Back in the 1960’s and ’70’s when scientists claimed that the Moon rocks returned by the Apollo astronauts would keep researchers busy for decades, they weren’t kidding. Analysis on one of the rocks collected during the Apollo 17 mission has helped to solve a longstanding puzzle about the Moon. Researchers at the Massachusetts Institute of Technology (MIT) carried out the most detailed analysis ever of the oldest pristine rock from the Apollo collection. Magnetic traces recorded in the rock provide strong evidence that 4.2 billion years ago the moon had a liquid core with a dynamo, like Earth’s core today, that produced a strong magnetic field.
Until the Apollo Moon missions, many scientists were convinced the moon was born cold and stayed cold, never melting enough to form a liquid core. Apollo proved that there had been massive flows of lava on the moon’s surface, but the idea that it has, or ever had, a molten core remained controversial. “People have been vociferously debating this for 30 years,” said Ben Weiss, Assistant Professor of Planetary Sciences in MIT’s Department of Earth, Atmospheric and Planetary Sciences and senior author of a paper on the new finding being published in Science on Jan. 16.
Many of the rocks returned from the Moon had magnetic qualities, which was perplexing to scientists. How could Moon rocks be magnetic if the Moon had no magnetic core?
One rock in particular was compelling. The rock was collected on Apollo 17, the last Apollo mission to the moon, by Harrison “Jack” Schmidt, the only geologist ever to walk on the moon. “Many people think that it’s the most interesting lunar rock,” said Weiss.
“It is one of the oldest and most pristine samples known,” said graduate student Ian Garrick-Bethell, who was the lead author of the Science paper. “If that wasn’t enough, it is also perhaps the most beautiful lunar rock, displaying a mixture of bright green and milky-white crystals.”
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The team studied faint magnetic traces in a small sample of the rock in great detail. Using a commercial rock magnetometer that was specially fitted with an automated robotic system to take many readings “allowed us to make an order of magnitude more measurements than previous studies of lunar samples,” Garrick-Bethell said. “This permitted us to study the magnetization of the rock in much greater detail than previously possible.”
And those data enabled them to rule out the other possible sources of the magnetic traces, such as magnetic fields briefly generated by huge impacts on the moon, which are very short-lived. But the evidence written in the lunar rock showed it must have remained in a magnetic environment for a long period of time — millions of years — and thus the field had to have come from a long-lasting magnetic dynamo.
That’s not a new idea, but it has been “one of the most controversial issues in lunar science,” Weiss said.
The magnetic field necessary to have magnetized this rock would have been about one-fiftieth as strong as Earth’s is today, Weiss said. “This is consistent with dynamo theory,” and also fits in with the prevailing theory that the moon was born when a Mars-sized body crashed into the Earth and blasted much of its crust into space, where it clumped together to form the moon.
The new finding underscores how much we still don’t know about our nearest neighbor in space, and which will soon be visited by humans once again under current NASA plans. “While humans have visited the moon six times, we have really only scratched the surface when it comes to our understanding of this world,” said Garick-Bethell.