Near-Earth asteroid Ryugu

First taste

With just 5.4 grams of asteroid Ryugu, space scientists make 2 huge discoveries

Thank you, Ryugu.

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This time last year, scientists got their hands on some very rare space rocks — specifically, 5.4 grams of material from the near-Earth asteroid 162173 Ryugu, which was returned to Earth via the Japanese space agency’s craft, Hayabusa 2, on December 6, 2020. And in a pair of papers published today in Nature Astronomy, those scientists detail the very first discoveries yielded up by these valuable grains.

What’s new — The discoveries alter our understanding of what Ryugu looks like — the diamond-shaped space rock is both darker and more porous than expected. This fragility is important in terms of assessing whether it or similar asteroids could one day threaten Earth.

The rocks also contain volatile compounds, suggesting Ryugu preserves material from the outer Solar System. This means scientists now have in their labs some of the most pristine stuff from the Solar System’s infancy — a time capsule from before the beginning of our world.

In the first paper, “Preliminary analysis of the Hayabusa2 samples returned from C-type asteroid Ryugu,” the researchers mostly confirmed some old predictions about Ryugu’s composition — but there were a few unexpected findings, too.

The Ryugu sample reflects just 2 percent of the light it receives. It is also exceptionally fragile — it is about 50 percent more porous than carbonaceous chondrite meteorites that have fallen to Earth.

“Such high-microporosity materials have not been discovered in any meteorites found on Earth probably due to break-up owing to their fragile nature during entry into the Earth’s atmosphere,” the researchers write.

The Hayabusa 2 spacecraft is the second-ever mission to return samples of an asteroid to Earth.All About Space Magazine/Future/Getty Images

The second paper titled “First compositional analysis of Ryugu samples by the MicrOmega hyperspectral microscope” details a chemical analysis of the Ryugu sample.

The rock contains a wide variety of volatile organic compounds, including non-water molecules made of oxygen and hydrogen atoms known as hydroxyls. These volatiles likely originated in the outer Solar System, the researchers say.

In other words, the Ryugu samples “appear to be among the most primordial materials available in our laboratories,” they write.

Material sampled from asteroid Ryugu. JAXA

How the discovery was made

The Hayabusa 2 craft launched on a nearly four-year mission to Ryugu in December 2014, arriving at the asteroid in June 2018.

Ryugu is a C-type, or “carbonaceous” asteroid, asteroids that contain large portions of carbon and are the most common asteroid type in the Solar System. It’s also a member of the Apollo group, near-Earth asteroids with orbits ranging from just outside Earth’s to inside our planet’s orbit, crossing Earth’s orbit during their trip around the Sun.

Hayabusa 2 collected surface and subsurface material from the asteroid in late 2019. It dropped the samples to Earth on December 3, 2020. Hayabusa 2 continued on for an extended mission to another near-Earth Apollo asteroid, 1998 KY26.

Once safely ensconced in the Extraterrestrial Samples Curation Center in Sagamihara, Japan, researchers began the first wave of nondestructive analysis of the Ryugu samples, using spectrographs and optical microscopes to probe the dust for clues to some of the biggest questions we have about our Solar System.

Why it matters — For scientists, asteroids like Ryugu represent unique archives from the early days of the Solar System, material kept in a deep freeze, and therefore unchanged, from the time before planets formed.

The Ryugu sample is the second of its kind ever returned to Earth. The first arrived in 2005 when the Japan Aerospace Exploration Agency mission Hayabusa managed to return a few micrograms of dust from the near-Earth asteroid 25143 Itokawa. Until NASA’s ORISIS-Rex returns with its samples of the near-Earth asteroid 101955 Bennu in 2023, these are the only samples we have.

“Because reflectance spectra and colors of surface rocks on Bennu look different from those of Ryugu, I expect that different kinds of samples will be returned from Bennu,” JAXA scientist and one of the paper’s co-authors, Toru Yada, tells Inverse.

