Beneath the Surface, the Moon Is Dry as a Bone

You can't squeeze blood from a turnip¹, and apparently² you can't wring³ water from moon rocks, either. A new analysis of samples returned by the Apollo astronauts suggests that there is virtually no chance that water ever existed beneath the lunar surface.

Scientists have been arguing for decades about whether the moon contains significant quantities of water. Theoretically, it should because the moon was once part of Earth. (It was created by a catastrophic collision between our planet and a Mars-size body over 4 billion years ago.) Yet initial chemical analyses in the 1970s and '80s of rocks that had been brought back by the Apollo missions turned up virtually no evidence of lunar water. More recently, using improved equipment to examine the molecular⁴ constituents⁵ of the rocks, researchers have found minute quantities of hydrogen—an indicator⁶ of water. (The LCROSS spacecraft, which slammed into a perpetually shaded lunar crater⁸ last fall, turned up evidence of water ice on the surface, but that ice was presumably deposited by an ancient comet impact.) It's an important riddle⁹ to solve, as researchers would like to use lunar water to manufacture rocket fuel for future deep-space missions.

Now, a team has attempted to solve the mystery by comparing the ratio of two isotopes¹¹, or forms, of chlorine in lunar and terrestrial rocks. The scientists had performed similar analyses on Earth rocks and on meteorites¹², so they decided¹³ to duplicate the research on the lunar samples. The researchers reported online 5 August in Science that based on the difference in chlorine ratios, the moon has basically zero subsurface water.

Geochemist and lead author Zachary Sharp of the University of New Mexico, Albuquerque, explains that the chlorine isotope¹⁰ ratio is a highly reliable indicator of the amount of hydrogen a planetary body contains. On Earth, primordial¹⁴ volcanic¹⁵ eruptions¹⁶ brought up lava¹⁷ containing huge amounts of volatile¹⁸ substances such as carbon dioxide, sulfur¹⁹, water vapor²¹, and chlorine. As the lava cooled, those gases leaked into the atmosphere—but they also interacted, and they left behind traces of that interaction, in the form of isotope ratios, which betray their relative quantities. On Earth's surface, the chlorine isotope ratio is uniform because the element interacts so readily with hydrogen. But on the moon, there's a wide variety in that ratio—what Sharp calls "scatter²²"—something that could not have happened if the moon contained significant amounts of hydrogen.

The ratio variability was a "completely unexpected and initially²³ quite bewildering" result, Sharp says. The bottom line, he says, is that "the only process we are aware of that could cause such large isotope scatter is if the hydrogen content of the moon is 10, 000 to 100, 000 times less than [on] Earth."

On its face, the paper presents a strong argument for a water-free moon, says planetary scientist David Stevenson of the California Institute of Technology in Pasadena. "But years of observation of geochemistry have taught me that caution is wise, " he says, "whether it be for the earlier claims on water or this latest work on chlorine isotopes."