Samples of the near-Earth asteroid (162173) Ryugu were brought to Earth by JAXA’s Hayabusa-2 spacecraft.
This image of the asteroid Ryugu was captured by the Optical Navigation Camera – Telescopic (ONC-T) on JAXA’s Hayabusa-2 spacecraft on June 26, 2018, from a distance of 13.7 miles (22 km). Image credit: JAXA / University of Tokyo / Kochi University / Rikkyo University / Nagoya University / Chiba Institute of Technology / Meiji University / Aizu University / AIST.
Ryugu, also known as 1999 JU3, is a near-Earth C-type asteroid. Its name refers to Ryūgū (Dragon Palace), a magical underwater palace in a Japanese folktale.
The asteroid was discovered in May 1999 by astronomers with the Lincoln Near-Earth Asteroid Research.
It measures approximately 900 m (0.56 miles) in diameter and orbits the Sun at a distance of 0.96-1.41 astronomical units (AU) once every 474 days.
On December 6, 2020, a total of 5.4 grams of material from Ryugu was returned to Earth by JAXA’s Hayabusa-2 spacecraft.
“There is enough evidence that Ryugu started in the outer Solar System,” said Dr. Esen Ercan Alp, a researcher at the Argonne National Laboratory.
“Asteroids found in the outer reaches of the Solar System would have different characteristics than those found closer to the Sun.”
Dr. Alp and colleagues analyzed seventeen Ryugu samples measuring 1-8 mm and found several pieces of evidence to support this hypothesis.
“For one, the grains that make up the asteroid are much finer than you would expect if it was formed at higher temperatures,” Dr. Alp said.
“For another, the structure of the fragments is porous, which means it once held water and ice.”
“Lower temperatures and ice are much more common in the outer Solar System.”
Optical micrograph of the largest sample of the asteroid Ryugu analyzed by Nakamura et al. Image credit: Tohoku University.
The researchers were able to take several measurements on each of the Ryugu fragments. They found the same porous, fine-grained structure across the samples.
They also were able to measure the amount of oxidation that the samples had undergone. This was especially interesting since the fragments themselves had never been exposed to oxygen — they were delivered in vacuum-sealed containers, in pristine condition from their trip across space.
While the team did find a chemical makeup similar to meteorites that have hit the Earth — specifically a group of them called CI chondrites, of which only nine are known to exist on the planet — they did discover something that set the Ryugu fragments apart.
The spectroscopy measurements found a large amount of pyrrhotite, an iron sulfide that is nowhere to be found in the dozen meteorite samples the authors also studied.
In one of the grains, they found a fine vein of magnetite (iron oxide mineral) and hydroxyapatite (phosphate mineral).
In areas of the samples containing hydroxyapatite, they detected rare earth metals — a group of chemical elements indispensable today for alloys and glassware for high-tech applications, among others.
“The rare earths occur in the hydroxyapatite of the asteroid in concentrations 100 times higher than elsewhere in the Solar System,” said Dr. Frank Brenker, a researcher in the Institute of Geoscience at Goethe University.
“What’s more, all the elements of the rare earth metals have accumulated in the phosphate mineral to the same degree — which is also unusual.”
“The results show that these asteroid samples are different from meteorites, particularly because meteorites have been through fiery atmosphere entry, weatherization and in particular oxidation on Earth,” said Dr. Michael Hu, a researcher at the Argonne National Laboratory.
“This is exciting because it’s a completely different kind of sample, from way out in the Solar System.”
With all of the data combined, the study lays out the multi-billion-year history of Ryugu.
It was once part of a much larger asteroid which formed about 2 million years after the Solar System did — roughly 4.5 billion years ago.
It was made of many different materials, including water and carbon dioxide ice, and over the next three million years, the ice melted. This led to an interior that was hydrated and surface that was dryer.
About a billion years ago, another chunk of space rock collided with this asteroid, breaking it apart and sending debris flying, and some of those fragments coalesced into the Ryugu asteroid we know today.
“For planetary scientists, this is first-degree information coming directly from the Solar System, and hence it is invaluable,” Dr. Alp said.
The study was published in the journal Science.
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T. Nakamura et al. Formation and evolution of carbonaceous asteroid Ryugu: direct evidence from returned samples. Science, published online September 22, 2022; doi: 10.1126/science.abn8671
