It has long been thought that our pale lunar companion was forged 4.5 billion years ago when a gigantic planetary body named Theia careened into the very young Earth, sending molten material into orbit around it and eventually cooling into the Moon we all stare up at today.

This ancient impact, however, was hardly the end of our planetary system’s violent history. As revealed in a new Nature Communications study, the next 10 to 200 million years featured a succession of asteroid impacts that may have delivered a vast amount of water to the Moon. This lunar water was first officially detected in 2008, and its origin has been baffling scientists ever since.

“We haven’t completely solved the mystery, but we’ve certainly added to the story,” Jessica Barnes, a postdoctoral research associate at the Open University and lead author of the paper, told IFLScience.

^{Two ways in which water could have been retained on the Moon. Either much of it survived its chaotic formation (a) or there was a later influx of water, via impactors, shortly afterward (b). They aren’t mutually exclusive; both could have happened. Barnes et al./Nature Communications}

Shortly after the Moon’s formation, it was covered in a magmatic ocean, much like  Earth was. This would have existed until at least 4.3 billion years ago; when it had solidified to a sufficient degree, the upper layers of the young Moon would have sunk down into the depths of its own mantle, dragging down any extra “contaminants” with it.

It is during this period that the team’s lunar samples, which date back to the time of this magmatic ocean, suggest that several major water-rich objects likely impacted into the surface. These would have added large volumes of water to the Moon, which sank down as the lunar surface began to significantly cool. Although the hotter inner segments of the early Moon would have encouraged this water to effuse back out again, a thermal lid – a cool, solid crust – at the surface must have prevented any water from doing so.

After obtaining the most precise values to date for the Moon’s water content between 4.5 and 4.3 billion years ago, the team used computer models to work out what type of impact events would have been required to deliver this much water to the Moon in the first place – and both comets and asteroids were clearly involved.

A chemical analysis of the team’s lunar samples showed that the water on the Moon was predominantly like that found in hydrous asteroids called carbonaceous chondrites, not comets, meaning these must have been the Moon’s primary water delivery system. Although comets are far more water-rich than any asteroid, these samples suggest that they account for no more than 20 percent of the lunar water budget.

^{The Moon was once covered in magma, much like all of the inner Solar System planets once were shortly after they formed. RHG/Shutterstock}

These hydrous asteroid impacts appear to predate the Late Heavy Bombardment (LHB), a destructive period of time between 4.1 and 3.8 billion years ago. Around then, much of the inner Solar System – including the Moon – was pummeled by a vast number of asteroids and comets.

The earlier hydrous impactors were likely to be an earlier phase of this LHB rather than a completely distinct event, but there’s much about this period of time that remains completely unknown to researchers. At the very least, though, this research brings up the possibility that water delivered to both the Moon and Earth near their fiery births may have come from hydrous asteroids, and not comets, as many have previously assumed.

“As a community of “lunatics” we’ve achieved a lot since the first detection of water in 2008,” Barnes added. “But there’s still so many unanswered questions, and many won’t be answered until we go back. The Moon is a stepping stone for other places in Solar System, so we need to know what it’s like before we go anywhere else.”