From Space.com. It’s worth reading between the lines of this one to see the perplexed furrows on the brows of the ‘surprised’  scientists.

The magnetic fields of planetary building blocks lasted for a surprisingly long time in the solar system’s early days, a new study suggests.

The magnetic fields of these big asteroids were apparently generated by the same process that drives Earth’s global magnetic activity, and could have persisted for hundreds of millions of years after the objects’ formation, researchers said.
The study team analyzed pallasites, iron-and-nickel meteorites believed to originate from an ancient rocky body about 250 miles (400 kilometers) wide. The pallasites contain tiny particles of ‪tetrataenite — a mineral that records a magnetic history of the parent body going back billions of years.