Bennu may yield more secrets of asteroids.NASA/Goddard/University of Arizona

“The samples constitute a uniquely precious collection which are now available for refined laboratory analyses with the potential to draw new insights into the formation and evolution paths of planetary bodies in our Solar System,” the researchers on the second paper write.

More practically speaking, it’s comforting to learn that at least some near-Earth asteroids that cross our path around the Sun may be too spongy and fragile to survive entering Earth’s dense atmosphere and remain intact enough to cause tremendous damage on impact.

What’s next — These studies are the first of many laboratory analyses of the Ryugu samples to come, and scientists will bring many more techniques to bear on the materials, too.

The successful return of samples of the asteroid Bennu in 2023 is the next really big step, however. A member of the Apollo group like Ryugu, Bennu is a B-type asteroid, with a more blue spectrum and likely a very different composition than Ryugu — it also has a one in 1,800 chance of hitting Earth sometime in the 2100s.

“Comparison of Ryugu samples with Bennu is very essential,” Yada says.

PAPER 1 ABSTRACT: C-type asteroids1 are considered to be primitive small Solar System bodies enriched in water and organics, providing clues to the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing and on-asteroid measurements with Hayabusa2 (ref. 10). However, the ground truth provided by laboratory analysis of returned samples is invaluable to determine the fine properties of asteroids and other planetary bodies. We report preliminary results of analyses on returned samples from Ryugu of the particle size distribution, density and porosity, spectral properties and textural properties, and the results of a search for Ca–Al-rich inclusions (CAIs) and chondrules. The bulk sample mainly consists of rugged and smooth particles of millimetre to submillimetre size, confirming that the physical and chemical properties were not altered during the return from the asteroid. The power index of its size distribution is shallower than that of the surface boulder observed on Ryugu11, indicating differences in the returned Ryugu samples. The average of the estimated bulk densities of Ryugu sample particles is 1,282 ± 231 kg m−3, which is lower than that of meteorites, suggesting a high microporos- ity down to the millimetre scale, extending centimetre-scale estimates from thermal measurements. The extremely dark optical to near-infrared reflectance and spectral profile with weak absorptions at 2.7 and 3.4 μm imply a carbonaceous composition with indigenous aqueous alteration, matching the global average of Ryugu and confirming that the sample is representative of the asteroid. Together with the absence of submillimetre CAIs and chondrules, these features indi- cate that Ryugu is most similar to CI chondrites but has lower albedo, higher porosity and more fragile characteristics.

PAPER 2 ABSTRACT: The characterization of objects that have best preserved the mineralogical and molecular phases formed in the earliest stages of the Solar System evolution is key to understanding the processes that led to the formation of the planets in their diversity. The Hayabusa2 mission of the Japan Aerospace Exploration Agency has returned for the first time samples collected at the surface of a C-type asteroid, Ryugu. They are now preserved at the Extraterrestrial Samples Curation Center of the Japan Aerospace Exploration Agency at the Institute of Space and Astronautical Science in Sagamihara, Japan, where they are submitted to a first round of purely non-destructive analyses. The MicrOmega hyperspectral microscope devel- oped at the Institut d'Astrophysique Spatiale (Orsay, France), which operates in the near-infrared range (0.99–3.65 μm), is performing their mineralogical and molecular characteriza- tion down to the scale of a few tens of micrometres. Strong features at 2.7 μm (indicating their OH-rich content) and at 3.4 μm (diagnostic of the presence of organics) dominate at a global scale, but key distinctive signatures have been identi- fied at a submillimetre scale. In particular, carbonates (a frac- tion of them enriched in iron) as well as NH-rich compounds have been detected. The occurrence of volatile-rich species, likely originating from the outer Solar System, would support Ryugu having preserved both pristine material and altered phases, which are now available for refined laboratory analy- ses with the potential to draw new insights into the formation and evolution paths of planetary bodies in our Solar System.
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