It's not just poetic to call it a silvery moon: In addition to water, a NASA probe that crashed into a lunar crater last year churned up unexpected concentrations of silver and mercury, aka quicksilver, a new study says.
The metals had been found before in moon rocks brought back by Apollo astronauts, but the elements had appeared in only trace amounts. (Also see "Water Found in Apollo Moon Rocks.")
The new data, derived from the Lunar Crater Observation and Sensing Satellite (LCROSS) mission, show much higher amounts of silver and mercury in debris from the crash, which happened inside the south-pole crater known as Cabeus.
The surprising find hints at out how water may have arrived on the moon and why it become concentrated at the poles, astronomers say.
When impactors strike the lunar surface, the moon's easily vaporized metals, such as mercury and silver, tend to migrate—atom by atom—toward the cooler poles, much as water vapor in Earth's atmosphere condenses on cold surfaces.
Water and other volatile compounds brought in by asteroids and comets would similarly experience this "cold sink" effect. (See "Water Discovered on an Asteroid—A First.")
"The silver is like a tracer," said study leader Peter Schultz of Brown University in Rhode Island. "It tells us where [moon water] probably came from, and I think it's telling us that it came from comets and asteroids colliding with the moon."
(Related: "Comet Swarm Delivered Earth's Oceans?")
Moon Ice Hiding Climate Clues?
For the LCROSS mission, NASA directed an empty Centaur rocket to smash into a permanently shadowed region of the southern crater. The spacecraft that delivered the rocket recorded the entire event before it was then sent crashing into the moon.
The rocket crash alone gouged a new crater within Cabeus that measures up to 100 feet (30 meters) across.
The impact also sent up to 13,000 pounds (6,000 kilograms) of lunar dust, vapor, and other debris hurtling into space.
In a new paper analyzing this ejecta, LCROSS principal investigator Anthony Colaprete calculates that about 342 pounds (155 kilograms) of water vapor and ice were expelled during the rocket impact.
His team also estimates that at least 5 to 8 percent of the material still inside the Cabeus crater is water ice.
In a separate study, Schultz and his colleagues recorded the silver and mercury abundances, as well as detection of other volatile compounds in the impact plume, including several light hydrocarbons, some sulfur-bearing molecules, and carbon dioxide.
Further studies of these compounds and their concentrations on the moon could provide new information about the history of the solar system, Schultz said.
"We go to Antarctica to study past atmospheres and to look for evidence of past impacts and changes of climate," he said.
"Ice on the moon is probably hiding similar clues, but instead of illuminating Earth's climate, it may tell us about the climate history of the solar system."
Such factors should be considered if humanity comes closer to colonizing the moon and using a lunar base as a way station for deeper jaunts into space, he added. (See "Moon Settlers May Be 'Ski' Racers, Helium Miners, TV Stars.")
"We've got to look at the bigger picture. Is [the moon] something we want to save and study before we start exploiting it?"
The new LCROSS findings appear in a suite of three studies published in this week's issue of the journal Science.