Did asteroids ever have magnetic fields?

Earth's magnetic field is generated by liquid iron flowing in its outer core, a process called a dynamo. Did small asteroids ever have dynamos? Meteorites suggest that some did.

When certain magnetic minerals form or cool, they can lock in the direction and strength of a surrounding magnetic field, like a tiny compass frozen in place. This is called remanent magnetisation. By carefully measuring it in meteorites, scientists can detect ancient fields.

Studies of angrites, CV chondrites, eucrites and pallasites have found magnetisation that seems to have been recorded in fields generated by internal dynamos. Angrites suggest a dynamo active within the first few million years. Allende and other CV chondrites have been interpreted as evidence that their parent body was partly melted inside, with a metallic core producing a field, while its outer layers stayed unmelted. Tiny nickel-rich grains in the cloudy zone of pallasite metal recorded fields that may have lasted for tens to hundreds of millions of years.

There are also signs of a magnetic field in the solar nebula itself, recorded by chondrules in some meteorites. This early field would have helped the disc of gas and dust spiral onto the young Sun.

These findings are challenging because meteorites can be remagnetised on Earth, for instance by hand magnets used by collectors. Scientists select carefully handled samples. NASA's Psyche mission is designed to search for remnant magnetisation on a possible metal core asteroid.

More about the science of meteorites

• How is shock in a meteorite measured?
• What does a meteorite's weathering grade mean?
• How do scientists tell which parent body a meteorite came from?
• How many different parent bodies do meteorites come from?
• Why do iron meteorites come from so many parent bodies?
• Can meteorites tell us what is inside Earth?

All questions