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The meteor showers bring geysers from the moon. A lunar orbiter spotted additional water around the moon as it passed through waves of cosmic dust that could cause meteor shower on the Earth.
The water has probably been released from the moon's soil by tiny meteorite impacts, planetary scientist Mehdi Benna of NASA's Goddard Space Flight Center in Greenbelt, Maryland said in a statement. April 15 Nature Geoscience. These random impacts suggest that water is buried everywhere in the moon, rather than being isolated in icy black craters – and that the moon has been moist for billions of years.
Lunar soil samples reported by Apollo astronauts suggest that the moon is dry as a bone. But over the last decade or so, several isolated missions have discovered water deposits on the moon, including traces of frozen surface water in areas of permanent shadow near the poles (SN: 24/10/09, p. ten).
"We knew that there was water in the soil," says Benna. Scientists did not know how widespread this water was or how long it had been there.
Benna and his colleagues used observations from NASA's LADEE spacecraft, which gravitated around the moon from November 2013 to April 2014 (SN online: 18/04/14). LADEE spectrometers have detected dozens of pronounced increases in the abundance of water molecules in the Moon's exosphere, the tenuous atmosphere of gas molecules clinging to the Moon. Twenty-nine of these measurements coincided with known fluxes of space dust.
When the Earth crosses these streams of water, dust is consumed in the atmosphere, producing annual meteor showers such as Leonids and Geminids. But since the moon does not have a real atmosphere, dust particles from the same showers hit the surface of the moon directly, stirring what is hidden below.
Benna and his colleagues calculated that only the heavier meteorites of about 0.15 gram could have released water. This means that the top eight centimeters of lunar soil is actually dry – smaller impacts would have released water if there were any. Under this dry coating is an overall layer of hydrated soil, with ice water adhered to the dust grains.
But the moon is far from soggy. By squeezing half a ton of lunar soil, one would only produce a small bottle of water, Benna said. "It's not much water, no matter what, but it's still water." And it's too much water to get to the moon recently, he says. The moon may have preserved at least some of this water since its formation (SN: 15/04/17, p. 18).
Future studies could help determine if and how this water could be useful to human explorers.
The discovery is "plausible and certainly provocative," says global scientist Erik Asphaug of the University of Arizona at Tucson. "This is the kind of paper that deserves to be published so we can debate it."
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