The inside of the moon isn’t as watery as previously reported, according to a new study that found a high variety of chlorine atoms in Apollo moon rocks.

For decades scientists had thought the moon is bone dry inside and out. But recent moon-impact missions found water ice on the lunar surface, and reanalysis of rocks brought back by Apollo astronauts found evidence for significant amounts of water inside the moon in the form of hydroxyl (-OH), a hydrogen compound formed by the breakdown of water (H2O).

In a new study of Apollo moon rocks, geochemist Zachary Sharp of the University of New Mexico and colleagues measured the moon rocks’ chlorine isotopes, or different forms of the chlorine atom.