National Geographic Daily News
Magnetic anomalies on the moon.
The moon's magnetic rocks are pictured to the left in dark red.

Image courtesy Science/AAAS

Rachel Kaufman

for National Geographic News

Published March 12, 2012

Unlike Earth, the moon has no global magnetic field, but patches of the satellite's surface are magnetic. What gives?

According to new models, these unusually magnetic pockets come from an asteroid that slammed into the moon when it had a magnetic field, billions of years ago, according to Mark Wieczorek, director of research at the Institute of Earth Physics of Paris.

In general, heating metal in a rock and then cooling it imparts magnetic properties—the most common way Earth rocks get magnetized.

"Many of the meteorites we see on the Earth contain large abundances of metallic iron. They're roughly a hundred times more magnetic than typical rocks you can see on the moon," said Wieczorek, who co-authored a new study on the phenomenon.

On the moon, "if we can get enough of these asteroid materials"—heated by impact—"in a certain region, that would create a magnetic anomaly that would be a hundred times stronger" than the rest of the moon.

(Take a moon mysteries and myths quiz.)

Moon Models Show Asteroid Impact

Wieczorek and his team also found that most of the magnetic material is on the northern rim of the South Pole-Aitken basin, the biggest crater on the moon and among the biggest in the solar system.

In the new models "you start out with a 200-kilometer [124-mile] sphere and smash it into the moon—on a computer, of course—at high velocities. Depending on the impact velocity and angle, we can get large quantities of material deposited on the rim of this impact basin."

An asteroid that size moving at 9.3 miles (15 kilometers) a second and hitting the moon at a 45-degree angle would spew out moon chunks along a 745-mile-wide (1,200-kilometer-wide) crater, the results showed.

These bits of magnetized asteroid would've then been deposited downward, along the rim of the crater.

(See "New Scars Found on Moon, Hint at 'Recent' Tectonic Activity.")

Other Magnetic Oddities Explained?

The study may also explain magnetic anomalies on other planets in our solar system, Wieczorek added. (See solar system pictures.)

For instance, "Mars does not have a magnetic field today, [but] like the moon, it has these strong magnetic fields on the surface."

Mars's northern half, which is made up of relatively new lowlands, could contain remnants of an even bigger impact event, he said. Fittingly, "it turns out these magnetic anomalies surround that basin."

Meanwhile, Mercury does have a global magnetic field, but it also shows evidence of having been bombarded by asteroids.

"It wouldn't surprise me" to find magnetic anomalies on Mercury's surface as well, Wieczorek said.

The magnetic-moon study was published March 9 in the journal Science.

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