That Magnetic Touch: Asteroids Hitting Asteroids

Art Courtesy of Annli Zhu.

In 2017, scientists unearthed a magnetic puzzle in our own solar neighborhood, beginning with samples taken from a series of iron-rich meteorites that had fallen to Earth. Upon analyzing the samples, the team detected evidence of magnetism. This discovery was important because it meant that the parent asteroid of these meteorites was somehow capable of internally generating its own magnetic field—a phenomenon that was difficult to explain. 

The same year this discovery was made, planetary scientist Zhongtian Zhang, now at the Carnegie Institution for Science’s Earth and Planets Laboratory, began his graduate studies at Yale. Zhang was intrigued by the results of the meteorite analysis, but like the rest of the scientific community, he was confused as to how an asteroid like this one could feasibly generate a magnetic field. “The community had been puzzled with this as well, and I hadn’t been able to come up with a solution for a long time,” Zhang said.

Six years later, in a paper published in the Proceedings of the National Academy of Sciences this July, Zhang and David Bercovici, Frederick William Beinecke Professor of Earth and Planetary Sciences, may have figured out the origin of these magnetic meteorites. The secret may lie in asteroids, and what happens when they collide with one another.

The Paradox of Magnetism

Planetary bodies generate magnetic fields through mechanisms known as ‘dynamos.’ In general, dynamos rely on convective motion, in which less dense material rises up as more dense material sinks down. Take Earth, for example: the iron-nickel core at the heart of our planet solidifies from the inside out in a process called outward solidification, causing the convective motion necessary to generate a magnetic field. Understanding dynamos can provide insights into a planetary body’s internal structures and evolutionary histories.

The cores of asteroids, however, are a different matter altogether. Meteorites originate from asteroids, which are fragments of rocks in space that date back nearly 4.5 billion years. Meteorites that are rich in iron, specifically, come from the cores of asteroids. There are approximately 1.3 million asteroids in our solar system, most of which reside in the Asteroid Belt between Jupiter and Mars. Of these, only eight percent are made of metal. The liquid cores of these metal asteroids are known to cool from the outside in through a process called inward solidification. This is why these metal asteroids were not thought to be capable of generating their own magnetic fields—inward solidification directly inhibits convection and suppresses the traditional magnetic field dynamo.

When Asteroids Collide

When thinking about this paradox, Zhang turned to a previous project of his on rubble-pile asteroids, which are formed when asteroid fragments coalesce into new objects due to gravitational forces. “I started to think of things in terms of collisions and formation of rubble piles,” Zhang said. “I was thinking that this may be the solution to the problem that’s been on my mind for quite a while.”

Zhang deduced that in order for the metallic core of an asteroid to become exposed in the first place, a collision must have taken place in a process termed ‘mantle unstripping’ by means of another asteroid. The force of an asteroid hitting another asteroid would cause the mantle of the original asteroid to be broken down, exposing the resulting asteroid fragments, alongside the core, directly to the environment of space.

In the aftermath of a collision, an asteroid’s molten core would have broken apart and reformed, and if a small portion of metal fragments were able to cool down sufficiently before falling back into the molten core, they would sink downwards. Bercovici compared the process to dropping ice cubes in hot tea, except the ice cubes sink. These cold fragments that sink to the center would then extract heat from the overlying liquid and cause the outward solidification capable of driving a magnetic field. Meanwhile, the inward solidification that occurred from the surface would produce cold material to preserve this field. “It provides an implication about how asteroids work, [how they were] formed and disrupted,” Zhang said. “It provides a new scenario for people studying magnetic fields.”

Initially, Zhang set out to determine the size of the asteroid fragments necessary to power a dynamo in this fashion. The ideal fragment would be small enough to cool efficiently in the vacuum of space, but also large enough to remain sufficiently cold after sinking through the hot liquid region of the core, according to the two researchers. Zhang modeled the thermal regulation of the fragments and determined that the ideal fragment size is approximately ten meters, which coincided with his calculations for the average fragment size created by these collisions. “It turns out that fragmentation size is right in the Goldilocks regime for having the “right” ice cubes,” Bercovici said. “Bottom line—that was cool, pun not really intended.”

To Psyche And Beyond

Zhang performed additional modeling and determined that the convection generated from this theory would be adequate to power a magnetic field for at least one million years. This research could have important implications for what we understand about asteroids, including NASA’s future Psyche Mission.

Psyche is an asteroid that has long been a subject of fascination for some members of the scientific community, as it may be the iron-nickel core of a planet that formed billions of years ago. The mission recently launched on October 13, 2023, and is anticipated to reach Psyche in 2029. Once in orbit, the hope is that the mission will allow scientists to develop a deeper understanding of our solar system’s history, as well as that of our own planet, through the information collected from Psyche. Zhang and Bercovici’s research could be crucial to understanding Psyche’s origin, as well as planetary evolution as a whole. Bercovici is also a principal investigator on the Psyche Mission, which was the source of funding for this project.

“I decided to be part of the mission because of my interest in planetary sciences in the first place,” Zhang said. “It was also a personal interest in these kinds of things and being part of the Psyche mission bolstered me to look at this as a problem of magnetic fields and meteorite observations.” Zhang hopes to expand this work in new directions in the future, hopefully involving information about metal asteroids obtained from the Psyche Mission to understand the asteroid’s history.

Bercovici enjoyed working with Zhang over the course of the project, citing Zhang’s tenacity after having published several ‘hard-won’ papers. “Zhongtian is one of the most creative, deep-thinking, and versatile students or colleagues I’ve had the pleasure of working for,” Bercovici said. “Sometimes he was like a mustang bolting into the hills with new ideas, and my job was to help him close the loop and explain his ideas clearly. Having students and postdocs much smarter than me is always fun, and my job is to make sure they communicate well with mere mortals, like myself.”

To understand the universe, one must acknowledge its mysteries—including the ones that exist in our own solar neighborhood. After six years of mystery, this magnetic meteorite puzzle may finally have been solved, and its lessons applied forward, thanks to the work of these two Yale researchers.