The inside of the moon isn't as watery as previously reported, according to a new study that found a high variety of chlorine atoms in Apollo moon rocks.

For decades scientists had thought the moon is bone dry inside and out. But recent moon-impact missions found water ice on the lunar surface, and reanalysis of rocks brought back by Apollo astronauts found evidence for significant amounts of water inside the moon in the form of hydroxyl (-OH), a hydrogen compound formed by the breakdown of water (H2O).

In a new study of Apollo moon rocks, geochemist Zachary Sharp of the University of New Mexico and colleagues measured the moon rocks' chlorine isotopes, or different forms of the chlorine atom.

Chlorine is strongly attracted to hydrogen. As magma cools and solidifies, hydrogen and chlorine present in the molten rock tend to bond to form hydrogen chloride gas.

On Earth, volcanic magma contains more hydrogen than chlorine, so most of the chlorine bonds with hydrogen. Due to the nature of the bond, the ratio of chlorine isotopes left behind in the cooling rock is about the same as the ratio that gets released as gas.

"We analyzed the moon, and we found that the chlorine isotope values vary by 25 times more than the Earth's," Sharp said.

The best way to explain this result is that the rocks, formed as the moon cooled 4.5 billion years ago, are low in hydrogen.

Instead of becoming mostly hydrogen chloride gas, the chlorine in lunar magma was free to bond with other elements and form salts such as iron chloride and zinc chloride, leading to a wider range of chlorine isotopes in the rock.

And if moon rocks lack hydrogen, they must lack water, the study concludes.

Moon's Lack of Water "Mostly Semantics"

Sharp doesn't dispute the recent studies that found more hydrogen than anticipated in Apollo moon rocks.

But he thinks the reason those moon rocks have a range of chlorine isotopes but also lots of hydrogen is that the astronauts were trained to pick up odd-looking rocks.

This would mean the rocks brought back to Earth are actually not typical of moon rocks overall. It's as if a person walked up a stream and picked up a few rocks that glittered with gold, he said. Based on that sample alone, the rock collector would think Earth's insides are full of gold.

While it might sound like the chlorine study conflicts with his results, McCubbin said the two papers are actually in "complete agreement."

"The apparent difference is mostly a semantics issue," he said in an email.

Both teams show the moon is dry relative to the Earth—but the "moon does have some water," McCubbin argues. (Take a moon quiz.)

The new paper shows that the moon and Earth actually started off with the same chlorine isotope values, he noted, backing up the notion that the moon formed from a chunk of Earth rock blown off by a large impact event.

But, as far as McCubbin is concerned, it's still a mystery why the chlorine isotope values for Earth and the moon are different now.

"Oceanographers like to state that we know far more about the moon than our own oceans," McCubbin said. "But this is a gross overestimation of what is truly known about our closest planetary neighbor."