The lasting impression left by the Apollo missions is of a moon that is gray, dusty, desolate and dead. But instruments left behind by Apollo astronauts recorded moonquakes and wobbles in its rotation that gave hints of a still molten core.
Now, a rock collected more than 36 years ago during Apollo 17, the last human visit to the moon, reveals that the molten core may have once churned and generated a magnetic field.
Given that lava plains on the lunar surface indicate a volcanic past that may have lasted nearly 2 billion years, “I don’t think it’s that surprising,” said Ian Garrick-Bethell G, who just finished his doctorate at the Massachusetts Institute of Technology.
Nonetheless, the findings of Garrick-Bethell and his colleagues, which appear in the current issue of the journal Science, may help resolve a longstanding debate.
Many of the rocks brought back from the moon have a faint magnetic signal, suggesting that they originally cooled from magma when the moon had a magnetic field. That was a surprise to many scientists who thought the moon was too small and too cold to have ever possessed a geomagnetic dynamo where electric currents from the convection of molten iron generate a field.
The evidence was inconclusive because the moon’s surface has been repeatedly bashed by meteorites, and the shock of impact can also leave a magnetic signature on rocks.
But a 4.2-billion-year-old rock named troctolite 76535, collected by Harrison Schmitt, the only trained geologist among the Apollo moon walkers, offers a pristine view of the moon’s early history. (Troctolite is a type of rock made of the minerals olivine and plagioclase.) The rock is significant because it formed when the moon was only 300 million years old, and previous studies showed that it had never been shocked, or altered, by the force of an impact.
Garrick-Bethell’s research showed two distinct magnetic fields within the rock, or at least within the small fragments they examined. The first field, he said, was set when the rock first crystallized perhaps 30 miles beneath the surface over several million years. Then it appears that a meteorite impact knocked the rock close to the surface without shocking it, but heating it enough to erase part of its magnetic field and imprint a second field at a 140-degree angle to the first as it cooled a second time over thousands of years.
The slow cooling time appears to rule out the possibility that the fields were caused by meteorite impacts, the researchers said.
The researchers arrived at their findings by placing the rock chips in an increasingly strong magnetic field, which “erased” the rock’s magnetism bit by bit. That allowed them to see whether the magnetic atoms had lined up in the same direction, as would be expected if the magma had cooled in a magnetic field.