2018ApJ...861...38G

This work presents theoretical studies that 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(g) produced abiotically via water photolysis followed by hydrogen escape. If these two constituents were present simultaneously, such large amounts of H₂(g) and O₂(g) can react via photochemistry to form up to ∼10 Earth oceans. In cases where photochemical removal is slow, hence O₂(g) can indeed build up abiotically, the atmosphere could reach the combustion-explosion limit. Then, H₂(g) and O₂(g) react extremely quickly to release energy and form liquid water together with modest amounts of hydrogen peroxide. These processes set constraints for H₂(g) and O₂(g) atmospheric compositions in Super-Earth atmospheres. Our initial study of the gas-phase oxidation pathways for modest conditions (Earth's insolation and ∼10th of a percent of H₂(g)) suggests that H₂(g) is oxidized by O₂(g) into H₂O(g) mostly via HOx and mixed HOx-NOx catalyzed cycles. Regarding other pairs of atmospheric species, we find that CO-O₂ could attain explosive-combustive levels on mini gas planets for midrange C/O in the equilibrium chemistry regime (p > ∼1 bar). Regarding (CH₄-O₂), 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.