An asteroid that has been wandering through space for billions of years will be bombarded by everything from rocks to radiation. Billions of years of travel through interplanetary space increase the chances of colliding with something in the vast void, and at least one of those impacts was powerful enough to leave asteroid Ryugu forever altered.
When the Japanese Space Agency’s Hayabusa2 spacecraft touched down at Ryugu, it collected surface samples that revealed that the magnetite particles (which are normally magnetic) in the asteroid’s regolith were devoid of magnetism. A team of researchers from Hokkaido University and several other institutions in Japan now offer an explanation for how this material lost most of its magnetic properties. Their analysis showed that this was caused by at least one collision with a high-velocity micrometeorite that destroyed the chemical structure of the magnetite so that it was no longer magnetic.
“We hypothesized that pseudomagnetite was created [as] a result of space weathering from the impact of a micrometeorite,” the researchers, led by Hokkaido University professor Yuki Kimura, said in a study recently published in Nature Communications.
What is left…
Ryugu is a relatively small object with no atmosphere, making it more susceptible to space weathering – a change from micrometeoroids and the solar wind. Understanding space weathering can actually 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 fall to Earth, and most of those meteorites are chunks of rock from the interior of an asteroid, so they haven’t been exposed to the brutal environment of interplanetary space. . They can also be changed as they fall in the atmosphere or by physical processes at the surface. The longer it takes to find a meteorite, the more information can potentially be lost.
Once part of a much larger body, Ryugu is a C-type asteroid, or carbonaceous asteroid, meaning it is made mostly of clay and silicate rocks. These minerals normally need water to form, but their presence is explained by Ryugu’s history. The asteroid itself is thought to have been born from debris after its parent body was blown to pieces in an impact. The parent body was also covered in water ice, which explains the magnetite, carbonates, and silicates found in 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 residual or remanent magnetization. The residual magnetization of an asteroid can reveal how intense the magnetic field was at the time and place of magnetite formation.
Kimura and his team were able to measure the remanent magnetization in two magnetite fragments (known as framboids because of their peculiar shape) from the Ryugu sample. This is evidence of a magnetic field in the nebula where our solar system formed, and shows the strength of that magnetic field at the time the magnetite formed.
However, three other analyzed magnetite fragments are not magnetized at all. This is where space weathering comes into play.
…and what was lost
Using electron holography, which is done with a transmission electron microscope that sends high-energy electron waves through a sample, the researchers found that the three framboids in question do not have magnetic chemical structures. This makes them drastically different from magnetite.
Further analysis with scanning transmission electron microscopy showed that the magnetite particles were made mostly of iron oxides, but there was less oxygen in these particles that had lost their magnetism, indicating that the material had undergone a chemical reduction in which electrons were donated of the system. This loss of oxygen (and oxidized iron) accounts for the loss of magnetism, which depends on the electron organization in magnetite. That’s why Kimura calls it “pseudomagnetite.”
But what caused the reduction that demagnetized the magnetite in the first place? Kimura and his team found that there were more than a hundred metallic iron particles in the part of the sample from which the demagnetized framboids came. If a micrometeorite of a certain size had struck this region of Ryugu, then it would have produced roughly that many iron particles from the magnetite framboids. Researchers believe that this mysterious object must have been quite small or must have been moving incredibly fast.
“As impact velocity increases, the expected projectile size decreases,” they said in the same study.
Pseudomagnetite may sound like an impostor, but it will actually help upcoming investigations that seek to discover more about what the early Solar System was like. Its presence indicates the past presence of water on an asteroid, as well as atmospheric effects in space, such as micrometeorite bombardment, that affected the asteroid’s composition. How much magnetism is lost also affects the overall residual resistance of the asteroid. The remainder is important in determining the magnetism of the object and the intensity of the magnetic field around it as it forms. What we know about the early magnetic field of the Solar System has been reconstructed from remnant records, many of which come from magnetite.
Some magnetic properties of these particles may have been lost centuries ago, but much more may be gained in the future from what remains.
Nature Communications, 2024. DOI: 10.1038/s41467-024-47798-0
