Electron irradiation of methanol ices: Implications for organic chemical diversity on Europa's surface

Europa is an icy moon of Jupiter with an internal, salty liquid water ocean and a young surface that shows signs of exchange of material from the subsurface ocean and collection of exogenous material from Io. Organic compounds, if detected on the surface of Europa, could help in understanding the conditions of its internal ocean. Any possible organic molecules that could be present on Europa's surface will, however, be affected by the constant irradiation of ions and electrons coming from Jupiter's magnetosphere. This experimental work aims to study the fate of organic compounds under conditions anticipated for Europa's heavily irradiated surface regions to provide laboratory data in support of the Europa Clipper mission. These laboratory data will help discriminate between molecules that have been modified by surface irradiation and those that record interior conditions and exchange. For that, we irradiated ~5 μm thick samples of methanol ice at 50 K, 80 K, and 130 K with 10 keV electrons. We monitored the infrared spectra of the ice, then analyzed the volatile products using Gas Chromatography Mass Spectrometry (GCMS). Carbon monoxide, carbon dioxide, methane and various alcohols, ethers, aldehydes, esters, ketones, alkenes, and heterocycles were detected in the ice and gas phases. The abundances of compounds were especially affected by irradiation dose and substrate temperature. Ethers and aldehydes are the most prominent families observed in the gas phase, with ethers being formed at early times in the irradiation and aldehydes appearing later. With electrons, longer-chain products are formed compared to UV irradiation. Furthermore, some organic compounds observed in UV irradiation experiments are not formed under electron irradiation. A large fraction of organic molecules, if detected by future missions on Europa's surface, could therefore be the products of surface irradiation and not necessarily indicators of the conditions of Europa's ocean.