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The Moon's water may have come mostly from asteroids, not comets

Shortly after the Moon was formed 4.5 billion years ago

Luc Viatour

Scientists have long debated the origin of the Moon’s water — whether it came from our planet, comets, or asteroids. Now, a new study is saying that the water found inside the Moon came mostly from asteroids. The researchers say that knowing where the Moon’s water came from can tell us about the origins of water on Earth, and even help us better understand how life formed on our planet.

Knowing where the Moon’s water came from can tell us about the origins of water on Earth

Most scientists agree that the Moon’s water probably came from different sources, with so-called wet asteroids — asteroids that contain water — playing a major role. Today’s study, published in the journal Nature Communications, is the first one to single out exactly which types of asteroids are responsible for giving water to the Moon. It also suggests that comets only contributed a very small amount of water.

"If you want to understand where water came from on the Earth, it’s very helpful to understand where water from your nearest neighbor also originated from," says James Day, a geoscientist at Scripps Institution of Oceanography, who did not work on the study. "That’s going to have collateral effects on understanding the origin of life."

Lead study author Jessica Barnes, a post-doc at the Open University, says the implications go even further. "If we want to look for life elsewhere in the Solar System and beyond, following water is the easiest way we have to understanding if a planet was habitable in the past, or if it’s habitable today," she says.

An artist's illustration of what the Moon looked like when it was made of magma, soon after it was formed.

The Moon was long thought to be bone-dry. But recently, scientists have realized that there’s actually some water both inside the Moon and on its surface. The Moon’s water isn’t liquid; some ice can be found in the craters that don’t receive any sunlight, but mostly, traces exist inside the Moon’s minerals. In recent years, evidence of lunar water was found on the rock samples brought back by the Apollo missions, opening up the debate of where the water actually came from. "There’s been a long-standing debate in the community as to whether the water came from comets, whether it came from asteroids, whether it came from Earth," Barnes says.

The researchers identified the origin of the water by analyzing the composition of elements like hydrogen and nitrogen in lunar rocks and comparing it to that of asteroids and comets. They found that the chemical composition of water inside the Moon matches that of water-rich meteorites called carbonaceous chondrites, but not that of most comets, and concluded that comets contributed less than 20 percent of the water found inside the Moon. They also used data from a range of previous studies to model the water’s arrival, finding that it likely got to the Moon soon after its creation about 4.5 billion years ago, when the Moon was an ocean of magma.

"The modeling is very straight forward," says Erik Hauri, a planetary scientist at the Carnegie Institution for Science who has researched lunar water and did not work on this study. "This group of researchers is doing a really first-rate job at studying the Moon and its water budget."

"This group of researchers is doing a really first-rate job at studying the Moon and its water budget."

Others caution that the Moon’s chemical composition could be very different today than it was when water arrived. The Moon was an ocean of magma billions of years ago, and the process of erupting, degassing, and solidifying likely changed the water’s composition, says Alberto Saal, an associate professor of earth, environmental, and planetary sciences at Brown University, who’s researched water on the Moon. "They did the best with what we have, that’s for sure," Saal says. "Now the question is, how close is this from what is reality?"

Day, of Scripps Institution of Oceanography, agrees and says that the researchers’ model is based on a lot of assumptions. The study used data collected from the lunar rocks brought back by the Apollo missions, but only about 2 percent of those samples have been analyzed for water. "I think it’s definitely a thought-provoking model. It’s definitely not 100 percent watertight," he says. "There are some major uncertainties that still exist."

"Now the question is, how close is this from what is reality?"

Barnes agrees that there are a lot of questions left to answer. The lunar rocks we have were brought back by Apollo astronauts who covered a pretty short distance on the Moon’s surface. "Imagine you were an alien and you went to Earth and you only went from Glasgow to Edinburgh and back again," Barnes says. "You wouldn’t understand a whole lot about the Earth. You’d be missing the pyramids, you’d be missing the Grand Canyon. Although we’ve done a lot with the material that we have, there’s still much of the Moon we haven’t explored yet."

Until recently, we didn’t even know the Moon had water. But we’ve come a long way to understanding where it came from, Barnes says. "This is a really exciting and young, if you like, field of research," she says. "We’re at that stage where we’re beginning to understand a lot, but there are still so many questions left to answer."