Ancient impact may explain moon’s magnetic mystery

Anomalies near crater suggest scattering of iron-rich debris

Iron-rich fragments from an ancient impact could explain puzzling magnetic fields measured in various places on the moon.

Lunar maps illustrate the relationship between the South Pole-Aitken basin and recorded magnetic anomalies (shown on left). Scientists suggest that the crater-forming impact blew chunks of easily magnetized material across the lunar surface. © Science/AAAS

The magnetic anomalies are perplexing because unlike metallic minerals deposited by an asteroid, normal lunar rocks cannot record a magnetic field. “Those things are really non-magnetic,” says Ian Garrick-Bethell, a planetary scientist at the University of California, Santa Cruz. “The puzzle has been, how do you form these anomalies — how do you get anything that’s that magnetic?”

Perhaps, scientists report in the March 9 Science, the magnetic material arrived billions of years ago when a huge object rocked the moon, digging a hole in the farside and splashing debris across the surface. The impact formed the South Pole-Aitken basin, perhaps the largest impact crater in the solar system. Most of the anomalies are next to the mammoth crater, but scientists hadn’t linked the locations until now.

The basin averages about 2,200 kilometers in diameter — or roughly 10 times as big as the crater left by the dino-killing asteroid slammed into Earth 65 million years ago. “You definitely would not have wanted to be standing on the surface of the moon at that point,” says study coauthor Mark Wieczorek, a planetary geophysicist at the Institute of Earth Physics of Paris.

Wieczorek and colleagues used magnetic and topographic maps to identify the association. Simulating the collision then showed where easily magnetized debris might have landed. “The moon should’ve been basically contaminated with magnetic material from this thing,” says study coauthor Benjamin Weiss, a planetary scientist at MIT. “Throw a little bit of meteorite fairy dust in there, and it can really basically control where the magnetic fields are recorded.”

Generating magnetized patches without the fairy dust is tricky. Fields as strong as those observed would require deposits of farside lunar rocks more than 100 kilometers thick. “That’s thicker than the crust of the moon itself,” Wieczorek says.

But simulating the footprints left behind by plausible impactors matched the observations. Iron-containing minerals would have accumulated at the northern ridge of the crater, and maybe even splattered the moon’s nearside, explaining some of the anomalies observed there. As the rocks cooled, they stored within them a record of the young moon’s magnetic field. “Here’s a model that explains not only the magnetic anomalies near the crater, but also potentially a lot of the anomalies all over the moon, which people have really struggled with,” says Garrick-Bethell.

Lon Hood, a planetary scientist at the University of Arizona, has also considered how nonlunar materials might be responsible for anomalous observations. He points to six large craters, younger than the South Pole-Aitken basin, that may have generated magnetic fields directly opposite the point of impact. Hood says his hypothesis and this new idea could fit together, if the South Pole impact provided material that could record magnetic fields from later impacts. “The fields are enhanced,” he says. “They’re stronger than they might have been.”

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