Possible favored Great Oxidation Event scenario on exoplanets around M-Stars with the example of TRAPPIST-1e

Abstract

The Great Oxidation Event (GOE), which marked the transition from an anoxic to an oxygenated atmosphere, occurred 2.4 billion years ago on Earth, several hundreds of millions of years after the emergence of oxygenic photosynthesis. This long delay implies that specific conditions in terms of biomass productivity and burial were necessary to trigger the GOE. It could be a limiting factor for the development of oxygenated atmospheres on inhabited exoplanets. In this study, we explore the specificities of a terrestrial planet in the habitable zone of an M dwarf for a GOE. Using a 1D coupled photochemical-climate model, we simulate the atmospheric evolution of TRAPPIST-1 e, an Earth-like exoplanet, exploring the effect of oxygen sources (biotic or abiotic). Our results show that the stellar energy distribution promotes O3 production at lower O2 concentrations compared to Earth, and the ozone layer on TRAPPIST-1 e forms more efficiently. This lowers the threshold for atmospheric oxidation, suggesting that the GOE on TRAPPIST-1 e would occur quickly after the rise of oxygenic photosynthesis, up to 1Gyrs earlier than on Earth, and would reach O2 enabling oxygenic respiration and thus the development of animals. We may question whether this is a general behavior around several M-stars. Furthermore, we discuss how the overproduction of ozone could make O3 detection possible using the James Webb Space Telescope, providing a potential method to observe oxygenation signatures on exoplanets in the near future. Previous studies predicted that for an Earth-like atmosphere O3 would require over 150 transits for detection, but our results show that significantly fewer transits could be needed.

Figures (10)

• Figure 1: Methane oxidation cycle and net reaction. The net reaction consumes methane to the benefice of CO$_2$ accumulation. However, this process actually involves a sequence of fast bimolecular reactions that interconvert short-lived radicals. These radicals are consumed as soon as produced, so that their concentrations remain stable but low in the atmosphere. They can be considered as catalysts of the net reaction. The catalytic cycle (B1) is illustrated on the figure: OH and CH$_3$ radicals are inter-converted, leading to the net consumption of CH$_4$ and O$_2$, and the production of CH$_2$O and H$_2$O.
• Figure 2: Stellar irradiation received at the top of the atmosphere on Earth 2.7Ga ago (blue) Claire2012 and on TRAPPIST-1e (orange) Peacock2019. Cross section per molecules of O$_2$, O$_3$ and H$_2$O$_2$ are represented in grey. Knowing O$_3$ less abundant than O$_2$, their photolysis shifting point is around 200nm, close to where the flux balance change between the Sun and TRAPPIST-1.
• Figure 3: Ozone profiles in mass mixing ratio (mmr) for different levels of surface O$_2$ from the 1D G-PCM. 1 PAL represent the present atmospheric level. Solid lines are early Earth simulations and Earth in black. Dashed lines are TRAPPIST-1e analogue simulations. Both planets differ mainly by their stellar irradiation. TRAPPIST-1e is favorable to the production of more ozone compare to Earth.
• Figure 4: Oxygen atmospheric loss ($F_{O_2}$), methane atmospheric loss ($F_{CH_4}$) and O$_3$ column density as a function of surface O$_{2}$. Surface abundances of [CH$_4$] = 10$^{-4}$ and [CO$_2$] = 10$^{-2}$. Results of early Earth model on Earth (solid) and on TRAPPIST-1e (dash).
• Figure 5: Top: oxygen and methane abundance as a function of the time. Results of early Earth model on Earth Jaziri2022 (solid) and on TRAPPIST-1 e (dash). The Pasteur point (green pointed line) shows the oxygen level necessary to develop aerobic respiration. Bottom: evolution of the oxygenation parameter K$_{oxy}$=F$_{source}$/F$_{sink}$ at the surface (blue line). The critical value of K$_{oxy}$ corresponding to the triggering of the GOE is represented with a red line for the Earth (solid) and Trappist-1 e (dashed). K$_{oxy}$ and its critical value are expressed in the Methods section