e xtensiv e application of this model, and further development.…”

3D modelling of the impact of stellar activity on tidally locked terrestrial exoplanets: atmospheric composition and habitability

“…The increased rate of photolysis leads to significantly higher abundances of atomic oxygen, which leads to significantly faster growth in ozone through the three-body reaction O 2 + O( 3 P) + M − −− → O 3 + M, where M denotes a third body. Braam et al (2022) used the same spectrum as this work to simulate a similar 'Earth-like' planet as Yates et al (2020), and found that when compared to Yates et al (2020) (who used the BT-Settl spectrum) they had significantly higher amounts of ozone. This was due to two factors, the change in spectrum to one which has higher UV increases the amount of ozone significantly, and an improved calculation of photolysis rates as compared to the work done by Yates et al (2020).…”

“…In reality, to make progress we need a range of approaches. In this work we introduce a new model, complementary in its approach to that of Chen et al (2021) and Braam et al (2022) in examining ozone chemistry in 3D, but are aware that this first step motivates extensive follow-up both in terms of more extensive application of this model, and further development.…”