Descent without Modification? The Thermal Chemistry of H₂O₂ 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 NH4+ and SO2 making SO(4)2- 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.

Electron irradiation and thermal chemistry studies of interstellar and planetary ice analogues at the ICA astrochemistry facility

“…Previous studies have revealed that thermal reactions take place in mixed ices of SO 2 and H 2 O, with the primary product being bisulphite (HSO − 3 ) alongside a smaller quantity of meta-bisulphite (S 2 O 2− 5 ) [28,29]. In the presence of oxidant species, the SO 2 may be oxidised to bisulphate (HSO − 4 ) or sulphate (SO 2− 4 ) [30][31][32][33].…”

Methane: Fuel or Exhaust at the Emergence of Life?

“…As with the early Earth, the icy moons would, while producing copious reductants, soon face an oxidant crisis. Denitrifying methanotrophic acetogenesis might well be the emergent metabolism on such worlds, with nitrate/nitrite produced in the ice shell through the oxidation of ammonia with hydrogen peroxide entrained from the surface as electron acceptors (Loeffler and Hudson, 2015 ; Russell et al , 2017 ).…”

Thermal Reactions between H2S and O3: Implications for Europa Surface Chemistry

“…In addition, H 2 S has been tentatively identified on the surface of the Galilean satellites (McCord et al 1998). Though H 2 S does not react with the oxidant H 2 O 2 in the solid phase (Loeffler & Hudson 2015), H 2 S reacts readily with the oxidant O 3 in the gas phase (Cadle & Ledford 1966;Hales et al 1969;Glavas & Toby 1975). A low-temperature thermal oxidation reaction between an additional sulfur compound and the strong oxidant O 3 may have implications for the surface and subsurface chemistry of the Galilean satellites.…”