The 4.6-Billion-Year-Old Tape Recorder Hidden Inside Asteroid Dust

By Andy Tomaswick - March 06, 2026 02:03 PM UTC | Planetary Science
Images of the surface of Ryugu taken by the navigation camera on Hayabusa-2. Credit - JAXA, Chiba Institute of Technology, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Meiji University, University of Aizu, AIST
Images of the surface of Ryugu taken by the navigation camera on Hayabusa-2. Credit - JAXA, Chiba Institute of Technology, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Meiji University, University of Aizu, AIST

Asteroids are critical to unlock our understanding of the early solar system. These chunks of rock and dust were around at the very beginning, and they haven’t been as modified by planetary formation processes as, say, Earth has been. So scientists were really excited to get ahold of samples from Ryugu when they were returned by Hayabusa-2 a few years ago. However, when they started analyzing the magnetic properties of those samples, different research groups came up with different answers. Theorizing those conflicting results came from small sample sizes, a new paper recently published in JGR Planets from Masahiko Sato and their colleagues at the University of Tokyo used many more samples to finally dig into the magnetic history of these first ever returned asteroid samples.

So why would this study be important for understanding the early solar system? When asteroids formed, they were in part affected by the prevailing magnetic fields in the solar system at the time. These magnetic fields are what brought the gas and dust together that would eventually form planets, so understanding their strength (or weakness) is a key input to planetary formation theory.

There is a chance that on a planet itself, the current magnetic fields could impact the measurements. For example, meteorites, which are mainly asteroid samples returned by more natural means, are too affected by their time in Earth’s magnetic field to provide the early solar system insights scientists are looking for. To prevent this contamination, the Ryugu samples were isolated during descent and reentry, and handled extremely carefully once opened.

Fraser talks about the possibility of us mining asteroids.

Even with all those precautions, several different groups that looked at the samples from a magnetic perspective came to wildly different conclusions. One said the samples had a stable magnetic “memory” of the early solar system. Another found that the asteroid had formed in a “dead zone” with no magnetic field to speak of. And yet another argued that whatever magnetic field signals were found in the other studies were just caused by accidental contamination by Earth-based magnetic fields anyway.

According to the new paper, the problem was the small sample size the original papers were based on. In total, other research had only looked at 7 samples returned from the asteroid. To alleviate this problem, the new paper decided to look at 28 of them - four times the amount that had been previously studied, and much better for statistical relevance tests.

Determining if a rock “remembers” the magnetic field it was created in is a delicate process. When magnetic minerals form or cool down inside a magnetic field, their internal microscopic structures, called domains, align in the direction the field was pointing. Once the rock solidifies, those directions are locked in, allowing scientists to see which way the magnetic field was pointing, and how strong it was. But first, the weaker, more modern magnetic contamination must be stripped away, which the scientists did using a process called Stepwise Alternating Field Demagnetization.

John Michael Godier discusses the possibility of contamination of Ryugu samples. Credit - John Michael Godier YouTube Channel

After being cleaned of modern contaminants, the 28 samples told a relatively clear-cut story. Twenty-three of them had stable magnetic memories locked inside of them, while five didn’t. Interestingly, the strength of the field in the ones that did ranged from 16.3 microTeslas (uT) up to 174 uT - for comparison Earth’s magnetic field is around 50uT. And some of those samples had magnetic memories that pointed in multiple different directions in the same sample.

That last point proved that the memories were not caused by contamination, since Earth’s magnetic field points in only one direction consistently. Those samples in particular must have been magnetized before they were mashed together into Ryugu. When they were, they might have been smashed together surrounded by liquid water. The material holding these magnetic memories, known as framboidal magnetite, forms when liquid water interacts with rock in a process called aqueous alteration. So, at some point in Ryugu’s past, there was flowing liquid water in its core that chemically altered the rock and then locked the magnetic field in when the rock solidified.

The authors estimate that process happened around 3.1 to 6.8 million years after the very first solids were formed in the solar system. So Ryugu truly is an exemplar of the early solar system. Now that we have a better understanding of the magnetic environment in those early times, the next step will be updating planetary formation models with this new information.Who would have thought that a few specks of dirt from a rubble pile floating in space would have such an impact on our wider understanding of the universe.

Learn More:

Tokyo University of Science - Asteroid samples offer new insights into conditions when the solar system formed

M. Sato et al. - Characteristics of Natural Remanence Records in Fine-Grained Particles Returned From Asteroid Ryugu

UT - Tiny Fragments of a 4-Billion Year Old Asteroid Reveal Its History

UT - Samples of Asteroid Ryugu Contain More Than 20 Amino Acids

Andy Tomaswick

Andy Tomaswick

Andy has been interested in space exploration ever since reading Pale Blue Dot in middle school. An engineer by training, he likes to focus on the practical challenges of space exploration, whether that's getting rid of perchlorates on Mars or making ultra-smooth mirrors to capture ever clearer data. When not writing or engineering things he can be found entertaining his wife, four children, six cats, and two dogs, or running in circles to stay in shape.