Detection of organic compounds in freshly ejected ice grains from Enceladus’s ocean

Saturn’s moon Enceladus ejects a plume of ice grains and gases originating from a subsurface ocean via fractures near its south pole. The chemical characterization of organic material in such ice grains was previously conducted via the analysis of mass spectra obtained in Saturn’s E ring by Cassini’s Cosmic Dust Analyzer at impact speeds below 12 km s−1. Here we present a comprehensive chemical analysis of organic-bearing ice grains sampled directly from the plume during a Cassini fly-by of Enceladus (E5) at an encounter speed of nearly 18 km s−1. We again detect aryl and oxygen moieties in these fresh ice grains, as previously identified in older E-ring grains. Furthermore, the unprecedented high encounter speed revealed previously unobserved molecular fragments in Cosmic Dust Analyzer spectra, allowing the identification of aliphatic, (hetero)cyclic ester/alkenes, ethers/ethyl and, tentatively, N- and O-bearing compounds. These freshly ejected species are derived from the Enceladus subsurface, hinting at a hydrothermal origin and involvement in geochemical pathways towards the synthesis and evolution of organics. Fresh ice grains from Enceladus’s plume were analysed during the highest-speed fly-by of the Cassini spacecraft. Organic compounds with a range of chemical structures were discovered, suggesting comprehensive subsurface chemistry on Enceladus.