Moon's tiny core hints at earthly origin.

Two new studies support the notion that the moon is essentially a chip off Earth's mantle, formed when a Mars-size body crashed into our planet.

Both studies, which rely on strikingly different measuring techniques, find that the moon has a tiny core, no bigger than about 400 kilometers in radius and containing less than 4 percent of the moon's mass. In contrast, Earth's core holds about one-third the planet's mass.

Theorists have predicted that the moon would have at most a tiny core if it formed from the debris kicked up when a giant body plowed into Earth a few billion years ago. By that time, our planet's iron-rich material had sunk toward its center (SN: 8/16/97, p. 107). The collision would have gouged material from Earth's low-density mantle, providing little matter for a dense lunar core.

Had a fully formed moon been captured by Earth, or had the moon and Earth formed simultaneously from the same cloud of gas and dust, the moon would have a larger core more in proportion to that of our own planet, says Robin M. Canup of the Southwest Research Institute in Boulder, Colo.

The idea that the moon is an offspring of Earth has been around for decades, ever since the first samples brought back from the moon revealed that lunar rocks have a composition similar to Earth's mantle. The new findings, described last week at the annual Lunar and Planetary Science Conference in Houston, lend further credence to a terrestrial lineage.

Lon L. Hood of the University of Arizona in Tucson and his collaborators used NASA's Lunar Prospector spacecraft to measure Earth's magnetic field in the vicinity of the moon. The craft's magnetometer detected variations in the field such as would be expected if the moon has a small, electrically conducting core, perhaps made of iron. Hood and his colleagues estimate that the radius of the core is 300 to 425 km.

These numbers agree with an earlier estimate of core size based on a gravity map of the moon, also generated by Lunar Prospector. Alex Konopliv of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and his collaborators reported that analysis in the Sept. 4, 1998 Science.

Recent experiments have measured minute variations in the moon's rotation and wobble by bouncing laser beams off reflectors that were left on the lunar surface some 3 decades ago. These variations result from a loss of energy. Earth's gravitational influence, which robs the moon of energy by flexing it, can't account for all of the fluctuations.

James G. Williams, J. Todd Ratcliff, and their JPL colleagues conclude that the variations can best be explained if the moon has a small, at least partially molten core that is no bigger than 374 km. Friction between a small, fluid core and an overlying layer of rock could account for the extra energy loss.

"The existence of a small lunar core favors the impact hypothesis," says Canup. She adds that further modeling will be required to determine the size and velocity of the body that struck Earth some 4 billion years ago.