Much of the present research on water on the Moon has concentrated on what we can see most immediately – the surface. We’ve heard of the strange production of a thin film of water created by the solar wind and other processes, we’ve heard of the potential of relatively large amounts of comet borne ice hiding in shadowy craters. Recently, news also came of apatite, a mineral that is taken as a signature of hydration in rock formation. However, a new study of chlorine isotopes present in Moonrocks now suggests the Moon didn’t form with a lot of water in its composition.
The present best hypothesis for the creation of the Moon suggests a Mars sized object hit the Earth and combined with it, scraping off a load of material into orbit that eventually became the Moon. The question is, how much water went from the Earth to the Moon in that event? The researchers used two isotopes of chlorine – two different sized atoms of the stuff – to try and get an idea. Chlorine 35 is the least massive of the two and so more likely to be transferred as it takes less energy to move atoms of the stuff. But, chlorine 37 preferentially bonds to hydrogen to form HCl, which is distributed differently to Cl and so alters the ratio of the two isotopes. On the Earth, the ratio is very stable. On the Moon, however, it was found that the ratio varied quite a bit and was twenty-five times lower than the terrestrial counterpart. The explanation? There wasn’t any hydrogen around for Cl 37 to bond with, and if there’s no H, there’s no H2O.