Lauren Fuge
In December 2020, Japan’s Hayabusa2 spacecraft dropped its treasure through the Earth’s atmosphere and onto the red dirt carpet of the South Australian desert. This package held the result of Hayabusa2’s 6-year, 5.24-billion-kilometre journey: five grams of dust and rock from the primordial asteroid Ryugu.
This was only the second time that an asteroid sample had been returned to Earth.
Now, the results are in of the first preliminary analysis of this other-worldly material, and they show that we could soon discover whether asteroids like Ryugu brought water and organic materials to the ancient Earth.
How did we get a piece of asteroid back to Earth?
Launched in 2014, Hayabusa2 rendezvoused with Ryugu in 2018 and spent 18 months orbiting the asteroid, before taking samples and heading for home.
Excitingly, Hayabusa2 sampled two different parts of Ryugu: from on the surface and below the surface, picking up material that had never been exposed to the Vacuum of space.
All up, the space probe collected just over five grams of dust, equivalent to about a teaspoonful.
This doesn’t sound like much, but it’s a large sample, especially when scientists estimated from images that Hayabusa2 had only grabbed a single gram. Even then, they were stoked about it.
“One gram may sound small for some of you, but for experts, one gram is huge – it’s enough to address the science questions,” explained Masaki Fujimoto from JAXA, who led the team retrieving the return capsule.
In fact, the mission had only aimed to get a tenth of the gram. This far surpassed the only other asteroid sample ever returned to Earth, by the Hayabusa1 mission in 2010. It brought back just 1500 tiny dust grains, weighing less than a milligram in total.
More than five grams means that the sample contained several thousand individual grains, all of which will be exhaustively analysed. But since the grains are irreplaceable, the analysis is starting off cautiously, with non-invasive and non-destructive observations.
So, what do the results say?
The two new studies – published in Nature Astronomy – focused on the physical properties and composition of Ryugu.
Together, they confirm that it’s a C-type asteroid – a dark and rocky world, rich in carbon and water. These types of asteroids are ancient, left over from the birth of our Solar System.
Ryugu formed about 4.5 billion years ago and has retained its primitive composition – but C-type asteroids may not have remained completely unchanged during this time.
Funnily enough, evidence for this comes from Earth. While the sample from Hayabusa2 is the first sample ever returned to Earth, asteroid samples rain down on us every day as meteorites. Scientists think that a type of meteorites known as carbonaceous chondrites may have come from C-type asteroids.
These meteorites look like they have been altered by fluids, which could fit with what we know about C-type asteroids – formed in the far reaches of the asteroid belt, they contain ice that could have melted and helped produced clay minerals and carbonates (salts).
“One of the aims of the Hayabusa 2 mission was to investigate the link between C-type asteroids and carbonaceous chondrites,” explains planetary scientist Monica Grady in an article in The Conversation.
“Were C-type asteroids really the parent bodies from which carbonaceous chondrites originated? This is important because carbonaceous chondrites are probably the sort of objects that brought water and organic compounds to Earth, enabling life to emerge here.”
So what are the initial results from the mission?
The first paper found that the sample was darker in colour than expected, reflecting just 2% of solar radiation – less reflective even than asphalt.
The material also had a low density and a high porosity, which is surprising. Hayabusa2 had measured the asteroid itself to have a low density – an expected result, since an asteroid is basically a collection of rubble, with lots of spaces between the rocky components.
But the team thought the density of the sample material would be higher, because the collection and return process should have shaken the material up and collapsed the gaps between grains.
The sample’s density is also much lower than that of carbonaceous chondrites – perhaps because the meteorites that end up on Earth have to be hardy enough to survive a fiery plunge through the atmosphere, and so more fragile chunks don’t make it through.
“Ryugu may also contain more low-density material, such as organic molecules, than such meteorites,” Grady adds.
“This is extremely important, because it implies that the material from Ryugu has preserved a component of carbonaceous material that we have not been able to study before. This should allow us to learn more about the primordial building blocks of life.”
The second group specifically looked at the sample’s composition, and found that it was rich in not only carbon but also hydrated minerals and clays.
But it was a little different to other carbonaceous chondrites that scientists have studied on Earth. The sample had a fine, uniform texture, and didn’t contain any chondrules – molten spherical droplets usually found in carbonaceous chondrites.
This may suggest that Ryugu is the parent body of a type of meteorite called a CI chondrite – which are so rare that only five have ever been found on Earth.
CI chondrites have a chemical composition very similar to the Sun, and give us a snapshot of what the Solar System was like when it first formed.
According to Grady, together these papers “have shown us that the material from Ryugu is primitive and sufficiently different from known meteorites to make us think again about how representative meteorites are of asteroids.
“This might come to change some aspects of our view of early Solar System history.”
But these two studies are just the beginning.
Astronomers around the world are keen to learn more about these precious samples – and to compare them to a sample of the C-type asteroid Bennu, which will arrive back on Earth in 2023.