The inside of the moon might not be all wet after all. A new study suggests that, contrary to recent work, the lunar interior is as bone-dry as scientists thought 40 years ago, when NASA astronauts lugged home the first moon rocks.
New analyses of chlorine in those rocks, published online August 5 in Science, indicate that the moon contains just one-10,000th to one-100,000th the water that the Earth’s interior does.
Studying the wateriness of different worlds can illuminate how they evolved, says geochemist Zachary Sharp of the University of New Mexico in Albuquerque, lead author of the new paper. “It’s a window into processes that shaped the solar system soon after it formed.”
Researchers have long argued over whether the moon contains water on its surface — frozen in shadowy craters, for instance. Such water would not be native to the moon, but instead delivered there over time by comet impacts. The new studies tackle a more fundamental question: How much water did the moon contain inside when it formed, 4.5 billion years ago?
Most scientists think the moon was born when a huge object roaming the inner solar system — something about the size of Mars — smashed into the embryonic Earth. Debris from the collision coalesced to form the moon. As it cooled, an ocean of magma covering its surface began to crystallize. Sharp and his colleagues studied what happened to two isotopes of the element chlorine during that process.
Chlorine-35 has two fewer neutrons in its nucleus than chlorine-37, and hence is lighter and was more prone to vaporizing out from the magma ocean. But if the magma also contained a lot of hydrogen — perhaps in the form of water, H2O — a competing process would also take place. Chlorine-37 likes to bond with hydrogen and vaporize out as hydrogen chloride. So if hydrogen were present, more chlorine-37 would escape the magma along with chlorine-35.
But that’s not what Sharp’s team saw when analyzing 11 samples of moon rocks and soil. Instead, they found a wide range in the ratios of chlorine-35 to chlorine-37. The best explanation, Sharp says, is that there was hardly any hydrogen in the moon’s magma ocean. No hydrogen means no water.
Lunar scientists reached the same conclusion 40 years ago, when they first cracked into the Apollo samples and found them full of metallic iron, with no sign of having been chemically altered by water. But in 2008, an analysis of a handful of lunar volcanic glass beads suggested they might have formed in a watery environment. Since then, several research groups have also looked at the mineral apatite, which can lock up water in its chemical structure, in lunar rocks. Using newly developed analytical techniques, some of these groups reported — including in a paper in the July 22 Nature — that the moon could have contained quite a bit of water, perhaps almost as much as Earth’s interior did.
Such estimates are hard to reconcile with the new chlorine work suggesting a bone-dry moon. One possible explanation: The research teams are all looking at separate parts of the same problem, and some parts of the moon may have been wetter than others. “I think we’re dealing with a case of three blind men and an elephant,” says James Greenwood, a planetary scientist at Wesleyan University in Connecticut who has worked on apatite studies.
Another apatite researcher, Francis McCubbin of the University of New Mexico, points out that one person’s “bone-dry” could be another person’s “relatively damp.” Many researchers now agree that the moon contains some water, he says, but it’s “still very dry in comparison to other planetary bodies, like Earth and Mars.”
More studies will be needed to pinpoint exactly how much water the moon might have contained, says Lindy Elkins-Tanton, a geologist at MIT. “I think the moon is a little wetter than we used to think it was,” she says. “But there are many questions about how much water there was, and where it was residing.”
