January 20, 2009

Tiny Moon Rock Reveals Molten Lunar Core

A small, 4.2 billion-year-old moon rock collected by an astronaut during Apollo 17, the last manned mission to the Moon, provides evidence that the Moon once had a molten core that generated a magnetic field.

Harrison Schmitt, Apollo 17's only trained geologist, gathered the rock, known as troctolite 76535, in 1972. Troctolite is a type of rock consisting of the minerals olivine and plagioclase.

Considering that lava plains on the Moon's surface suggest a volcanic past that may have lasted two billion years, "I don't think it's that surprising," Dr. Ian Garrick-Bethell, who just completed  his doctorate at the Massachusetts Institute of Technology, told the New York Times.

However, the findings of Dr. Garrick-Bethell and his colleagues may cause scientists to rethink prevailing wisdom that objects smaller than the planet Mars are not able to maintain a stable magnetic field.

Many of the rocks retrieved from the Moon have a weak magnetic signal, suggesting that they originally cooled from magma when the Moon had a magnetic field.   That surprised many researchers who believed the Moon was too small and cold to have had conditions where electric currents from the convection of molten iron generated a magnetic field.

However, evidence was inconclusive since the lunar surface has been repeatedly hit by meteorites, the impact of which can also leave a magnetic signature on rocks.  But troctolite 76535 provides a pristine view of the Moon's early history, and is significant because it formed when the Moon was only 300 million years old. Furthermore, previous research had found that the rock was never altered by the force of an impact.

Dr. Garrick-Bethell's research revealed two distinct magnetic fields within the rock. The first field was set when the rock first crystallized 30 miles below the Moon's surface several million years. It then appears that a meteorite impact bumped the rock close to the surface without shocking it.  This heated the rock enough to remove part of its magnetic field and imprint a second field at a 140-degree angle to the first as the rock cooled for a second time over thousands of years.

According to researchers, the slow cooling time would seem to discount the possibility that the fields were generated by meteorite impacts.

The researchers drew their conclusions by placing the rock chips in an increasingly strong magnetic field, which "erased" the rock's magnetism incrementally. That allowed the scientists to observe whether the magnetic atoms had lined up in the same direction.  This would be expected if the magma had cooled in a magnetic field.

The researchers noted that a magnetic field about two percent of the strength of Earth's current magnetic field would generate the observed magnetism of troctolite 76535.

The findings of Dr. Garrick-Bethell and his colleagues appear in the current issue of the journal Science.


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