Enlarge / The surface of Ryugu. Image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, Aizu University, AIST (credit: JAXA)

An asteroid that has been wandering through space for billions of years is going to have been bombarded by everything from rocks to radiation. Billions of years traveling through interplanetary space increase the odds of colliding with something in the vast emptiness, and at least one of those impacts had enough force to leave the asteroid Ryugu forever changed.

When the Japanese Space Agency’s Hayabusa2 spacecraft touched down on Ryugu, it collected samples from the surface that revealed that particles of magnetite (which is usually magnetic) in the asteroid’s regolith are devoid of magnetism. A team 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-velocity micrometeoroid collision that broke the magnetite’s chemical structure down so that it was no longer magnetic.

“We surmised that pseudo-magnetite was created [as] the result of space weathering by micrometeoroid impact,” the 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 space weathering—alteration 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 were not exposed to the brutal environment of interplanetary space. They can also be altered as they plummet through the atmosphere or by 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 made of mostly clay and silicate rocks. These minerals normally need water to form, but their presence is explained by Ryugu’s history. It is thought that the asteroid itself was born from debris after its parent body was smashed to 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 (iron-containing 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 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 remanent magnetization in two magnetite fragments (known as framboids because of their particular shape) from the Ryugu sample. It is proof of a magnetic field in the nebula our Solar System formed in, and shows the strength of that magnetic field at the time that the magnetite formed.

However, three other magnetite fragments analyzed were not magnetized at all. This is where space weathering comes in.

…and what was lost

Using electron holography, which is done with a transmission electron microscope that sends high-energy electron waves through a specimen, the researchers found that the three framboids in question did not have magnetic chemical structures. This made them drastically different from magnetite.

Further analysis with scanning transmission electron microscopy showed that the magnetite particles were mostly made of iron oxides, but there was less oxygen in those particles that had lost their magnetism, indicating that the material had experienced a chemical reduction, where electrons were donated to the system. 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 refers to it as “pseudo-magnetite.”

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 specimen that the demagnetized framboids had come from. If a micrometeorite of a certain size had hit that region of Ryugu then it would have produced approximately that many particles of iron from the magnetite framboids. The researchers think this mystery object was rather small, or it would have had to have been moving incredibly fast.

"With increasing impact velocity, the estimated projectile size decreases," they said in the same study.

Pseudo-magnetite might sound like an imposter, but it will actually help upcoming investigations that seek to find out 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 was lost also affects the overall remanence of the asteroid. Remanence is important in determining an object’s magnetism and the intensity of the magnetic field around it when it formed. What we know of the Solar System’s early magnetic field has been reconstructed from remanence records, many of which come from magnetite.

Some magnetic properties of those particles might have been lost eons ago, but so much more could be gained in the future from what remains.

Nature Communications, 2024.  DOI: 10.1038/s41467-024-47798-0

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I don’t know how to have a war against China.

Yes, I know about China’s militarization of the South China Sea and the sword-rattling against Taiwan. We can have naval battles over enhanced sandbars. And attacking a heavily militarized, mountainous island is not trivial.

But let’s say China attacks Taiwan. Whether they take it or not, then what?

I haven’t seen Clausewitz quoted for some time. War is the continuation of politics by other means. So which politics justify a war? China’s nuclear buildup? That’s been proposed before: Nuke ‘em before they get as many as we have. But they have about 500 nuclear weapons and we have 1500 deployed and more in reserve. Economic reasons? Are you kidding?

And what would victory look like? What is the objective? An American march into Beijing to demand surrender from Xi Jinping?

Taiwan is pretty much the only reason for a war, and that’s up to China. So far, they seem to recognize how difficult that would be.

But we have folks in the US practically salivating for a war. Or, lacking that, heavy duty economic conflict. Crush them economically! Or all the books proclaiming that we have a NEW COLD WAR with China.

There are indeed problems with China. They would like to export their system of authoritarian capitalism to the rest of the world, or at least gain positive appreciation for it. They control supplies of things like strategic minerals. They have been ramping up their military, including those bases in the South China Sea and their nuclear arsenal. Paul Krugman and Anne Stevenson-Yang describe poor internal economic decisions that result in dumping products on the international markets.

THE NEW COLD WAR is a particularly poor way to describe the situation between the United States and China. China has some aspects of the Soviet Union – a controlled economy and human rights violations in Xinjiang and Tibet – but it is integrated economically into many parts of the world, including the United States and Europe.

The Cold War was marked by proxy wars – in Angola, Vietnam, and Afghanistan for example – whereas the United States and China have avoided such entanglements and likely will continue to do so, unless China strikes Taiwan. China’s support of Russia in Ukraine and of North Korea’s nuclear buildup have a level of difficulty for China that the Soviet Union’s clients seldom offered.

It’s prudent to prepare for a war, in case China tries to take Taiwan, but the United States military, famously as large as the next eight or ten in line, combined, should be prepared without extra gobs of money. US and other ships have regularly made passage through the waters that China might want to contest.

The nuclear buildup is not surprising. The US should be (and, I think, is) attempting to talk to China about arms control, although that topic is particularly difficult with Russia’s warlike stance.

On the economic side, Krugman attributes Biden’s tariffs against Chinese products to a stand against China’s exporting their economic difficulties to the world. It would seem that economic issues can be managed without resort to war.

Michael Hirsh, in a long article in Foreign Policy, calls the situation “a cold peace,” which seems more appropriate. China has its own problems – in the economy and in its attempts at outreach to other countries through economic development. Its language isolates it; even Russia is not moving to require Chinese in its schools. Its ideology and thoroughgoing surveillance have had a mixed reception in other countries.

The leap to expecting a war over Taiwan and casting what President Joe Biden calls a competition into a cold war militarizes foreign policy thinking in a way that is more likely to lead to a war. Tariffs, yes, and diplomacy that works toward reining in other sorts of competition, like avoiding an arms race.

Cross-posted to Nuclear Diner

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