Water-rich asteroids that bombarded the Moon between 4.5 billion and 4.3 billion years ago were the main source of water in the Moon's interior, suggests a new study.
But the paper, published today in Nature Communications, also lends support to previous work that found water was delivered by comets and was also present when the Moon first formed.
Co-author Dr Romain Tartese, from the National Museum of Natural History in Paris, said recent studies of lunar samples had overturned the Apollo-era view of a dry Moon.
These studies had also shown the ratio of heavy hydrogen to normal hydrogen in water from the Moon's interior was consistent with that found in the interior of the Earth, suggesting a common origin.
However, Dr Tartese said, the source of the water and timing of its delivery to the Earth and Moon was still up for debate.
The Moon is believed to have formed from the debris generated by the collision of a Mars-sized planet with the Earth about 4.5 billion years ago.
In the latest study, the researchers argue that in order for the water to be delivered to the interior of the Moon, it had to have been present when a magma ocean still covered the surface.
"It is estimated it took the magma ocean 10 to 200 million years to solidify, so the water had to arrive before a crust formed on the Moon's surface," Dr Tartese said.
"Once delivered into the magma ocean the water dissolved into that and would have been incorporated into minerals that crystallised and then the crust on top of magma would have prevented it escaping."
Chemicals reveal source of water
To determine the source of the water, the researchers compared the chemical compositions of various meteorite types and comets with the known chemical signature of the Moon water.
They determined that a water-rich class of asteroids known as carbonaceous chondrites were responsible for most of the water in the lunar interior, with comets accounting for less than 20 per cent of the total water budget of the Moon.
Although these results suggest that most of the water may have come from asteroids, some of it was also likely to have been derived from the early Earth during the Moon-forming impact event, the authors add.
Associate Professor Craig O'Neill, from the Department of Earth and Planetary Sciences at Macquarie University, said the latest paper was the first to take an overview of the Moon-water question.
"It's like the parable of the blind men and the elephant and they are all looking at different parts of the elephant," he said.
"Studies into the Moon's water have been done in dribs and drabs and what has been missing is an overall view.
"In this paper they have taken that step back and done the maths and modelling of the existing database to give that overview."
Moon is like an 'archive of early Earth'
Dr O'Neill said it was not surprising the study found a number of sources for water on the Moon because the "early solar system was a jumble with [planetary] bodies being flung around".
He said one of the "great unanswered questions of earth sciences was how the Earth got its water".
"The truth is we don't know because part of the problem is we don't have a geological record for the first 500 million years".
Dr Tartese said the Moon could help fill this gap in understanding because it "has preserved everything that happened from 4.5 billion years ago" and was like an "archive of the early Earth".
"The movement of plate tectonics and climate and liquid water means the surface of the Earth is relatively young, whereas the surface of the moon is as it was [at formation], except for a few crater impacts," he said.
Dr Tartese said future planned Moon missions aimed to investigate water at the Moon's poles.
"Surface water is really key for space exploration," Dr Tartese said.
He said water could not only provide hydration, it could be split into oxygen to breathe and hydrogen for fuel.
"Hydrogen from water is an excellent fuel propellant and getting off the surface of the moon is easier with one-sixth of the gravity. Utilising the water could be a way to go further and explore into the solar system from a moon base," he said.