The moon probably had water when it first formed four and a half billion years ago, according to a new study.
Research reported in the journal Nature Geoscience, found evidence of water that was brought to the surface from deep within the lunar mantle by a series of ancient impacts.
"I think it would be very tough to have this water be anywhere other than original to the material that formed the moon," said the study's lead author Rachel Klima of Johns Hopkins University.
"I don't think this was cometary water that was somehow mixed in and excavated back out, or solar wind water. I think this had to be water that was initially there when the materials forming the moon accreted, and what we found supports that idea."
The new water signatures, in the form of hydroxyl molecules, were detected in the central peak of Bullialdus Crater on the moon's near side, according to Klima.
Hydroxyls are molecules consisting of an oxygen atom connected to a hydrogen atom. The pairing is often seen as a substructure of a water molecule.
"Hydroxyls can form when hydrogen in the solar wind flux hits the minerals in the rocks on the lunar surface," said Klima.
"That tends to happen in cooler areas, and there have been signs that it migrates with the lunar day. So basically when it's cooler it will form and stick to the surfaces, and when it gets warmer later in the lunar day it will move."
When Klima first detected hydroxyl in her spectroscopic readings, she assumed it was solar wind generated surface hydroxyls, similar to what had been seen previously.
"But looking at the crater in more detail and at different times during the lunar day, I found there was no change in the hydroxyl signature," said Klima.
Klima and colleagues were also unable to detect any hydroxyls in the surrounding lunar soil. The only hydroxyls were in the crater's central peak, indicating that it had been dredged up from deep underground.
"It was only in the center of the crater where the rocks from the deepest part of that area had been brought up to the surface, that we saw this hydroxyl signature," said Klima.
"This crater is only about 60 kilometers across, but it occurs on the rim of a larger impact basin, which would have excavated much deeper.
"So we have a two-stage excavation with a big-impact event bringing material up from very deep, and the smaller crater impacting into this material bringing it up to the surface."
Klima estimates the hydroxyl-embedded rocks may have been up to 69 kilometers below the lunar surface, prior to impact.