An asteroid that has been wandering in space for billions of years will be bombarded with everything from rocks to radiation. Billions of years of traveling through interplanetary space increase the odds of colliding with something in the vast void, and at least one of those impacts had enough force to leave the Ryugu asteroid forever altered.
When the Japanese Space Agency’s Hayabusa2 spacecraft landed on Ryuga, it collected samples from the surface that showed that magnetite particles (which are normally magnetic) in the asteroid’s regolith are devoid of magnetism. A group of researchers from Hokkaido University and several other institutions in Japan are now offering an explanation for how this material lost most of its magnetic properties. Their analysis showed that it was caused by at least one high-speed collision of micrometeorites, which disrupted the chemical structure of the magnetite so that it was no longer magnetic.
“We assumed that pseudomagnetite had been created [as] the result of space weathering from the impact of micrometeorites,” researchers led by Hokkaido University professor Yuki Kimura said in a study recently published in Nature Communications.
What remains…
Ryugu is a relatively small object with no atmosphere, which makes it more susceptible to cosmic weathering – alteration by micrometeoroids and the solar wind. Understanding space weathering can really help us understand the evolution of asteroids and the solar system. The problem is that most of our information about asteroids comes from meteorites that fell to Earth, and most of those meteorites are pieces of rock from inside the asteroid, so they haven’t been exposed to the harsh environment of interplanetary space. They can also be modified as they fall through the atmosphere or by physical processes at the surface. The longer it takes to find a meteorite, the more information can be lost.
Once part of a much larger body, Ryugu is a C-type asteroid, or carbonaceous, meaning it is composed mostly of clay and silicate rocks. These minerals normally require water to form, but their presence is explained by Ryugu history. The asteroid itself is believed to have been born from debris after its parent body was shattered into pieces by a collision. The parent body was also covered in water ice, which explains the magnetite, carbonates and silicates found on Ryugu – they need water to form.
Magnetite is a ferromagnetic (iron-containing and magnetic) mineral. It is found in all C-type asteroids and can be used to determine their remanent magnetization. An asteroid’s residual magnetization can reveal how strong the magnetic field was at the time and place the magnetite formed.
Kimura and his team were able to measure the remanent magnetization in two fragments of magnetite (known as framboids because of their peculiar shape) from the Ryugu sample. This is evidence for the existence of a magnetic field in the nebula where our solar system formed, and shows the strength of that magnetic field at the time magnetite formed.
However, the other three analyzed magnetite fragments are not magnetized at all. This is where space weathering comes into play.
…and what was lost
Using electron holography, performed with a transmission electron microscope that sends high-energy electron waves through the sample, the researchers found that the three framboids in question do not have magnetic chemical structures. In this way, they differed sharply from magnetite.
Further analysis using scanning transmission electron microscopy showed that the magnetite particles were mostly iron oxides, but those particles that had lost their magnetism had less oxygen, indicating a chemical reduction of the material where electrons were transferred to the system. . This loss of oxygen (and oxidized iron) explained the loss of magnetism, which depends on the organization of electrons in magnetite. This is why Kimura calls it “pseudomagnetite”.
But what triggered the reduction that demagnetized the magnetite in the first place? Kimura and his team found that the part of the sample from which the demagnetized framboids originated contained more than a hundred particles of metallic iron. If a micrometeorite of a certain size had fallen into this Ryugu region, it would have produced approximately that many iron particles from the magnetite framboids. Researchers believe that this mysterious object was quite small, otherwise it must have been moving incredibly fast.
“As the impact velocity increases, the estimated size of the projectile decreases,” they said in the same study.
Pseudomagnetite may seem like an impostor, but it will actually help future investigations that seek to learn more about what the early solar system was like. Its presence indicates the former presence of water on the asteroid, as well as cosmic weathering, such as micrometeorite bombardment, that affected the asteroid’s composition. How much magnetism has been lost also affects the total residual magnetization of the asteroid. Remanence is important in determining the magnetism of an object and the intensity of the magnetic field around it when it was formed. What we know about the early magnetic field of the Solar System has been reconstructed from records of the residual magnetic field, many of which come from magnetite.
Some of the magnetic properties of these particles may have been lost years ago, but much more can be gained in the future from what remains.
Nature Communications, 2024. DOI: 10.1038/s41467-024-47798-0