Effects of space weathering on asteroid Ryugu revealed by Hayabusa2 samples

Scientists studying samples collected by the Japan Aerospace Exploration Agency’s (JAXA) asteroid explorer Hayabusa2 have made an exciting discovery about the effects of space weathering on asteroid 162173 Ryugu. Using a technique called electron holography, the team examined the magnetic properties of the samples to gain insights into the magnetic field in the early solar system and how space weathering influenced the asteroid’s surface.

Space weathering refers to the changes that occur on the surfaces of planetary bodies without atmospheres, such as the Moon, Mercury, and asteroids like Ryugu, due to meteoroid impacts and other space-related influences. While most meteorites that land on Earth are not suitable for studying space weathering since they originate from an asteroid’s interior, certain meteorites do contain surface material. However, their properties are altered by the heat from atmospheric entry and Earth’s environment.

To accurately study an asteroid’s surface, a pristine sample like the one collected by Hayabusa2 was necessary. The Team examined a 6.7 milligrams particle, approximately three millimeters in size, and a high number of tiny particles that likely broke off from the larger particle.

But why are these tiny particles crucial for understanding the properties of the early solar system? The sample contains grains of a magnetic mineral called magnetite, a form of iron oxide. Scientists believe that magnetite found in asteroids formed during the early stages of the Solar System’s development. As the magnetic field within the Solar System weakened over time, the magnetization remained in magnetite as remnant magnetization.

By studying the remnant magnetization of asteroids, scientists can infer properties of the magnetic field at the time and location where the magnetite formed. This information is invaluable for understanding the evolution of planetary systems. Additionally, these magnetite properties can be altered later through space weathering processes.

“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 investigated small grains known as framboids that contained magnetite in the sample. Surprisingly, they discovered that some framboids had lost their magnetic properties. They named these particles pseudo-magnetite and found that the mineral shared characteristics with both magnetite and another mineral called wüstite.

The team believes that these particles, along with thousands of metallic iron nanoparticles surrounding the framboids, were formed as a result of space weathering caused by micrometeoroid impacts. Computer simulations allowed the scientists to estimate that the meteoroids responsible for these changes were around two to 20 micrometers in size and impacted Ryugu at velocities exceeding five kilometers per second.

Hayabusa2 collected samples from Ryugu twice, first on February 22, 2019, by touching down on the asteroid’s surface, and later by creating an artificial crater through a projectile impact on July 11. The material obtained from the first sample was analyzed by Kimura’s team. In December 2020, Hayabusa2 successfully delivered a total of 5.4 grams of material to Earth in a protective capsule that landed in Australia. The spacecraft is now embarking on extended missions to explore two more asteroids – 2001 CC21 in 2026 and 1998 KY26 in 2031.

As part of an agreement to exchange samples and jointly study Ryugu and asteroid Bennu, JAXA provided a portion of the sample to NASA. NASA’s OSIRIS-Rex mission successfully returned a sample from Bennu in 2023. Kimura’s team hopes to analyze the samples from Bennu in future studies to further uncover the asteroid’s magnetic history.

“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,” Kimura said. “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.”

The research conducted by Kimura et al. was published in the journal Nature Communications on April 29.

This remarkable study shedding light on the effects of space weathering on asteroid Ryugu serves as another milestone in our understanding of the early solar system and planetary evolution. The analysis of pristine samples collected by Hayabusa2 provides valuable insights into the magnetic properties of asteroids, helping scientists infer the magnetic field’s characteristics during the formation of magnetite in the early Solar System. By studying the magnetic history of asteroids, researchers can unravel critical information about the evolution of planetary systems.

Furthermore, the discovery of pseudo-magnetite particles and metallic iron nanoparticles surrounding framboids offers additional evidence of space weathering caused by micrometeoroid impacts. These findings not only contribute to a better understanding of phenomena occurring in the Solar System but also have potential applications in estimating the degradation that space dust impact can cause on spacecraft in high-velocity environments.

The collaboration between JAXA and NASA, exchanging samples from Ryugu and Bennu, opens up even more exciting possibilities for future research. Analyzing the samples from Bennu could reveal further insights into the asteroid’s magnetic history and facilitate accurate interpretation of remote sensing data obtained from airless bodies. Additionally, by studying the relative ages of surfaces on these bodies, scientists can better understand their geological processes and history.

This study conducted by Yuki Kimura’s team presents a significant contribution to the field of planetary science and deepens our understanding of the early solar system. As more missions like Hayabusa2 and OSIRIS-Rex continue to gather crucial data and bring back samples from asteroids, our knowledge of the cosmos steadily expands. With the extended missions planned for Hayabusa2, upcoming explorations of asteroids 2001 CC21 and 1998 KY26 will undoubtedly offer new insights and add to our understanding of our celestial neighborhood.