Jupiter’s moon Europa has a predominantly water-ice surface that is modified by exposure to its space environment. Charged particles break molecular bonds in surface ice, thus dissociating the water to ultimately produce H₂ and O₂, which provides a potential oxygenation mechanism for Europa’s subsurface ocean. These species are understood to form Europa’s primary atmospheric constituents. Although remote observations provide important global constraints on Europa’s atmosphere, the molecular O₂ abundance has been inferred from atomic O emissions. Europa’s atmospheric composition had never been directly sampled and model-derived oxygen production estimates ranged over several orders of magnitude. Here, we report direct observations of H₂⁺ and O₂⁺ pickup ions from the dissociation of Europa’s water-ice surface and confirm these species are primary atmospheric constituents. In contrast to expectations, we find the H₂ neutral atmosphere is dominated by a non-thermal, escaping population. We find 12 ± 6 kg s⁻¹ (2.2 ± 1.2 × 10²⁶ s⁻¹) O₂ are produced within Europa’s surface, less than previously thought, with a narrower range to support habitability in Europa’s ocean. This process is found to be Europa’s dominant exogenic surface erosion mechanism over meteoroid bombardment.

木星的卫星木卫二的表面主要是水冰，由于暴露在太空环境中而发生了变化。带电粒子破坏表面冰中的分子键，从而使水分解，最终产生H ₂和O ₂，​​这为欧罗巴地下海洋提供了潜在的氧化机制。这些物质被认为构成了木卫二的主要大气成分。尽管远程观测对欧罗巴大气层提供了重要的全球限制，但分子 O ₂丰度是根据原子 O 排放推断出来的。木卫二的大气成分从未被直接采样过，模型得出的氧气产量估计值范围在几个数量级上。在这里，我们报告了对木卫二水冰表面解离中的H ₂ ⁺和 O ₂ ^{+拾取离子的直接观测，并确认这些物质是主要的大气成分。}与预期相反，我们发现 H ₂中性大气主要由非热逃逸群体主导。我们发现木卫二表面产生了 12 ± 6 kg s ^-1 (2.2 ± 1.2 × 10 ²⁶  s ^-1 ) O ₂，​​比之前想象的要少，范围更窄，足以支持木卫二海洋的宜居性。该过程被发现是木卫二在流星体轰击中占主导地位的外生表面侵蚀机制。