Limitation of atmospheric composition by combustion-explosion in exoplanetary atmospheres

TL;DR

This paper explores how combustion and explosion processes could limit atmospheric compositions on exoplanets, especially Super Earths, by analyzing chemical reactions and potential explosive limits involving H2, O2, CO, and CH4.

Contribution

It combines combustion theory with exoplanetary atmospheric science to identify conditions leading to atmospheric explosions and constraints on chemical compositions.

Findings

H2 and O2 can react rapidly to form water and hydrogen peroxide.

CO and O2 mixtures may reach explosive levels on mini gas planets.

Certain CH4 and O2 atmospheres could be near explosive thresholds on rocky planets.

Abstract

This work presents theoretical studies which combine aspects of combustion and explosion theory with exoplanetary atmospheric science. Super Earths could possess a large amount of molecular hydrogen depending on disk, planetary and stellar properties. Super Earths orbiting pre-main sequence-M-dwarf stars have been suggested to possess large amounts of O2 produced abiotically via water photolysis followed by hydrogen escape . If these two constituents were present simultaneously, such large amounts of H2 and O2 can react via photochemistry to form up to about 10 Earth oceans. In cases where photochemical removal is slow so that O2 can indeed build up abiotically, the atmosphere could reach the combustion explosion limit. Then, H2 and O2 react extremely quickly to form water together with modest amounts of hydrogen peroxide. These processes set constraints for H2 O2 atmospheric compositions in Super Earth atmospheres. Our initial study of the gas-phase oxidation pathways for modest conditions with Earth insolation and about a tenth of a percent of H2 suggests that H2 is oxidized by O2 into H2O mostly via HOx and mixed HOx NOx catalyzed cycles. Regarding other atmospheric species-pairs we find that CO,O2 mixtures could attain explosive combustive levels on mini gas planets for mid range for the C to O ratio in the equilibrium chemistry regime for p more than about 1bar. Regarding CH4,O2 mixtures, a small number of modeled rocky planets assuming Earth like atmospheres orbiting cooler stars could have compositions at or near the explosive combustive level although more work is required to investigate this issue.