Water molecules released from the surface suggest water is buried in the moon’s soil
Meteor showers bring moon geysers. A lunar orbiter spotted extra water around the moon when the moon passed through streams of cosmic dust that can cause meteor showers on Earth.
The water was probably released from lunar soil by tiny meteorite impacts, planetary scientist Mehdi Benna of NASA Goddard Space Flight Center in Greenbelt, Md., and colleagues report April 15 in in Nature Geoscience. Those random impacts suggest water is buried all over the moon, rather than isolated in freezing dark craters — and that the moon has been wet for billions of years.
Samples of lunar soil brought back by the Apollo astronauts suggested that the moon is bone 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 in the soil,” Benna says. What scientists didn’t know was how widespread that water was, or how long it had been there.
Benna and colleagues used observations from NASA’s LADEE spacecraft, which orbited the moon from November 2013 to April 2014 (SN Online: 4/18/14). LADEE’s spectrometers detected dozens of sharp increases in the abundance of water molecules in the moon’s exosphere, the tenuous atmosphere of gas molecules that clings to the moon. Twenty-nine of those measurements coincided with known streams of space dust.
When Earth passes through those streams, the dust burns up in the atmosphere, producing annual meteor showers like the Leonids and the Geminids. But because the moon has no true atmosphere, bits of dust from the same showers strike the moon’s surface directly, stirring up what lies beneath.
Benna and colleagues calculated that only meteorites heavier than about 0.15 grams could have released the water. That means the top eight centimeters or so of lunar soil are indeed dry — smaller impacts would have released water if any was there. Beneath that dry coating is a global layer of hydrated soil, with water ice clinging to dust grains.
But the moon is by no means soggy. Squeezing half a ton of lunar soil would yield barely a small bottle of water, Benna says. “It’s not a lot of water by any measure, but it’s still water.” And it’s too much water to have arrived at the moon recently, he says. The moon may have held on to at least some of this water since the time of its formation (SN: 4/15/17, p. 18).
Future studies could help figure out whether 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 is good to see published so we can debate it.”
M. Benna et al. Lunar soil hydration constrained by exospheric water liberated by meteoroid impacts. Nature Geoscience. Published online April 15, 2019. doi:10.1038/s41561-019-0345-3.
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