Ryugu Asteroid Particles Carry a 4.5-Billion-Year-Old Magnetic Memory
Japan's Hayabusa2 spacecraft returned to Earth in December 2020 carrying something no robotic mission had delivered before: pristine samples from a primitive, carbon-rich near-Earth asteroid. The roughly 5.4 grams of material from asteroid Ryugu had been sealed from contamination since collection - minimal exposure to Earth's magnetic field, careful curation, and a documented handling history. For scientists trying to read ancient magnetic records preserved in space rocks, those conditions matter enormously.
A team led by Associate Professor Masahiko Sato at Tokyo University of Science has now extracted magnetic data from 28 individual Ryugu particles - the largest such dataset from these samples to date - and used them to reconstruct conditions in the early solar system.
Locking Magnetism Into Ancient Minerals
The phenomenon at the center of this research is called natural remanent magnetization, or NRM. When magnetic minerals form in the presence of an ambient magnetic field, they can lock in a record of that field's direction and intensity - a record that, in the right geological conditions, persists for billions of years. The same principle underlies paleomagnetic studies of Earth rocks, but reading it in extraterrestrial samples presents unique challenges.
Ryugu's parent body - the larger object that was disrupted to eventually produce Ryugu - experienced liquid water activity early in solar system history. That water drove chemical reactions that produced tiny magnetic minerals called framboidal magnetite. As those minerals grew, they recorded the magnetic field present at the time. The particles returned by Hayabusa2 potentially preserve those records intact.
Previous studies had measured NRM in only seven Ryugu particles, and conflicting interpretations emerged from that limited dataset. Sato's team expanded the measurement set to 28 submillimeter-sized particles, using a superconducting quantum interference device (SQUID) magnetometer at the University of Tokyo. The SQUID instrument is sensitive enough to detect the tiny magnetic signals in these microscopic samples.
What 28 Particles Revealed
The measurements showed that 23 of 28 Ryugu samples exhibited stable NRM components. Eight of those particles showed two distinct stable components, suggesting they recorded magnetic information at different times or from different physical processes. One particle displayed spatially inhomogeneous NRM directions - magnetic orientations that varied across different locations within a single particle.
That spatial variation turned out to be an important clue. If the magnetization had been acquired after the asteroid was sampled - during spacecraft handling or after arrival on Earth - it would have overprinted the particles uniformly. Spatially inhomogeneous directions cannot result from a late, uniform overprinting event; they require that the magnetization was acquired before the particles fully solidified, during active mineral growth on Ryugu's parent body.
The results strongly suggest the NRM is a chemical remanent magnetization, acquired as framboidal magnetite grew in the presence of liquid water on Ryugu's parent body during the early solar system. "This means that these particles preserve a record of the magnetic field of the very early solar system, potentially within approximately 3 to 7 million years after its formation," Sato noted.
Reading the Solar Nebula's Field
Why does the magnetic field strength and configuration of the early solar nebula matter? The nebular magnetic field influenced how material was transported through the protoplanetary disk - the spinning disk of gas and dust from which the planets formed. A stronger field facilitates the outward transport of angular momentum, which in turn affects how quickly material falls onto the forming Sun and how long the disk persists. These parameters shape the final architecture of planetary systems.
Current estimates of the solar nebula's magnetic field come from paleomagnetic measurements of primitive meteorites. Ryugu samples add a new type of evidence: particles from an asteroid that has experienced minimal alteration since the early solar system, handled with contamination controls unavailable for meteorites that fell through Earth's atmosphere and spent time on the surface.
The study does carry limitations. The 28 particles, while representing a significant expansion over prior datasets, are still a small sample from a single asteroid. Whether the magnetic records they preserve are representative of the parent body as a whole - or are concentrated in specific regions that experienced particular water-activity histories - cannot yet be determined. And converting NRM measurements into absolute magnetic field strengths requires assumptions about the properties of framboidal magnetite that are still being refined through laboratory work on synthetic analogs.
Future analyses of Ryugu samples, as well as comparisons with samples from asteroid Bennu returned by NASA's OSIRIS-REx mission, should help resolve these uncertainties and sharpen the picture of what the magnetic environment of the early solar system actually looked like.