The isotopic composition of water in Earth’s oceans is challenging to recreate using a plausible mixture of known extraterrestrial sources such as asteroids—an additional isotopically light reservoir is required. The Sun’s solar wind could provide an answer to balance Earth’s water budget. We used atom probe tomography to directly observe an average ~1 mol% enrichment in water and hydroxyls in the solar-wind-irradiated rim of an olivine grain from the S-type asteroid Itokawa. We also experimentally confirm that H⁺ irradiation of silicate mineral surfaces produces water molecules. These results suggest that the Itokawa regolith could contain ~20 l m⁻³ of solar-wind-derived water and that such water reservoirs are probably ubiquitous on airless worlds throughout our Galaxy. The production of this isotopically light water reservoir by solar wind implantation into fine-grained silicates may have been a particularly important process in the early Solar System, potentially providing a means to recreate Earth’s current water isotope ratios.

使用小行星等已知地外来源的合理混合物来重建地球海洋中水的同位素组成具有挑战性——需要一个额外的同位素轻质水库。太阳的太阳风可以为平衡地球的水预算提供答案。我们使用原子探针断层扫描直接观察了来自 S 型小行星 Itokawa 的橄榄石颗粒的太阳风辐照边缘中平均约 1 mol% 的水和羟基富集。我们还通过实验证实硅酸盐矿物表面的 H ⁺辐照会产生水分子。这些结果表明，糸川风化层可能含有约 20 l m ^{- 3}来自太阳风的水，并且这样的水库可能在我们整个银河系的无空气世界上无处不在。通过将太阳风注入细粒硅酸盐中产生这种同位素轻水库可能是早期太阳系中一个特别重要的过程，可能为重建地球当前的水同位素比率提供了一种手段。