The Apollo moon missions ended almost 40 years ago. But for lunar scientists, they're gifts that keep on giving. Researchers studying rocks brought back by astronauts have found that the moon’s scarce water has a different chemical signature than Earth water. Which leads to the conclusion that the water probably came from comets. The study appears in the journal Nature Geoscience. [James Greenwood et al., "Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon"]

The researchers used what’s called an ion microscope to compare the amount of normal hydrogen in the moon rocks to the amount of the hydrogen isotope deuterium, which carries an extra neutron. They found deuterium at higher levels than it's found in Earth water—but at levels similar to the comets Hale-Bopp, Hyakutake and Halley. Which suggests comets deposited water on the ancient moon, shortly after its formation four-and-a-half billion years ago.

The finding could also explain a mystery of water on Earth—how the oceans got here. Because if comets battered the moon, they probably hit the Earth, too. Ocean water does have more deuterium than water in the Earth's mantle. Maybe, the researchers say, that's because the oceans have an extra dose of melted comet ice.

—Christopher Intagliata

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[Scientific American is part of Nature Publishing Group.]