One of the moon's oddest anomalies is how parts of its crust have a magnetic field and others don't. But now scientists say they have a relatively simple theory for this inconsistency—a 120-mile-wide asteroid that smashed into the moon's south pole 4.5 billion years ago left behind magnetic rock, reports the Los Angeles Times and Red Orbit. The collision left "this huge, whopping crater that's roughly half the size of the US," says the paper's lead author.
The asteroid may have struck in an area where the moon once had a magnetic field, which would have magnetized the asteroid's iron-rich rock. "I think it's a good idea," says a planetary science professor at UC Santa Cruz. "I'm surprised no one really explored it earlier." But not everyone is convinced, as other astronomers think the magnetic anomalies are the result of smaller and more recent impacts. "I wish the moon was simpler than it is," says one.