Asteroid collisions may be responsible for mysteriously magnetic meteorites on Earth

Rubble-pile asteroids form when asteroids collide and their shards reassemble into new, shuffled asteroids. And when they do, according to a new study, they might give the renewed asteroid a temporary magnetic field.

This result might address a mystery that has baffled astronomers for years: Some metallic meteorites hold traces of magnetism as if they carry remnants of internal magnetic fields. That shouldn’t be possible. Even if a meteorite does contain iron, it isn’t expected to have a circulating dynamo, like the one in Earth’s inner core, which scientists think is necessary to generate a magnetic field.

“I had been aware of this puzzle for some time,” Zhongtian Zhang, a planetary scientist at Yale University, said in a statement. 

And as he studied rubble-pile asteroids, the puzzle returned to his mind. So, Zhang and fellow Yale planetary scientist David Bercovici turned to modeling asteroid collisions. 

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When two iron-rich asteroids smashed into each other and scattered into shards, they discovered, some of those shards would coalesce into a chillier inner core that was coated by a warmer layer of molten rock. Then, if the shards were just the right size, the cold core would start stripping elements, such as sulfur, from the hot liquid.

This model illustrated that the resulting heat transfer could create circulation sufficient to trigger a dynamo and, thus, a magnetic field. And if such a dynamo formed, it could last for millions of years — and its traces would be detectable by astronomers long afterward.

The study was published on July 31 in the journal Proceedings of the National Academy of Sciences.