Glassy grains within lunar soils contain significant amounts of water, according to a study published online in Nature Geoscience. Once thought to be completely dry, the lunar surface owes this trace of wetness to interactions with the solar wind.

Yang Liu and colleagues analysed lunar soil samples returned by the Apollo missions and detected water in some of the grains. Further analyses revealed geochemical similarities to hydrogen ions in the solar wind. This suggests that the solar wind delivers hydrogen ions to the lunar surface that are then converted and stored as water in the glassy grains.

In an accompanying News and Views article, Marc Chaussidon writes that these findings of lunar surface water linked to the solar wind is “opening the door to another source of water for inner Solar System bodies.”