Hayabusa2 samples reveal the effects of space weathering on asteroid Ryugu

by Martijn Luinstra
Conceptual illustration of the study. From left to right, a micrometeoroid impacts Ryugu, Hayabusa2 returns a sample, and the sample is analyzed using electron holography.

Using samples collected by Japan Aerospace Exploration Agency’s (JAXA) asteroid explorer Hayabusa2, scientists have found the effects of micrometeoroid bombardments on asteroid 162173 Ryugu. The team used a technique called electron holography to study the sample’s magnetic properties, to learn about the magnetic field in the early solar system, and how space weathering affected the asteroid’s surface. Space weathering is the name astronomers use to describe changes to the surface of planetary bodies without an atmosphere, such as the Moon, Mercury, or asteroids like Ryugu, caused by meteoroid impacts or other influences from space.

Most meteorites that land on Earth’s surface are not suitable for studying space weathering, as the material originated in an asteroid’s interior. Some meteorites do contain material from an asteroid’s surface, but their properties are changed by the heat from atmospheric entry and the environment on Earth.

Thus, the team needed a pristine sample — collected by a spacecraft such as Hayabusa2 — to study an asteroid’s surface properly. The studied material consisted of a particle with a mass of 6.7 milligrams about three millimeters in size and many tiny particles that the scientists speculate had broken off the larger particle.

But how can these tiny particles help scientists understand the properties of the early solar system?

The sample contains grains of a magnetic mineral called magnetite, which is a form of iron oxide. The magnetite found in asteroids formed during the early stages of the Solar System’s formation. The magnetic field in the Solar System has since decreased in strength, but the magnetization is still left behind in the magnetite as remanent magnetization.

Magnetite particles from a Ryugu sample. A: Electron microscopy image. B: Magnetic flux distribution image obtained by electron holography. The concentric lines show the magnetic field. (Credit: Kimura et al.)

By measuring the remanent magnetization of asteroids, scientists can infer properties of the magnetic field at the time and place where the magnetite was formed. This information helps astronomers study the evolution of planetary systems.

However, the properties of magnetite in asteroids can later be changed, by processes such as space weathering. “The signatures of space weathering we have detected directly will give us a better understanding of some of the phenomena occurring in the Solar System,” says study lead Yuki Kimura of Hokkaido University in Sapporo, Japan.

The scientists analyzed small grains, or framboids, of magnetite in the sample. Surprisingly, they found that some framboids had lost their magnetic properties. They named the particles pseudo-magnetite and found that the mineral shared properties with magnetite and another mineral called wüstite.

The team suggests that these particles and the thousands of metallic iron nanoparticles surrounding the framboids were created as the result of space weathering by micrometeoroid impact. Using computer simulations, the scientists estimated the meteoroids that could have caused these results were about two to 20 micrometers in size. The meteoroids must have impacted Ryugu at a high velocity of five kilometers per second or more.

Hayabusa2 sampled Ryugu twice. On Feb. 22, 2019, it touched down on the asteroid’s surface to collect the first sample. Material from this first sample was studied by Kimura’s team. Later, the spacecraft created an artificial crater by shooting a projectile at the asteroid’s surface. This allowed the spacecraft to collect subsurface material from the crater on July 11.

Pseudo-magnetite surrounded by metallic iron nanoparticles (white and black arrows). A: Electron microscopy image. B: Iron distribution in the sample. C: Magnetic flux distribution image of the same sample, in which no magnetic field lines can be seen in the pseudo-magnetite while the magnetic fields of the iron nanoparticles are clearly visible. (Credit: Kimura et al.)

In December 2020, Hyabusa2 successfully delivered a total of 5.4 grams of material to Earth in a protective capsule containing that landed in Australia. After this, Hayabusa2 entered its extended mission to explore two more asteroids. It is now set to explore asteroid 2001 CC21 in 2026 and asteroid 1998 KY26 in 2031.

JAXA delivered a part of the sample to NASA as part of an agreement to exchange samples and jointly study Ryugu and asteroid Bennu. NASA’s OSIRIS-Rex mission successfully returned a sample from Bennu in 2023.

Kimura’s team hopes to analyze samples collected from Bennu in a future study. The presence of pseudo-magnetite can then reveal the magnetic history of the asteroid.

“In future work, our results could also help to reveal the relative ages of surfaces on airless bodies and assist in the accurate interpretation of remote sensing data obtained from these bodies,” said Kimura. “Although our study is primarily for fundamental scientific interest and understanding, it could also help estimate the degree of degradation likely to be caused by space dust impacting robotic or crewed spacecraft at high velocity.”

Kimura et al.’s results were published in the journal Nature Communications on April 29.

(Lead image: Illustration of the study. From left to right, a micrometeoroid impacts Ryugu, Hayabusa2 returns a sample, and the sample is analyzed using electron holography. Credit: Yuki Kimura)

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