Meteor showers bring moon geysers. A lunar orbiter spotted additional water around the moon when the moon pbaded through currents of cosmic dust that can cause meteor showers on Earth.
The water was probably released from the lunar soil by small meteorite impacts, planetary scientist Mehdi Benna of NASA's Goddard Space Flight Center in Greenbelt, Md., And his colleagues reported on April 15 in Geosciences of nature. These random impacts suggest that the water is buried in the entire moon, instead of being isolated in the frozen dark craters, and that the moon has been wet for billions of years.
The samples of lunar soil brought by the Apollo astronauts suggested that the moon is completely dry. But in the last decade or so, several remote missions have found water deposits on the moon, including signs of frozen surface water in regions of permanent shadow near the poles (SN: 10/24/09, p. 10).
"We knew there was water on the ground," says Benna. What the scientists did not know was how widespread the water was or how long it had been there.
Benna and his colleagues used observations from NASA's LADEE spacecraft, which orbited the Moon from November 2013 to April 2014 (SN Online: 04/18/14). LADEE's spectrometers detected dozens of sharp increases in the abundance of water molecules in the moon's exosphere, the faint atmosphere of gas molecules clinging to the moon. Twenty-nine of these measures coincided with known currents of space dust.
When the Earth pbades through these currents, the dust burns in the atmosphere, producing annual rains of meteorites such as the Leonids and the Gemnides. But because the moon does not have a true atmosphere, dust fragments of the same rains hit the surface of the moon directly, shaking what is below.
Benna and his colleagues calculated that only meteorites heavier than about 0.15 grams could have released the water. That means that the eight centimeters or so of the lunar soil are in fact dry; smaller impacts would have released water if there were any. Under that dry coating there is a global layer of hydrated soil, with water ice adhering to the dust grains.
But the moon is by no means soaked. Squeezing half a ton of lunar soil would hardly produce a small bottle of water, says Benna. "It's not much water at all, but it's still water." And it's too much water to have reached the moon recently, he says. It is possible that the moon has retained at least part of this water from the time of its formation (SN: 4/15/17, p. 18).
Future studies could help determine if and how that water could be useful for human explorers.
The finding is "plausible and certainly provocative," says planetary scientist Erik Asphaug of the University of Arizona in Tucson. "It's the kind of paper that's good to see published so we can discuss it."