Limitation of atmospheric composition by combustion-explosion in exoplanetary atmospheres.
GRENFELL J.L., GEBAUER S., GODOLT M., STRACKE B., LEHMANN R. and RAUER H.
Abstract (from CDS):
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.