Magnetizing the moon

(CN) — Some rocks on the moon show signs of strong magnetism, even though the moon doesn’t have a magnetic field today. For decades, scientists have debated how that could be possible. Now, researchers at MIT say a massive asteroid impact may be part of the answer.

In their study published Friday in the journal Science Advances, they suggest a collision like the one that formed the Imbrium basin — one of the moon’s largest and most visible craters — could have briefly amplified the moon’s weak magnetic field, leaving a lasting mark in surface rocks on the opposite side.

According to researchers, surface rocks in that region could have recorded the stronger field before it quickly faded.

“There are large parts of lunar magnetism that are still unexplained,” said Isaac Narrett, lead author of the paper, in a press release. “But the majority of the strong magnetic fields that are measured by orbiting spacecraft can be explained by this process — especially on the far side of the moon.”

Samples from the Apollo missions have shown that some lunar rocks hold traces of magnetism. According to researchers, spacecraft have also detected strong magnetic signals in certain areas of the surface, especially near the moon’s south pole on the far side. However, those readings don’t match what researchers would expect from the moon’s small, ancient core, which likely generated only a weak magnetic field early in its history.

Earlier models tested whether the sun’s magnetic field or an impact alone could explain the readings, but researchers say those ideas didn’t hold up. In the new study, they started with a different assumption.

The team proposed that the moon once had a weak internal dynamo producing a magnetic field of about 1 microtesla — much weaker than Earth’s.

Next, they modeled a major asteroid impact using existing data on crater formation. That helped them simulate how a cloud of plasma would spread around the moon. Researchers say the plasma could have briefly intensified the moon’s magnetic field on the opposite side of the impact, causing a short-lived magnetic spike that lasted around 40 minutes.

That brief window might have been enough to magnetize rocks, but only if something had first shaken up the electrons inside them. A seismic shock from the impact could have done just that.

According to researchers, a collision the size of the one that formed the Imbrium basin would have sent a pressure wave through the moon. That wave would have converged on the far side, jolting rocks just as the magnetic field peaked and locking in the signal.

“It’s as if you throw a 52-card deck in the air, in a magnetic field, and each card has a compass needle,” said Benjamin Weiss, professor of planetary sciences at MIT, in the press release. “When the cards settle back to the ground, they do so in a new orientation. That’s essentially the magnetization process.”

The study offers a middle ground in a long-running debate about the moon’s magnetic history. Some researchers have pointed to an internal dynamo, while others have blamed impacts.

Future missions, including NASA’s Artemis program, may be able to confirm that. The rocks in question lie near the lunar south pole, a key target for upcoming exploration.

“For several decades, there’s been sort of a conundrum over the moon’s magnetism — is it from impacts or is it from a dynamo?” said Rona Oran, another co-author of the study, in the press release. “And here we’re saying, it’s a little bit of both. And it’s a testable hypothesis, which is nice.”