Recent studies have found vast amounts of water ice at or near the lunar surface. But the inside of the moon is bone dry, a new study finds.

A recent study of lunar rock samples from NASA's Apollo missions could mean the moon's interior harbors less water than thought. In fact, a new examination of the lunar rocks' chlorine composition indicates that the moon is essentially dry ? without any water deep inside at all.

"Lots of publications about water being found on the moon talk about ice that resides on the lunar surface," Zachary Sharp, a professor at the University of New Mexico in Albuquerque, N.M., and the study's lead researcher, told SPACE.com. "This is not what we're talking about. We're talking about water that was initially in the moon as it formed."

Secrets of moon rocks

Sharp and his colleagues compared the composition of Earth rocks, primitive meteorite samples, and lunar volcanic rocks. In particular, they measured the prevalence of two kinds of chlorine atoms, called isotopes ? chlorine-35 and chlorine-37.

They found that the ratio of chlorine-35 to chlorine-37 was very constant, and only varied by about 0.1 percent. Similar results were found with the primitive meteorite samples.

But when the researchers performed the same measurements on lunar samples, they found surprising results. [10 Coolest New Moon Discoveries]

"We quickly found huge variability in the lunar chlorine isotope ratio," Sharp said. "It was 25 times that of Earth. We were confounded."

This information helped the scientists calculate how widespread other elements are on the moon ? including hydrogen, a key ingredient in water. "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."

What chlorine tells us about water

Because of its chemical properties, chlorine is very attracted to water (this is called being hydrophilic). Therefore, it is an extremely sensitive indicator of the presence of hydrogen, and thus, water.

If the moon contained large amounts of water in its interior, that would have greatly affected the properties of chlorine there, too, the researchers said. In particular, there wouldn't be so many different ratios of the two types of chlorine ? rather, they would be found at steady levels all over the moon, as they are on Earth.

On Earth, chlorine bonds with hydrogen to create the compound hydrogen chloride (HCl), which is released as a gas during volcanic processes.Chlorine-35 and chlorine-37 are atoms that contain the same number of protons, but chlorine-37 has two more neutrons in its nucleus. Thus, the latter flavor of chlorine weighs more, as it has a slightly greater atomic mass.

During volcanic processes on Earth, as chlorine vaporizes from the cooling molten rock, the volatile gas hydrogen chloride (HCl) is released. This compound is more often made with chlorine-37 than chlorine-35. That means that when the gas is emitted into the atmosphere, more of the former atom is lost than the latter, leaving behind cooled volcanic rocks that are enriched with chlorine-35.

But, this is only half of the process, Sharp said. The lighter element is also released as a vapor in other processes on Earth.

"The net effect on Earth is that there is essentially no fractionation ? the two cancel each other out," Sharp said.

However, on the moon, this cancelling out process hasn't occurred, so there are wildly different ratios of the two isotopes in different areas. That must be because there wasn't enough hydrogen around for chlorine to bond to, to create HCl, the scientists reason.

Instead, on the moon, chlorine has bonded with metals to create compounds such as sodium chloride, magnesium chloride and zinc chloride, Sharp said.

"The levels of hydrogen had to be less than the chlorine, otherwise we would have HCl rather than these chloride salts," Sharp said. "If we had made HCl, we wouldn't have fractionation. Knowing the chlorine content gives us an upper limit on the hydrogen content. So, we know that there had to be less hydrogen than chlorine."

With such low concentrations of hydrogen, the researchers suggest that the interior of the moon is anhydrous, or without water, as scientists had initially proposed long ago.

Furthermore, if this explanation is correct, Sharp and his colleagues would also hope to find salts on the lunar surface that were re-crystallized following the vaporizing process.

"We do see this on a number of lunar materials," Sharp said. "We find these little decorations of salt crystals on the surface."

Previous studies of water in the moon's interior

In 2008, research of volcanic glass beads from the moon brought attention to the fact that there might be more water in the lunar interior than scientists had previously thought.

But, Sharp claims that these samples are unusual, and might not be representative of the whole moon.

"It would be like if I asked you to bring back half a dozen rock samples from a streambed, and one of those samples glittered like gold," Sharp said. "I could look at that and think that Earth contained a lot of gold, but it's really just an anomalous sample. It's a wonderful discovery, but to be able to extrapolate that to the entire moon may not be valid, and that could explain the discrepancy in that paper and ours."

The new discovery is detailed in the August 6 issue of the journal Science.