Young asteroids generated long-lasting magnetism

Ancient meteorites reveal that young asteroids may have generated powerful magnetic fields for hundreds of millions of years longer than once thought. The finding could explain long-lasting magnetism elsewhere in the early solar system, such as on the young moon (SN Online: 12/4/14).

Planetary scientist James Bryson of the University of Cambridge and colleagues examined two South American meteorites left over from asteroids roughly 400 kilometers wide (about a ninth of the moon’s diameter). Tiny pockets of iron and nickel embedded in the space rocks aligned with their parent asteroid’s magnetic field as they formed billions of years ago, providing a datable snapshot of the asteroids’ magnetism. Inspecting the iron and nickel using X-rays, the team discovered that each asteroid produced a strong magnetic field for well more than 100 million years.

This is significantly longer than can be explained by heat mixing an asteroid’s molten interior and generating a magnetic field, which would last at most 10 million to 50 million years until the rock cooled.

The researchers propose in the Jan. 22 Nature that as an asteroid’s core solidifies, lighter elements such as sulfur push outward and form swirling patterns. The swirling sustains the magnetic field for as much as 350 million years after the asteroid has cooled too much for thermal convection, the researchers say.

Editor’s Note: This article was updated January 22, 2015, to correct the metals used to study the asteroid’s magnetic field history. They were iron and nickel, not lead.