An asteroid that has been drifting through space for billions of years will be bombarded by 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 asteroid Ryugu forever changed.
When Japan’s Ryugu space agency’s Hayabusa2 spacecraft landed, it collected surface samples that showed that particles of magnetite (which is mostly magnetic) in the asteroid’s regolith show no 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 it was caused by at least one high-speed micrometeoroid collision, which broke down the chemical structure of the magnetite so that it was no longer magnetic.
“We suspected that pseudo-magnetite had been created [as] the result of space weathering by micrometeoroid impacts,” said the researchers, led by Professor Yuki Kimura of Hokkaido University, in a study recently published in Nature Communications.
What remains…
Ryugu is a relatively small object with no atmosphere, making it more sensitive to space weathering: changes caused by 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 the majority of those meteorites are chunks of rock from the inside of an asteroid, so they aren’t exposed to the unforgiving environment of interplanetary space . They can also be changed as they descend through the atmosphere or by physical processes at the surface. The longer it takes to find a meteorite, the more information may be lost.
Ryugu was once part of a much larger body and is a C-type, or carbonaceous, asteroid, meaning it is composed mainly of clay and silicate rocks. These minerals normally require water to form, but their presence is explained by the history of Ryugu. The asteroid itself is thought to have formed from debris after its parent body was smashed into pieces in a collision. The parent body was also covered in water ice, which explains the magnetite, carbonates and silicates found on Ryugu; these need water to form.
Magnetite is a ferromagnetic (ferrous and magnetic) mineral. It is found in all C-type asteroids and can be used to determine their remanent or residual magnetization. An asteroid’s remanent magnetization can reveal how intense the magnetic field was at the time and place of the magnetite’s formation.
Kimura and his team were able to measure the remanent magnetization in two magnetite fragments (known as framboids because of their specific shape) from the Ryugu sample. It is evidence of a magnetic field in the nebula in which our solar system formed, and shows the strength of that magnetic field at the time the magnetite was formed.
However, three other magnetite fragments analyzed were 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 had no magnetic chemical structures. This made them drastically different from magnetite.
Further analysis using scanning transmission electron microscopy showed that the magnetite particles were largely composed of iron oxides, but there was less oxygen in the particles that had lost their magnetism, indicating that the material had undergone a chemical reduction, adding electrons to the system donated. . This loss of oxygen (and oxidized iron) explained the loss of magnetism, which depends on the organization of the electrons in the magnetite. This is why Kimura calls it ‘pseudo-magnetite’.
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 where the demagnetized framboids came from. If a micrometeorite of a certain size had struck that area of Ryugu, it would have produced about the same number of iron particles from the magnetite framboids. The researchers think this mysterious object was quite small, otherwise it would have been moving incredibly fast.
“With increasing impact speed, the estimated projectile size decreases,” they said in the same study.
Pseudo-magnetite may sound like a cheat, but it will actually help in upcoming studies that try to learn more about what the early solar system was like. Its presence indicates the former presence of water on an asteroid, as well as space weathering, such as micrometeoroid bombardment, that affected the asteroid’s composition. How much magnetism is lost also affects the asteroid’s overall remanence. 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 solar system’s early magnetic field has been reconstructed from remanence data, many of which come from magnetite.
Some of the magnetic properties of these particles may have been lost centuries ago, but so much more could be gained from what remains in the future.
Nature communication, 2024. DOI: 10.1038/s41467-024-47798-0