The near-Earth asteroid Ryugu is a small, primitive, carbon-rich rubble pile thought to be a remnant of a larger parent body that was shattered early in solar system history. Because of its primitive nature, Ryugu preserves astromaterials that may retain NRM acquired shortly after the solar system formed. Samples returned from Ryugu to Earth by Japan's Hayabusa2 spacecraft in 2020 were handled and curated with great care, minimizing contamination from Earth's magnetic field and allowing any such effects to be tracked and corrected.
Earlier work used stepwise alternating field demagnetization, or AFD, to probe NRM in seven Ryugu particles, but the limited sample size left room for different interpretations of the results. To resolve these discrepancies, a team led by Associate Professor Masahiko Sato of the Department of Physics at Tokyo University of Science conducted a new set of highly sensitive magnetic measurements on a much larger suite of particles. Their study, published on February 10, 2026 in the Journal of Geophysical Research: Planets, reports systematic paleomagnetic measurements on 28 submillimeter-sized grains using a superconducting quantum interference device, or SQUID, magnetometer at the University of Tokyo.
The expanded dataset shows that 23 of the 28 Ryugu particles carry stable NRM components that persist through AFD treatment. Among these, eight particles display two distinct stable components, indicating that they experienced at least two magnetization events. One particle exhibits spatially inhomogeneous NRM directions, meaning the magnetization varies within the grain itself rather than pointing in a single uniform direction. Such complexity is difficult to reconcile with a simple overprint acquired during spacecraft operations or after recovery on Earth.
Instead, the spatially inhomogeneous directions point to magnetization acquired before the particles finally solidified. This implies that late-stage processes, such as handling on the spacecraft or on Earth, cannot account for the observed NRM characteristics. The team concludes that the dominant signal is a form of chemical remanent magnetization that was locked in as new magnetic minerals grew within the particles.
In particular, the results indicate that tiny, raspberry-like aggregates of magnetite known as framboidal magnetite formed during water-driven alteration on Ryugu's parent body. As these framboidal magnetite grains grew in the presence of a magnetic field, they acquired a chemical remanent magnetization that faithfully recorded that field. According to Sato, the particles thus preserve a record of the magnetic environment of the very early solar system, potentially within about 3 to 7 million years after its formation.
By clarifying the nature and origin of the NRM in Ryugu samples, the study helps reconcile differing interpretations from earlier, smaller datasets. The work demonstrates that fine-grained asteroid material can retain detailed information about both magnetization timing and alteration processes on the parent body. These insights, in turn, sharpen constraints on the magnetic and dynamical evolution of the protoplanetary disk.
The improved understanding of Ryugu's magnetic properties feeds directly into broader efforts to model how planetary building blocks formed and migrated. Knowing when and how water-driven alteration occurred, and under what magnetic conditions, helps researchers refine scenarios for the assembly of rocky planets. Ultimately, the new results from Ryugu provide an important piece of the puzzle in explaining how the early solar system evolved into the planetary system we see today.
Research Report:Characteristics of Natural Remanence Records in Fine-Grained Particles Returned From Asteroid Ryugu
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