Research shows that the magnetic fields in the asteroid parent bodies of two meteorites lasted hundreds of millions of years after our solar system’s formation.
Today, active magnetic fields surround Earth, Mercury, all the outer planets, and Jupiter’s moon Ganymede. But a new study in last week’s Nature shows that during the first tens of millions of years in the solar system’s life, numerous small bodies also possessed magnetic fields. Like miniature Earths, these objects had dynamos (the circulation of conducting fluid) in their cores to power their magnetic fields.
Both are pallasites, which come from the core-mantle boundary of a once-molten asteroid. Research shows that they acted as “recording devices” for the magnetic fields that once coursed through their parent bodies.
How To Read A Meteorite
When the molten conducting fluid inside one of those parent bodies solidified, it locked in an imprint of the magnetic field that existed at that time. Using nanoscale imaging, an international team of researchers led by James Bryson (University of Cambridge) was able to measure these meteorites’ cooling and solidification rates and their magnetic activity. To measure these characteristics, the researchers looked at the physical structure of tetrataenite, an iron-nickel alloy, in each meteorite.
The team’s observations show that “asteroid magnetic fields probably were generated a lot like that of the Earth: by motion of iron metallic fluid in a core that is undergoing crystallization to form a solid core,” said planetary scientist Ben Weiss (MIT), who was not involved in the study. Previous paleomagnetic measurements were used to argue for the existence of such cores in small planetary bodies, but “beyond this fundamental inference, we don’t know much about ancient core convection within asteroids,” wrote Bryson on the project’s blog.
Analogous to the way tree rings chronicle droughts and times of plenty, the matrices of tetrataenite within Imilac and Esquel record changes of strength and direction of the magnetic field produced by their parent bodies over time — and the eventual shutoff of the field once theirs respective asteroid's core solidified. These are some of the first observations of how an asteroid’s magnetic field changes in time, notes Weiss.
Pallasite meteorites solidified slowly, at 2 to 9 Kelvin per million years, which allows for the tetrataenite “islands” to form snapshots of magnetic activity. The researchers saw that these islands exsolved and hardened over time as they cooled, so their sizes serve as rulers to measure the rate of cooling in the parent bodies, and thus the devolving of the magnetic fields.
Magnetic Fields Extend Longer Than Previously Thought
While the exact size of Imilac’s and Esquel’s parent bodies are unknown, the researchers modeled the cooling of a 400-km-wide body — a size consistent with small rocky objects existing in the early solar system.
Previous research assumed that convection in these bodies was thermally driven (like a boiling pot of water, which transfers heat with physical motion from the pot’s bottom to the top). However, the data imprinted within the two meteorites shows magnetic activity lasting well beyond what thermally-driven convection could have sustained. The extra heat would have come from the gradual solidification of the molten metallic core.
Both the technique used for measuring these fossil fields and the results themselves have implications for planetary science. As Weiss explains, the research shows magnetic fields can be recorded “not just as magnetization (what gives a refrigerator magnet its pull), as is traditionally understood but also in the [physical] structures of minerals themselves.”
Further, since the team has showed that solidification-driven magnetic fields existed in the early solar system, we now have evidence that “a widespread, intense, and long-lived epoch of magnetic activity likely existed among many small bodies,” Bryson notes in the project’s blog. This activity lasted tens to hundreds of millions of years after the solar system’s formation, a timescale that’s an order of magnitude greater than previous theories had assumed.
James F. J. Bryson et al. “Long-lived magnetism from solidification-driven convection on the pallasite parent body.” Nature, Published online 21 January 2015.