Wet. Dry. Wet. Dry. You’d think the moon were a vacuum cleaner infomercial.
A series of studies in the last few years has raised our hopes that the moon is not completely dry—researchers have said that it’s still drier than the driest places on Earth, but some small amount of water ice is there. Then, this afternoon, along comes another study to reassert that the interior of the moon is drier than bone-dry.
For his paper in Science, Zachary Sharp peered into the lunar samples brought back to Earth by the Apollo missions. Where previous studies of those Apollo rocks suggested water ice was locked inside the minerals, Sharp’s assessment focuses on the chlorine in the sample because it could tell him about the moon’s history.
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 [Science News].
The two isotopes are chlorine-35 and chlorine-37. What matters is the ratio between them. Here on Earth it’s pretty constant, varying by just about a tenth of a percent. On the moon, Sharp found, the ratio varies wildly—by as much as 25 times the variation seen on Earth.
Chlorine loves to bind to hydrogen, and because of this, you’d expect the chlorine isotope ratios to be pretty constant across the moon if it had been wet way back when.
“Knowing the chlorine content, we can back-calculate the amount of hydrogen,” Sharp said. “We found that the hydrogen content had to be really low, so essentially the moon was extremely dry relative to Earth” [Space.com].
So how do we get these conflicting studies about water on the moon? Sharp argues that they may not truly conflict. His study is about the history of moon—he says that the chlorine study shows it didn’t retain water after it formed. The water that other studies found on the surface may have gotten there through comets impacts. He says:
“There are two types of water on the moon.” The type most people hear about is the stuff hidden in dark craters near the lunar poles, which probably came from comet collisions. “That is completely different from what we’re talking about” [Discovery News].
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