According to the theory, the moon resulted from a collision between Earth and a Mars-sized body; a vast cloud of dust and vapor was ejected into space and coalesced into Earth's satellite. Such a collision, occurring after Earth had become differentiated into a metallic core and rockier outer layers, would have produced a body relatively lacking in metal.

Lunar Prospector's readings indicate that the moon's metal core accounts for less than 3 percent of the satellite's total mass. By contrast, the Earth's core constitutes one third of its mass.

"We have known for a long time that the moon is depleted in metal," says Lon L. Hood, a senior research scientist at the University of Arizona in Tucson. "But the new measurements tell us more accurately how depleted it is. It is extremely depleted."

And that, he says, is an "indicator that the moon had a unique origin" -- unlike Earth and other bodies that developed through slow accretion. An alternative theory -- that a fully formed moon was captured by Earth's gravity -- is less plausible given the small size of the moon's metal core, according to Hood.

The readings taken by Lunar Prospector with a magnetometer in April 1998 are not conclusive in themselves. Further proof of the moon's origin might come from actual seismometers the Japanese hope to deploy on the moon. Hood and his collaborators detail the results in the August 1 issue of Geophysical Research Letters.

Lunar Prospector's mission ended with a controlled crash landing in late July.