From the size and number of the ice crystals detected, the researchers can also estimate how quickly the asteroid loses sulphur. "The process is incredibly fast from a cosmic perspective," explains Toru Matsumoto. The crystals he analysed are up to two-and-a-half micrometres long, which is around one-fiftieth of the thickness of a human hair. "The tiny whiskers have already reached these sizes after around 1,000 years," adds the researcher from Kyushu University in Fukuoka. Over the long term, the analysis of the ice crystals can be used to gain a better understanding of weathering processes on other celestial bodies as well, and to determine their age.
To this end, the researchers already have specific asteroids in their sights. NASA’s OSIRIS-REx probe is currently preparing to take samples from asteroid Bennu, while JAXA’s Hayabusa2 is already on its way back to Earth. The Japanese probe visited the Ryugu asteroid last year and, as with Itokawa, it collected dust particles. The samples should land on Earth at the end of 2020 and the international team of Jena mineralogists and Toru Matsumoto are awaiting them with anticipation
Matsumoto T et al. Iron whiskers on asteroid Itokawa indicate sulfide destruction by space weathering. Nature Communications (2020), DOI: 10.1038/s41467-020-14758-3, https://www.nature.com/articles/s41467-020-14758-3
Quelle: Friedrich Schiller University Jena
The Hayabusa2 spacecraft measured the maximum temperatures during one full rotation of the asteroid Ryugu and found that most of the asteroid stays cool. Blue regions are around 300 kelvins (27° Celsius) and yellow are around 330 kelvins (57° C). Just a few hot spots, in red, were dense boulders that reached 360 kelvins (87° C).
The asteroid Ryugu is light and fluffy. Images taken by Japan’s Hayabusa2 spacecraft suggest the whole asteroid is highly porous, scientists report in Nature on March 16.
“It is something like freeze-dry coffee,” says planetary scientist Tatsuaki Okada of the Japanese Aerospace Exploration Agency. If early protoplanets had similar structures, that could mean planets formed quickly.
As an ancient, carbon-rich asteroid, Ryugu is thought to be a time capsule of solar system history. To read that history, Hayabusa2 explored Ryugu from June 2018 to November 2019, and grabbed two samples from the asteroid to bring back to Earth (SN: 7/11/19).
Hayabusa2 observed how the asteroid’s surface retained and released heat, a clue to its composition and structure. Dense rocks take in heat slowly and hold that heat for longer; more porous rocks change temperature quickly, like sand on a beach.
Ryugu’s heat map shows that it’s about 50 percent porous, meaning half of it is holes, Okada and colleagues report. Even most of the asteroid’s large boulders appear porous.
That airiness fits with the idea that Ryugu is a rubble pile formed after the breakup of a larger body some 700 million years ago (SN: 3/20/19). But the new observations suggest that parent body might have been porous, too.
“This might be common for the asteroids and even for planetesimals in the early solar system,” Okada says.
If true, it could have big implications for how quickly planets formed. Planetary scientists think the early solar system was a violent place, with protoplanets colliding, breaking up and re-accumulating all the time. Porous protoplanets might fall apart and come together again more easily than dense ones, Okada says. That means “the timescale of planetary formation may be changed drastically,” and planets might have formed faster than scientists thought, he says.
Rocks on Asteroid Ryugu Unexpectedly Porous: JAXA-Led Team
Thermal imaging data have shown that rocks on the surface of asteroid Ryugu are unexpectedly porous, a team of the Japan Aerospace Exploration Agency, or JAXA, and others said Tuesday.
The thermographic images indicate that "the asteroid's boulders and their surroundings have similar temperatures," the research team led by Tatsuaki Okada, associate professor at JAXA, said in an article published in the online version of science journal Nature.
"Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites," the team said.
The thermographic images were taken by the Hayabusa2 unmanned spacecraft using a mid-infrared camera. Rocks on the asteroid "look like instant coffee powder that will easily fracture if you pick it up," Okada said.
The images also suggest that there are some dense boulders with colder temperatures, according to the research team.
Bombing asteroid Ryugu reveals it is a spritely 9 million years old
Bombing an asteroid has taught us its age. In April, the Japanese Hayabusa 2 spacecraft launched a copper projectile at the asteroid Ryugu, and now we have the first data on the crater it left behind.
The main purpose of the experiment was to collect a pristine sample of the material underneath the asteroid’s surface. Masahiko Arakawa at Kobe University in Japan and his colleagues analysed images of the collision to study the crater formation process.
When the 2-kilogram projectile smashed into Ryugu, it blasted away the sand and rocks where it hit, and even moved a 5-metre wide boulder. The resulting crater was about 14.5 metres wide and 2.3 metres deep.
“This is the first time that we have observed the crater formation process in a microgravity environment,” says Arakawa. Most of our understanding of how craters are formed is from laboratory experiments here on Earth, he says.
That is important because we use the number and size of craters on asteroids to estimate how old they are. Previous studies of Ryugu determined that it was either about 9 million years old or about 160 million years old, depending on how much the size of craters there is limited by the strength of the asteroid’s surface.
From observing the formation of this new crater, the researchers found that underneath its blanket of boulders Ryugu’s surface isn’t very sturdy. It is more like sand than rock. They say this confirms that Ryugu is only about 9 million years old.
Asteroids like Ryugu form when larger objects get smashed up and the remnants reaccumulate into “rubble piles”. There is usually lots of debris left behind from such a smash-up, creating what astronomers call a family of objects that all come from the same parent body.
“We may recognise the family in the asteroid belt in the future,” says Arakawa. That may be easier than we expected, because the parent body broke up relatively recently, suggesting the shattered pieces should be closer together.