Moon's Silver Hints at Lunar Water Origins

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."