Decoding an asteroid’s magnetic history
Today most planets in the solar system have magnetic fields, as do some moons; asteroids also may have been part of the dynamo club early in their history. Earth’s magnetic field is continuously generated as the planet’s iron–nickel core solidifies from the inside out: The gradually expanding solid-iron inner core releases low-density elements that rise upward, creating convective currents in the surrounding molten material. Yet certain planet-like bodies, including some moons and asteroids, are thought to have cores that solidified from the outside in, which leads to questions about how they were still able to generate dynamos.
Using a combined microscopy and magnetometry technique, James Bryson from the University of Cambridge and his colleagues examined the microstructure of iron-rich meteorites like the one above to determine the magnetic history of their parent asteroid. The researchers found that the magnetic intensity of ferrous-mineral clusters was consistent with their having solidified in the parent body’s cool outer layers. The directional variation of the minerals’ magnetization provided evidence of a strong but directionally unstable dynamo.
How did such dynamos arise in the first place? The result supports the hypothesis that iron-rich meteorites originated from a planetary core that was stripped of its overlaying silicate mantle by collisions in the early solar system. With little to no thermally insulating rock on the surface, the molten ball likely cooled and solidified from the outside in. The authors propose that dynamo activity arises through nonconcentric solidification, in which tendrils of metallic crust detach and sink through the molten interior, which generates the magnetic field.
Bryson and his colleagues’ study presents evidence that iron meteorites can provide reliable paleomagnetic measurements. Studies of other meteorites and their outside-in solidification could prove useful in understanding solar-system bodies such as Jupiter’s moon Ganymede and the massive asteroid 16 Psyche. (J. F. J. Bryson et al., Earth Planet. Sci. Lett. 472, 152, 2017 .)