Pickup ions from the breakup of Europa’s water ice surface

Jupiter’s moon Europa has a predominantly water-ice surface, modified by exposure to its17 space environment. Charged particles break molecular bonds in surface ice, dissociating18 water to ultimately produce H2 and O2 (Johnson 1990) and provide a potential oxygenation19 mechanism for Europa’s subsurface ocean (Chyba 2000). These species are understood to20 form Europa’s primary atmospheric constituents (Smyth & Marconi, 2006). While remote21 observations provide constraints on Europa’s atmosphere (Hall et al. 1995; de Kleer et al.22 2023), molecular O2 has solely been inferred from atomic O emissions. Europa’s atmospheric23 composition had never been directly sampled and model-derived oxygen production24 estimates ranged over multiple orders of magnitude. Here, we report direct observations of25 H2+ and O2+ pickup ions from the dissociation of Europa’s water-ice surface, confirming26 these species as primary atmospheric constituents. In contrast to expectations, we find the27 H2 neutral atmosphere is dominated by a non-thermal, escaping population. We find 12 ± 628 kg s -1 (2.2 ± 1.2 x 10 26 s -1 ) O2 are produced within Europa’s surface, less than previously29 thought, with a narrower range to support habitability in Europa’s ocean. This process is30 found to be Europa’s dominant exogenic surface erosion mechanism over meteoroid31 bombardment.