The effect of lightning on the atmospheric chemistry of exoplanets and potential biosignatures

¹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, Graz 8042, Austria
e-mail: patrick.barth@simtech.uni-stuttgart.de
² Centre for Exoplanet Science, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
³ School of Earth & Environmental Sciences, University of St Andrews, Bute Building, Queen’s Terrace, St Andrews KY16 9TS, UK
⁴ SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
⁵ Stuttgart Center for Simulation Science, University of Stuttgart, Pfaffenwaldring 5a, 70569 Stuttgart, Germany
⁶ Fakultät für Mathematik, Physik und Geodäsie, TU Graz, Petersgasse 16, Graz 8010, Austria
⁷ Department of Earth and Planetary Sciences, University of California, Riverside, CA, USA
⁸ Blue Marble Space Institute of Science, Seattle, WA, USA
⁹ School of Natural and Environmental Sciences, Newcastle University, Newcastle upon Tyne NE1 7RU, UK

Received: 26 June 2023
Accepted: 19 February 2024

Abstract

Context. Lightning has been suggested to play a role in triggering the occurrence of bio-ready chemical species. Future missions such as PLATO, ARIEL, HWO, and LIFE, as well as ground-based extremely large telescopes (ELTs), will carry out investigations of the atmospheres of potentially habitable exoplanets.

Aims. We aim to study the effect of lightning on the atmospheric chemistry. We also consider how it affects false-positive and false-negative biosignatures and whether these effects would be observable on exo-Earth and TRAPPIST-1 planets.

Methods. We utilised a combination of laboratory experiments and photochemical and radiative transfer modelling. We conducted spark discharge experiments in N₂−CO₂−H₂ gas mixtures, representing a range of possible rocky-planet atmospheres. We investigated the production of potential lightning signatures (CO and NO), possible biosignature gases (N₂O, NH₃, and CH₄), and important prebiotic precursors (HCN and urea). Using the measured CO and NO production rates, we conducted photochemical simulations for oxygen-rich and anoxic atmospheres for rocky planets orbiting in the habitable zones of the Sun and TRAPPIST-1 for a range of lightning flash rates. Synthetic spectra were calculated using SMART to study the atmosphere’s reflectance, along with the emission and transmission spectra.

Results. Lightning enhances the spectral features of NO, NO₂, and (in some cases) CO through direct production; whereas CH₄ and C₂H₆ may be enhanced indirectly. Lightning at a flash rate slightly higher than on modern-day Earth is 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 at a flash rate at least ten times higher than on modern-day Earth is also able to mask the presence of ozone in the anoxic, abiotic atmosphere of a planet orbiting a late M dwarf, reducing the potential for a false-positive life detection.

Conclusions. The threshold lightning flash rates to eliminate oxygen (>0.1%) and ozone false positive biosignatures on planets orbiting ultra-cool dwarfs is up to ten times higher than the modern flash rate. This result indicates that lightning cannot always prevent these false-positive scenarios.