Scientists presented this result and other findings today in a series of papers at the 30th Lunar and Planetary Science Conference in Houston, TX. Their data show that the lunar core contains less than four percent of the Moon's total mass, with the probable value being two percent or slightly less. This is very small when compared with the Earth, whose iron core contains approximately 30 percent of the planet's mass.

"This is a critical finding in helping scientists determine how the Earth and Moon formed," said Dr. Alan Binder of the Lunar Research Institute, Tucson, AZ, principal investigator for Lunar Prospector.

Similarities in the mineral composition of the Earth and the Moon indicate that they share a common origin. However, if they had simply formed form the same cloud of rocks and dust, the Moon would have a core similar in proportion to the Earth's. A third theory suggests that the moon was captured fully intact by the Earth's gravity.

Based on information first gathered during the Apollo era, scientists suggested that the Moon was formed when a Mars-sized body hit the Earth during its earliest history. "This impact occurred after the Earth's iron core had formed, ejecting rocky, iron-poor material from the outer shell into orbit," Binder explained. "It was this material that collected to form the Moon.

"Further analysis of Lunar Prospector data to refine the exact size of the lunar core and the amounts of elements like gold, platinum and iridium in lunar rocks -- all of which are concentrated with metallic iron -- is required," Binder added. "This will do much to pin down for good if the 'giant impact' model of the formation of the Moon is correct, or if the Moon formed in a different manner."

The current data come from gravity measurements conducted by Dr. Alex Konopliv of NASA's Jet Propulsion Laboratory, Pasadena, CA. His results indicate that the Moon's core radius is between 140 and 280 miles (220 and 450 kilometers). This result is consistent with independent magnetic data, evaluated by Dr. Lon Hood of the University of Arizona, Tucson, which suggest that the core radius is between 180 and 260 miles (300 and 425 km).

In other results from Lunar Prospector, Dr. Robert Lin of the University of California at Berkeley, Dr. Mario Acuna of NASA's Goddard Space Flight Center, Greenbelt, MD, and Hood also found that a broad section of the southern far-side of the Moon has large localized magnetic fields in its crust. These fields occur opposite the large Crisium, Serenitatis and Imbrium basins -- three of the "seas" that cover much of the Moon's near side. This result supports earlier evidence linking strong magnetized concentrations on one side of the Moon with young, large impact basins on the other side.

Results of efforts to map the composition of the lunar crust have surpassed the expectations of the spectrometer team, led by Dr. William Feldman of the Department of Energy's Los Alamos National Laboratory in New Mexico. Data obtained are so good that the distribution of thorium has been mapped with a resolution of 36 miles (60 kilometers). At this amount of detail, scientists can detect individual deposits rich in thorium and related elements. Their current observations suggest that thorium was excavated by impacts of asteroids and comets, and then distributed around craters, rather than being deposited by volcanic activity.

Lunar Prospector conducted its primary mapping mission at an altitude of 63 miles (100 kilometers) for almost one year after its arrival in lunar orbit on Jan. 11, 1998. In December and January, the spacecraft's altitude was lowered to approximately 15 miles by 23 miles (24 kilometers by 37 kilometers). Analyses of data from the lower-altitude observations are expected to further improve scientific understanding of the origin, evolution and physical resources of the Moon.