Descent without Modification? The Thermal Chemistry of H 2 O 2 on Europa and Other Icy Worlds

Abstract

The strong oxidant H₂O₂ is known to exist in solid form on Europa and is suspected to exist on several other Solar System worlds at temperatures below 200 K. However, little is known of the thermal chemistry that H₂O₂ might induce under these conditions. Here, we report new laboratory results on the reactivity of solid H₂O₂ with eight different compounds in H₂O-rich ices. Using infrared spectroscopy, we monitored compositional changes in ice mixtures during warming. The compounds CH₄ (methane), C₃H₄ (propyne), CH₃OH (methanol), and CH₃CN (acetonitrile) were unaltered by the presence of H₂O₂ in ices, showing that exposure to either solid H₂O₂ or frozen H₂O+H₂O₂ at cryogenic temperatures will not oxidize these organics, much less convert them to CO₂. This contrasts strongly with the much greater reactivity of organics with H₂O₂ at higher temperatures, and particularly in the liquid and gas phases. Of the four inorganic compounds studied, CO, H₂S, NH₃, and SO₂, only the last two reacted in ices containing H₂O₂, NH₃ making \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \rm NH}_4^{ +}$$ \end{document} and SO₂ making \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \rm SO}_4^{2 -}$$ \end{document} by H⁺ and e⁻ transfer, respectively. An important astrobiological conclusion is that formation of surface H₂O₂ on Europa and that molecule's downward movement with H₂O-ice do not necessarily mean that all organics encountered in icy subsurface regions will be destroyed by H₂O₂ oxidation. Key Words: Europa—Laboratory investigations—Icy moons—Infrared spectroscopy—H₂O₂ resistance. Astrobiology 15, 453–461.