Using samples collected by the Japan Aerospace Exploration Agency (JAXA) Hayabusa2 asteroid explorer, scientists have found the effects of micrometeorite bombardments on asteroid 162173 Ryugu. The team used a technique called electron holography to study the magnetic properties of the sample, learn about the magnetic field in the early solar system and how space erosion 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 such as Ryugu, caused by meteorite impacts or other influences from space.
Most meteorites that fall to the Earth's surface are not suitable for studying space weathering, since the material originated inside an asteroid. Some meteorites contain material from the surface of an asteroid, but their properties are changed by heat from atmospheric input and Earth's environment.
Therefore, the team needed a pristine sample (collected by a spacecraft like Hayabusa2) to properly study an asteroid's surface. The material studied consisted of a particle with a mass of 6.7 milligrams, about three millimeters in size, and many tiny particles that, scientists speculate, had broken off from 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 was formed during the early stages of the formation of the Solar System. Since then, the strength of the magnetic field in the Solar System has decreased, but the magnetization still remains in the magnetite as remanent magnetization.
Magnetite particles from a Ryugu sample. A: Electron microscopy image. B: Image of magnetic flux distribution obtained by electronic holography. The concentric lines show the magnetic field. (Credit: Kimura et al.)
By measuring the remnant magnetization of asteroids, scientists can infer the properties of the magnetic field at the time and place where the magnetite 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 space erosion signals we have directly detected will give us a better understanding of some of the phenomena occurring in the Solar System,” says study leader Yuki Kimura of Hokkaido University in Sapporo, Japan.
The scientists analyzed small grains, or framboids, of magnetite in the sample. Surprisingly, they discovered that some framboids had lost their magnetic properties. They called the particles pseudomagnetite and discovered 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 a result of space erosion by the micrometeoroid impact. Using computer simulations, scientists estimated that the meteoroids that could have caused these results were between two and 20 micrometers in size. The meteoroids must have hit Ryugu at a high speed of five kilometers per second or more.
Hayabusa2 tested Ryugu twice. On February 22, 2019, he landed on the asteroid's surface to collect the first sample. The material from this first sample was studied by Kimura's team. The spacecraft later created an artificial crater by firing a projectile at the asteroid's surface. This allowed the spacecraft to collect material from the crater's subsurface on July 11.
Pseudomagnetite surrounded by metallic iron nanoparticles (black and white arrows). A: Electron microscopy image. B: Distribution of iron in the sample. C: Magnetic flux distribution image of the same sample, in which no magnetic field lines are seen in the pseudomagnetite 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 that landed in Australia. Following this, Hayabusa2 entered its extended mission to explore two more asteroids. It is now scheduled to explore asteroid 2001 CC21 in 2026 and asteroid 1998 KY26 in 2031.
JAXA delivered a portion of the sample to NASA as part of an agreement to exchange samples and jointly study Ryugu and the 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 pseudomagnetite may reveal the asteroid's magnetic history.
“In future work, our results could also help reveal the relative ages of the surfaces of airless bodies and assist in the accurate interpretation of remote sensing data obtained from these bodies,” Kimura said. “While our study is primarily aimed at fundamental scientific interest and understanding, it could also help estimate the degree of degradation likely to be caused by space dust impacting robotic or manned spacecraft at high speed.”
The results of Kimura et al. They were published in the journal Nature Communications on April 29.
(Main image: Study illustration. From left to right, a micrometeorite hits Ryugu, Hayabusa2 returns a sample, and the sample is analyzed using electron holography. Credit: Yuki Kimura)
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