What are asteroids made of?Samples returned to Earth reveal what the solar system is made of


Just 12 months ago, we were sitting in Woomera, in the Australian outback, waiting for a light in the sky to confirm that the Hayabusa2 spacecraft had returned from its voyage to collect a small piece of a near-Earth asteroid called Ryugu.Unfortunately for us, it was cloudy that day and we didn’t see the ship come in.But that’s the only imperfection we see in the comeback.We found and retrieved Hayabusa2 and took it back to Woomera for cleaning and inspection.The sample capsule is removed from the spacecraft.Good condition, re-entry did not exceed 60℃, when turned over the capsule rattles, indicating that we do have solid samples.Its vacuum has been maintained, allowing the collection of any gases released from the asteroid samples, which were initially analyzed at Woomera.Over the course of a year, we learned more about that sample.In the past month, three papers have been published on the first analysis of The Ryugu sample, including an article in This week’s Science on the relationship between material seen on an asteroid and samples returned to Earth.The observations open a window into the formation of the solar system and help solve the mystery of meteorites that has baffled scientists for decades.Fragile fragments In summary, the sample weighs about 5 grams and is distributed between the two landing sites sampled.The scientist prepared the sample for analysis.The first sample is from the exposed surface of Ryugu.To get a second sample, the spacecraft fired a small disk at the asteroid to create a small crater, then collected a sample near the crater in the hope that the second sample would contain material from below the surface, protected from space weathering.Landing sampling was recorded by a camera on Hayabusa2.A detailed analysis of the video revealed that the shape of the particles ejected from Ryugu during the landing was very similar to the particles recovered from the sample capsule.This suggests that these two samples do represent the surface — the second sample may also contain some subsurface material, but we don’t know yet.Back in the lab, we can see that these samples are very fragile and very low density, indicating that they are very porous.They had the constitution of clay, and they behaved like clay.Ryugu samples are also very dark in color.In fact, they’re darker than any recovered meteorite samples.Ryugu’s field observations also showed this.But now that we have a rock, we can examine it and learn about it in detail.The Mystery of meteorites The solar system is full of asteroids: chunks of rock that are much smaller than planets.By looking at asteroids through telescopes and analyzing the spectrum of light they reflect, we can classify most of them into three types: C-type (lots of carbon), M-type (lots of metal) and S-type (lots of silica).When an asteroid’s orbit puts it on a collision course with Earth, depending on its size, we might think of it as a meteor (a shooting star) streaking across the sky as it burns up in the atmosphere.If some of these asteroids survive to reach the ground, we might find a piece of rock left over later: these are called meteorites.Most of the asteroids we see orbiting the sun are dark C-shaped.Based on their spectra, Type C is very similar in composition to a type of meteorite called carbonaceous chondrites.These meteorites are rich in organic and volatile compounds, such as amino acids, and may be a source of seed proteins that gave rise to life on Earth.However, while about 75% of asteroids are C-type, only 5% of meteorites are carbonaceous chondrites.Until now, this has been a conundrum: If Type C is so common, why don’t we treat their remains as meteorites from Earth?Observations and samples from Ryugu have solved the mystery.The Ryugu sample (and possibly meteorites from other C-type asteroids) are too fragile to survive its entry into Earth’s atmosphere.If they arrive at more than 15 kilometers per second, which is typical for meteors, they will break up and burn up long before they reach the ground.Most carbonaceous chondrites (the Allende meteorite shown here) contain characteristically round particles called chondrules.So what are the chances that the first C-type asteroid we visit is so similar to one of the rarest meteorites?The rarity of these CI meteorites on Earth is likely related to their vulnerability.They would hardly survive their journey through the atmosphere, and if they did reach the surface, the first rainstorm would turn them into a puddle of mud.Asteroid missions like Hayabusa2, its predecessor Hayabusa, and NASA’s OsiRIS-Rex are gradually filling in some of the gaps in our knowledge of asteroids.By bringing samples back to Earth, they allow us to look back at the history of these objects and look back to the formation of the solar system itself.Dawn of the Solar System but the Ryugu sample is more interesting than that.The material is similar to a rare type of carbonaceous chondrite called CI, where the C is carbon and the I refers to the Ivuna meteorite discovered in Tanzania in 1938.These meteorites are part of the chondrite family, but they have very few defining particles, called chondrules, which are mainly round particles of olivine that apparently crystallized from molten droplets.CI meteorites are dark, uniform and fine.These meteorites are unique in that they are made of the same elements as the sun in the same proportions (except for elements that are usually gases).We think this is because CI chondrites formed in clouds of dust and gas that eventually collapsed to form the Sun and the rest of the solar system.But unlike rocks on Earth, where 4.5 billion years of geological processing have changed the proportions of elements we see in the crust, CI chondrites are very much original samples of what the planets in our solar system are made of.So far, no more than 10 CI chondrites have been found on Earth, with a known total weight of less than 20 kg.In our collection, these objects are rarer than Mars samples.

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