We’ve had a few years to get used to the idea that the Moon is wet. India’s Chandrayaan-1 probe created a detailed map of lunar resources in 2009, and subsequent analysis found that as much as 600 million metric tons of water are locked inside the Moon’s rocks. And the evidence has been piling up. Proponents of lunar exploration often cite the Moon’s water as the key to making a human presence on our satellite a reality. But as soon as lunar water was confirmed, the question facing scientists was where did it come from. Theories typically center on comets and meteorites delivering water to the Moon, but there’s a new theory that proposes a very different source: the Moon’s water might have come from the Earth.There are two kinds of water on our planet: regular water—the familiar H20—and heavy water, D20. Heavy water contains a doubly heavy hydrogen atom called deuterium, and while it does occur naturally, it’s a lot less common than regular water. The deuterium-to-hydrogen ratio in the Earth’s oceans is about 1.56 ×10-4. That’s the same ratio measured in carbonaceous chrondrites, a type of meteorite found on Earth that scientists think originated in the outer asteroid belt between Mars and Jupiter and that brought water to the Earth in the first place.It’s this ratio of hydrogen to deuterium that the new theory of lunar water’s source is based on. A team of researchers led by Alberto Saal, an associate professor of Geological Sciences at Brown University, analyzed the isotopic composition of hydrogen in rocks returned by the Apollo 15 and Apollo 17 crews. Specifically, Saal’s team looked at the water locked inside tiny bubbles of supercooled lava as well as melt inclusions, blobs of melted materials trapped in slowly cooling magma that finally solidified.It turns out the water found in the lunar samples has the same deuterium levels as the carbonaceous chrondrites meteorites. It’s a discovery that’s led the team to suspect the Moon’s water didn’t come from the same meteorites that hit the young Earth, but from the Earth itself.The simplest explanation of how this happened is the same as the leading explanation of how the Moon itself was formed: when a Mars-sized object smashed into the early Earth, it sent debris flying away from the planet. This debris eventually coalesced and cooled, forming the Moon. If the Earth was already wet when that happened, which scientists think might have been the case, it’s feasible that some water was transferred in that collision.
But how was water, on the Earth or the Moon, able to stick around? A collision powerful enough to break apart a planet would produce intense heat, more than enough to vaporize any water that might have been present.There are some possible explanations, though right now they’re only theories. It’s possible the newly formed Moon siphoned off some of the Earth’s water-vapour rich atmosphere and held on to it long enough for water to become locked in the lunar material. Or perhaps the main chunk of Earth that became the Moon had water locked inside its chemical composition, water that later melted thanks to the Moon’s own interior heat.There are more questions in need of answers before this theory of shared water can become widely regarded as fact. We might have to go as far as finding another explanation for how the Moon was formed.
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A close up of a melt inclusion inside a Moon rock. via
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