Ganymede’s atmosphere is produced by charged particle sputtering and sublimation of its icy surface. Previous far-ultraviolet observations of the O i 1,356 Å and O i 1,304 Å oxygen emissions were used to infer sputtered molecular oxygen (O₂) as an atmospheric constituent, but an expected sublimated water (H₂O) component remained undetected. Here we present an analysis of high-sensitivity spectra and spectral images acquired by the Hubble Space Telescope revealing H₂O in Ganymede’s atmosphere. The relative intensity of the oxygen emissions requires contributions from the dissociative excitation of water vapour, indicating that H₂O is more abundant than O₂ around the subsolar point. Away from the subsolar region, the emissions are consistent with a pure O₂ atmosphere. Eclipse observations constrain atomic oxygen to be at least two orders of magnitude less abundant than these other species. The higher H₂O/O₂ ratio above the warmer trailing hemisphere compared with the colder leading hemisphere, the spatial concentration in the subsolar region and the estimated abundance of ~10¹⁵ molecules of H₂O per cm² are consistent with sublimation of the icy surface as source.

木卫三的大气层是由带电粒子溅射和其冰冷表面的升华产生的。先前对 O i 1,356 Å 和 O  i 1,304 Å 氧排放的远紫外观测 用于推断溅射分子氧 (O ₂ ) 作为大气成分，但预期的升华水 (H ₂ O) 成分仍未被发现。在这里，我们对哈勃太空望远镜获得的高灵敏度光谱和光谱图像进行了分析，揭示了木卫三的大气中的 H ₂ O。氧气排放的相对强度需要水蒸气的解离激发的贡献，这表明 H ₂ O 比 O _2更丰富在太阳下点附近。远离太阳下区域，排放与纯 O ₂大气一致。日食观测将原子氧的含量限制为至少比这些其他物种少两个数量级。与较冷的前半球相比，较暖的尾半球上方的H ₂ O/O ₂比较高，太阳下区域的空间浓度和每 cm ^{2约 10 15个 H} ₂ O分子的估计丰度与升华一致冰冷的表面作为来源。