The ultra-thin dusting of water molecules was first detected in 2009 from spacecraft measurements. So where did the H2O come from? A chemical (analysis) of lunar samples now points to a likely culprit: the solar wind, a stream of charged particles emanating from the sun.

Researchers looked at agglutinates, a type of glassy lunar material brought back by Apollo astronauts. Agglutinates (formed) when micrometeorites hit the lunar surface and melted a bit of the powdery regolith. Tiny intact grains of soil were preserved within, like insects in amber.

Those agglutinates turn out to contain hydroxyl, or OH, a precursor molecule to water. And the OH's isotopic signature (indicates) that much of the hydrogen came from the solar wind. Add solar wind hydrogen to moon material that contains oxygen and, voila, water. The research is in the journal Nature Geoscience.

The researchers note that similar chemistry could be at work on Mercury, large (asteroids) and other airless bodies that endure the full blast of the solar wind. Meaning that even as the sun bakes those worlds, it delivers a bit of hydration.