Lunar Prospector Magnetic Data Supports Popular Theory of Unique Moon Formation

Aug. 9, 1999

Lon L. Hood

New data from the Lunar Prospector spacecraft supports mounting evidence
that the moon formed in a way unique to this solar system.

This latest report comes from scientists who measured the Earth_s magnetic
field near the moon using instruments mounted on Lunar Prospector. Their
results strengthen the theory that the moon has a metal core that is much
smaller than cores of the inner planets of the solar system.

The new data agrees with Apollo mission seismic and sample-return evidence
that suggests the moon is partly made of the same stuff as the Earth_s upper
crust, or mantle. And their findings agree with results released earlier
this year by NASA Jet Propulsion Lab scientists who used Lunar Prospector to
make a gravity map of the moon and who also conclude that the moon has a
small, partially molten core.

Lon L. Hood of the University of Arizona in Tucson and collaborators detail
the results of their magnetic measurements in the Aug. 1 issue of the
American Geophysical Union_s Geophysical Research Letters. Hood and his
colleagues on the Lunar Prospector magnetometer team estimate that the
moon_s metal core is roughly 420 miles (680 km) across _ plus or minus 112
miles (180 km). This makes it only one to three percent of the moon_s total
mass. By contrast, the Earth_s core constitutes one-third of its mass.

"We knew that the moon_s core was small, but we didn_t know it was this
small," Hood said. "This really does add weight to the idea that the moon_s
origin is unique, unlike any other terrestrial body _ Earth, Venus, Mars or
Mercury," Hood said. "The simplest hypothesis, and the most popular now, is
that a Mars-sized object collided with Earth after Earth had differentiated
into a core and mantle. The impact generated a vapor cloud, which was mostly
composed of silicate, and that became the moon."

Alternate theories surmise that the Earth captured a fully formed moon, or
that the moon formed in orbit around the Earth but that Earth somehow
prevented metal from accreting to the moon. Those theories now seem less
plausible, Hood said.

Collaborating with Hood on this research are David Mitchell and Robert P.
Lin, of the University of California at Berkeley, Mario Acuna, of the
Goddard Space Flight Center in Greenbelt, Md., and Alan B. Binder, of the
Lunar Research Institute in Tucson. Binder is the principal investigator for
the Lunar Prospector mission.

They measured changes in Earth_s magnetic field as Lunar Prospector orbited
the moon 21 times in April 1998. The measurements were made during a
longer-than-usual unbroken passage of the moon within Earth_s magnetic tail.
Electrical currents that flow through the moon_s hot metallic core create
magnetic fields that oppose Earth_s magnetic field. The metal spacecraft
moved slightly away from the moon as a result of the changed magnetic
forces. The spacecraft magnetometer detected these movements, which the team
analyzed to calculate the size for the moon_s core.

The Lunar Prospector magnetometer team has yet to analyze data that were
taken this spring, Hood said. The sun is approaching a period of maximum
solar activity, so it_s possible there will be more "noise" that could mask
some of the magnetic effects in the newer data.

While evidence for a very small lunar core is mounting, proof is not yet
absolute, Hood added. If Japanese scientists succeed in deploying new
seismometers on the moon, they may be able to provide absolute proof, he

Lunar Prospector, an 18-month, $63 million NASA mission ended July 31 with a
controlled crash landing at the moon_s south pole. The spacecraft was
designed to study the moon_s resources, gravitational and magnetic fields,
composition and geologic history.