By bombarding a meteorite with X-rays, scientists have managed to read ancient magnetic messages formed in the early solar system, some 4.5 billion years ago. These recordings, which were retained by microscopic “space magnets” embedded in the cosmic debris, tell us the tale of the moment when an asteroid’s metallic core froze, killing its magnetic field. Tantalizingly, these memories may allow us to peer into the distant future of our own planet, yielding new insights into what may happen to its core as it continues to freeze. The work has been published in Nature.
Scientists are interested in studying asteroids because they act as time capsules, containing important bits of information about the history of our solar system. Meteorites are fragments of these rocky worlds that have collided with Earth, offering scientists an opportunity to probe the space debris’ past.
The 4.5-billion-year-old meteorites used in the study were collected from Argentina, but they originated in the asteroid belt between Mars and Jupiter. These particular types of debris are known as pallasites, which are primarily composed of a mixture of iron and nickel, peppered with silicate crystals. It was long assumed that meteorites such as this would offer us little insight into the magnetic history of the parent asteroid because iron has a bad “memory,” meaning it doesn’t retain magnetic signals well.
Luckily for us, these meteorites contained tiny, unassuming particles, less than one thousandth the width of a human hair, of a magnetic mineral called tetrataenite. Unlike iron, this mineral is capable of maintaining magnetic records for billions of years, allowing scientists to peer back in time to the early solar system. In particular, these recordings trap the magnetic signal of the precise moment when the parent asteroid’s core formed, as well as the moment it froze, killing its magnetic field.
Using a beam of X-rays, scientists from the University of Cambridge were able to produce a detailed image of the magnetization of the tetrataenite particles, which essentially allowed them to “read” the magnetic memory stored within the meteorite. This provided a snapshot of the solidification of the asteroid’s liquid core, which yielded interesting insights into the history of its magnetic field.
Although core solidification did kill the asteroid’s magnetic field, they found that it hung around for much longer than originally believed, lasting for as long as several hundred million years after the asteroid was forged. Furthermore, they found that the magnetic field was generated by a similar mechanism to the one that gives our planet its own magnetic field. As the liquid metal core of the asteroid cooled and started to freeze, the swirling motions of the inner core created a long-lasting magnetic field, just like our planet.
Just like the asteroid, Earth’s core is also freezing, albeit much more slowly, and has been for around a billion years. What we don’t know is how this has affected its magnetic field, which is where this study could provide some useful insight. Eventually, the core will freeze and the protective magnetic field will die, but that won’t happen for billions of years.