An asteroid that has been wandering through space for billions of years is bombarded by everything from rocks to radiation. Billions of years traveling through interplanetary space increase the chances of colliding with something in the vast void, and at least one of those impacts had enough force to change the asteroid Ryugu forever.
When the Japanese space agency’s Hayabusa2 spacecraft landed on Ryugu, it collected samples from the surface that showed that magnetite particles (which are normally magnetic) in the asteroid’s regolith do not exhibit 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 that destroyed the magnetite’s chemical structure, making it no longer magnetic.
“We suspected that pseudomagnetite was formed [as] the result of space weathering caused by micrometeoroid impacts,” the researchers, led by Professor Yuki Kimura of Hokkaido University, said in a recent study published in Nature Communications.
What remains…
Ryugu is a relatively small object without an atmosphere, making it more susceptible 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 falling to Earth, and most of these meteorites are pieces of rock from the interior of an asteroid, so they have not been exposed to the brutal environment of interplanetary space. They can also change as they fall through the atmosphere or through physical processes on 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, or carbonaceous, asteroid, meaning it is composed mostly of clay and silicate rock. Normally these minerals require water to form, but their occurrence can be explained by the history of Ryugu. The asteroid itself is believed to have formed from debris after its parent body was shattered into pieces in a collision. The parent body was also covered with water ice, which explains the formation of magnetite, carbonates and silicates on Ryugu – these require 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 remaining magnetization. The remanent magnetization of an asteroid can provide information about how strong the magnetic field was at the time and location 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. 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 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 performed 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 mostly composed of iron oxides, but there was less oxygen in the particles that had lost their magnetism, suggesting that the material had undergone a chemical reduction in which electrons were donated to the system became . This loss of oxygen (and oxidized iron) explains the loss of magnetism, which depends on the organization of electrons in magnetite. For this reason, Kimura calls it a “pseudomagnetite.”
But what triggered 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, it would have produced approximately as many iron particles from the magnetite framboids. The researchers assume that this mysterious object was rather small, otherwise it would have had to move incredibly quickly.
“As impact velocity increases, the estimated projectile size decreases,” the same study states.
Pseudo-magnetite may sound like a fake, but it will actually help future studies that want to find out more about what the early solar system looked like. Its presence indicates the past presence of water on an asteroid, as well as space weathering, such as: B. micrometeorite bombardment, which influenced the composition of the asteroid. How much magnetism was lost also influences the asteroid’s overall remanence. Remanence is important for determining the magnetism of an object and the intensity of the magnetic field around it at the time of its creation. What we know about the Solar System’s early magnetic field has been reconstructed from remanence records, many of which come from magnetite.
Some magnetic properties of these particles may have been lost eons ago, but much more could be gained from what remains in the future.
Nature Communications, 2024. DOI: 10.1038/s41467-024-47798-0