There’s a tiny bit of water on the surface of the moon.
The ultrathin 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. [Yang Liu et al., Direct measurement of hydroxyl in the lunar regolith and the origin of lunar surface water]
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.
[The above text is a transcript of this podcast.]