Rocks collected by Apollo astronauts 40 years ago contain evidence that the moon's interior is bone dry.
Moon rocks contain chemical evidence that they are drier than any on Earth.
Any water on the moon is on the surface, in craters.
The discovery could help sort out how the moon formed and where Earth got its water.
Moon rocks collected by Apollo astronauts 40 years ago contain evidence that the moon's interior is bone dry, contrary to some more recent reports, say researchers. The conclusion is based on a new study focusing on chlorine isotopes in moon rocks, which are quite different from those found in terrestrial minerals.
The discovery could be a step towards solving the riddle of where Earth's water came from.
"There are two types of water on the moon," said Zachary Sharp of the University of New Mexico in Albuquerque, whose paper on the matter appears in the Aug. 5 issue of "Science Express."
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."
Sharp and his colleagues are looking inside rocks for clues to how much water was in the moon when it formed. On Earth, figuring the evidence of water is easy, since there are literally oceans of the stuff still around today. On the moon, however, any water that escaped the planet's interior went straight into space.
To discover the moon's original water content, Sharp and his colleagues looked at the variations in two types of chlorine isotopes -- chlorine-36 and the slightly heavier chlorine-37 -- in a wide array of moon rocks. The relative amounts of these two isotopes in Earth rocks varies just one-tenth of a percent, Sharp explained, because there are competing processes that keep them that way.
One of these processes, however, depends on the presence of hydrogen, which means water. When no water is present, the chlorine isotopes can be found in very unusual amounts.
"We find that the moon rocks have 25 times the variability of terrestrial rocks," said Sharp of the chlorine isotopes. "The only explanation is that the moon is anhydrous (waterless)."
Those rocks that have been reported, in recent years, to indicate the moon contains more water are not incorrect, said Sharp, but simply special cases. They probably do not represent the whole moon very well.
The discovery confirms, in a new way, what scientists found when they first looked at Apollo moon rocks: Every indication that the moon has always been a dry, dry place.
"It offers another way to quantify how much water is in lunar materials," said Kevin Righter of NASA's Johnson Space Center. "It places an upper limit."
That, in turn, is useful parameter for folks trying to crack another mystery: How the Earth-moon system came into being, said Righter. Perhaps even more significantly, it also has big implications back on Earth, where it could help sort out where all of Earth's water came from.
"There is an ongoing debate on the origin of water on Earth," said Righter. Was it from comet impacts about 500 million years after Earth was created? Or was it all vented from molten materials that cooled to form the planet? "Those two issues -- lunar and Earth water -- are related and linked."