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<em>Science</em>: Ancient Impact May Explain the Moon’s Magnetic Quirks

A giant asteroid that struck the moon long ago, creating what is possibly the largest and oldest impact crater in the entire solar system, may explain the surprisingly strong magnetic fields that emanate from the lunar crust today, researchers say in the 8 March issue of Science.

Moon rocks generally don’t contain much metallic iron, making them poor recorders of magnetic events. As a result, researchers have been hard-pressed to explain the moon’s patchy magnetism since it was first observed in lunar samples and from orbital measures taken by the Apollo space missions in the 1960s and 1970s.

Now, Mark Wieczorek from the Institut de Physique du Globe de Paris in Paris, France, and colleagues from the United States have completed detailed computer simulations that suggest an origin for these magnetic anomalies on the moon. According to their results, a major impact—specifically, the one that produced the crater known as the South Pole-Aitken basin—could have recorded the signature of an ancient magnetic field and spread it out across the surface of the moon.


The largest grouping of magnetic anomalies (shown on the left) on the Moon are located near the northern rim of the largest impact basin. These anomalies are the result of highly magnetic materials from the projectile that formed this 2,000 kilometer-diameter basin. | Image © Science/AAAS

“We used a shock physics code that simulates two objects being smashed together at high velocities and found that, in order to form a crater as big as the South Pole-Aitken basin on the moon, an asteroid that is 124 miles (200 kilometers) wide and traveling more than nine miles (15 kilometers) per second would have been necessary,” said Wieczorek. “We then calculated different scenarios at various angles and velocities to discover how the moon’s magnetic anomalies could have gotten where they are today.”


The researchers wanted to know exactly where the flying debris from the asteroid’s impact may have landed, because such asteroid materials are known to contain large amounts of metallic iron, making them highly magnetic compared to lunar rocks. A hot, high-speed collision like the one that formed the South Pole-Aitken basin, they say, may have imprinted the signature of an ancient lunar magnetic field on the asteroid material as it cooled down.

“Imagine that there were a whole bunch of little magnets in the asteroid rocks, spinning around wildly after their hot, high-pressure collision with the moon,” Wieczorek said. “Eventually, the rocks cooled to a temperature known as the ‘Curie temperature’ and those little magnets slowed down and froze in place, recording the signature of the magnetic field that they cooled in.”

Since the South Pole-Aitken basin appears to be elongated in the north-south direction, researchers have proposed that the impact that created the basin must have been oblique, or angled. With this in mind, Wieczorek and his colleagues were able to show how projectile materials from the asteroid’s impact could explain the vast majority of the moon’s magnetic anomalies—as long those projectiles were magnetized by an ancient, lunar magnetic field.

“Most experts now believe that the moon once had a global, bipolar magnetic field that persisted for tens of thousands, hundreds of thousands, or even a million years,” said Wieczorek. “But one of the biggest problems right now is explaining how such a strong magnetic field was generated.”

One popular theory posits that the ancient, lunar magnetic field was powered by a circulation of molten iron in the moon’s core, or alternatively by differential rotation between the moon’s core and its mantle. (Such differential rotation could have been caused by giant impacts or by a shift in the orientation of the moon’s rotation axis early in its history.) But researchers might not know for sure until spacecraft return to the moon to collect more samples.

“It’s much more complicated and expensive to put a lander on the ground,” Wieczorek explained. “But now is really the time to return to the lunar surface, take some samples, or at least put some geophysical stations on the ground.”

If astronomers accomplish this, they might eventually get the chance to determine the age of the South Pole-Aitken basin and confirm that the moon’s magnetic anomalies were born from the asteroid that created it.


Read the abstract, “An Impactor Origin for Lunar Magnetic Anomalies,” by Mark Wieczorek and colleagues.

Listen to a Science Podcast interview with Mark Wieczorek.