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

Marrick Braam
The University of Edinburgh
Tuesday, May 10, 2022
1:30 pm
NS2 1201
Determining the habitability of an exoplanet and interpreting atmospheric spectra requires an understanding of its atmospheric physics and chemistry. We use a 3-D Coupled Climate-Chemistry Model, consisting of the Met Office Unified Model and the UK Chemistry and Aerosols framework, to study the emergence of lightning and its chemical impact on tidally-locked Earth-like exoplanets, for a setup of Proxima Centauri b. 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 supports a global stratospheric ozone layer, peaking between 20 and 50 km. We parameterise lightning flashes as a function of cloud-top height, following Earth sciences, and the resulting production of nitric oxide (NO) from the thermal dissociation of N2 and O2. Rapid dayside convection over and around the substellar point results in lightning flash rates of up to four flashes km-2 yr-1. These flashes induce a dayside atmosphere rich in NOx below altitudes of 20 km. Changes in dayside ozone are determined mainly by UV irradiance and the HOx catalytic cycle. 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. In this talk, I will discuss the distribution of lightning flashes and the subsequent spatial variability in lightning-induced chemistry around the exoplanet. Furthermore, I will emphasize the need for accurate stellar UV spectra to study the photochemistry of exoplanet atmospheres.
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