Asteroid grains shed light on the outer solar system's origins

Scientists at MIT and elsewhere have analyzed particles of the asteroid Ryugu, which were collected by the Japanese Aerospace Exploration Agency's (JAXA) Hayabusa2 mission and brought back to Earth in 2020. Scientists believe Ryugu formed on the outskirts of the early solar system before migrating in toward the asteroid belt, eventually settling into an orbit between Earth and Mars.

The team analyzed Ryugu's particles for signs of any ancient magnetic field that might have been present when the asteroid first took shape. Their results suggest that if there was a magnetic field, it would have been very weak. At most, such a field would have been about 15 microtesla. (The Earth's own magnetic field today is around 50 microtesla.)

Even so, the scientists estimate that such a low-grade field intensity would have been enough to pull together primordial gas and dust to form the outer solar system's asteroids and potentially play a role in giant planet formation, from Jupiter to Neptune.

The team's results, which are published today in the journal AGU Advances, show for the first time that the distal solar system likely harbored a weak magnetic field. Scientists have known that a magnetic field shaped the inner solar system, where Earth and the terrestrial planets were formed. But it was unclear whether such a magnetic influence extended into more remote regions, until now.

"We're showing that, everywhere we look now, there was some sort of magnetic field that was responsible for bringing mass to where the sun and planets were forming," says study author Benjamin Weiss, the Robert R. Shrock Professor of Earth and Planetary Sciences at MIT. "That now applies to the outer solar system planets."

The study's lead author is Elias Mansbach PhD '24, who is now a postdoc at Cambridge University. MIT co-authors include Eduardo Lima, Saverio Cambioni, and Jodie Ream, along with Michael Sowell and Joseph Kirschvink of Caltech, Roger Fu of Harvard University, Xue-Ning Bai of Tsinghua University, Chisato Anai and Atsuko Kobayashi of the Kochi Advanced Marine Core Research Institute, and Hironori Hidaka of Tokyo Institute of Technology.

A far-off field

Around 4.6 billion years ago, the solar system formed from a dense cloud of interstellar gas and dust, which collapsed into a swirling disk of matter. Most of this material gravitated toward the center of the disk to form the sun. The remaining bits formed a solar nebula of swirling, ionized gas. Scientists suspect that interactions between the newly formed sun and the ionized disk generated a magnetic field that threaded through the nebula, helping to drive accretion and pull matter inward to form the planets, asteroids, and moons.

"This nebular field disappeared around 3 to 4 million years after the solar system's formation, and we are fascinated with how it played a role in early planetary formation," Mansbach says.

Scientists previously determined that a magnetic field was present throughout the inner solar system -- a region that spanned from the sun to about 7 astronomical units (AU), out to where Jupiter is today. (One AU is the distance between the sun and the Earth.) The intensity of this inner nebular field was somewhere between 50 to 200 microtesla, and it likely influenced the formation of the inner terrestrial planets. Such estimates of the early magnetic field are based on meteorites that landed on Earth and are thought to have originated in the inner nebula.

"But how far this magnetic field extended, and what role it played in more distal regions, is still uncertain because there haven't been many samples that could tell us about the outer solar system," Mansbach says.

Rewinding the tape

The team got an opportunity to analyze samples from the outer solar system with Ryugu, an asteroid that is thought to have formed in the early outer solar system, beyond 7 AU, and was eventually brought into orbit near the Earth. In December 2020, JAXA's Hayabusa 2 mission returned samples of the asteroid to Earth, giving scientists a first look at a potential relic of the early distal solar system.

The researchers acquired several grains of the returned samples, each about a millimeter in size. They placed the particles in a magnetometer -- an instrument in Weiss' lab that measures the strength and direction of a sample's magnetization. They then applied an alternating magnetic field to progressively demagnetize each sample.

"Like a tape recorder, we are slowly rewinding the sample's magnetic record," Mansbach explains. "We then look for consistent trends that tell us if it formed in a magnetic field."

They determined that the samples held no clear sign of a preserved magnetic field. This suggests that either there was no nebular field present in the outer solar system where the asteroid first formed, or the field was so weak that it was not recorded in the asteroid's grains. If the latter is the case, the team estimates such a weak field would have been no more than 15 microtesla in intensity.

The researchers also reexamined data from previously studied meteorites. They specifically looked at "ungrouped carbonaceous chondrites" -- meteorites that have properties that are characteristic of having formed in the distal solar system. Scientists had estimated the samples were not old enough to have formed before the solar nebula disappeared. Any magnetic field record the samples contain, then, would not reflect the nebular field. But Mansbach and his colleagues decided to take a closer look.

"We reanalyzed the ages of these samples and found they are closer to the start of the solar system than previously thought," Mansbach says. "We think these samples formed in this distal, outer region. And one of these samples does actually have a positive field detection of about 5 microtesla, which is consistent with an upper limit of 15 microtesla."

This updated sample, combined with the new Ryugu particles, suggest that the outer solar system, beyond 7 AU, hosted a very weak magnetic field, that was nevertheless strong enough to pull matter in from the outskirts to eventually form the outer planetary bodies, from Jupiter to Neptune.

"When you're further from the sun, a weak magnetic field goes a long way," Weiss notes. "It was predicted that it doesn't need to be that strong out there, and that's what we're seeing."

The team plans to look for more evidence of distal nebular fields with samples from another far-off asteroid, Bennu, which were delivered to Earth in September 2023 by NASA's OSIRIS-REx spacecraft.

"Bennu looks a lot like Ryugu, and we're eagerly awaiting first results from those samples," Mansbach says.

This research was supported, in part, by NASA.