In June 2018, the Japanese mission Hayabusa 2 reached the asteroid 162173 Ryugu. It studied the asteroid for about 15 months, using small rovers and a lander, before collecting a sample and returning it to Earth in December 2020.
The Ryugu sample contains some of the oldest, most primitive and unaltered materials in the solar system, opening a window into its earliest days about 4.6 billion years ago.
The Ryugu sample is small, weighing only about 5.4 grams (0.19 ounces). However, scientific instruments that study the chemical properties of the sample do not require a large sample.
In new research, scientists examined tiny fragments of Ryugu using Argonne National Laboratory's Advanced Photon Source (APS). The APS is a particle accelerator that accelerates photons to nearly the speed of light. These photons release X-rays, which are used for a variety of scientific purposes. (The APS has even been involved in the development of COVID-19 vaccines.) In this research, the APS X-rays were used in a special technique called Mössbauer spectroscopy, which can determine the oxidation rate of iron in the Ryugu sample.
The research is titled “Formation and evolution of the carbonaceous asteroid Ryugu: Direct evidence from returned samples.” It was published in the journal Science and the lead author is Tetsuya Nakamura of Tohoku University in Sendai, Japan.
Ryugu is a rare type of asteroid. As a Cb spectral type, it exhibits characteristics of both C-type carbonaceous asteroids, by far the most common type, and B-type asteroids, a rarer type of carbonaceous asteroid.
5.4 grams is not a large sample, but it is large enough to reveal the nature and history of the asteroid Ryugu. Image source: Yada et al./Nature Astronomy 2021
JAXA, the Japan Aerospace Exploration Agency, chose Ryugu for its sampling mission for several reasons. As a near-Earth asteroid (NEA), Ryugu was easier to reach. Because it was also classified as a primitive, carbon-rich asteroid, they hoped it would contain organic chemicals that would provide clues to the early solar system. Ryugu is also relatively small (900 meters) and rotates slowly, making sampling easier. The asteroid's orbit also brings it close to Earth, making it easier to return the sample.
Ryugu was able to answer certain questions, all related to the history of the solar system. Ryugu's structure and composition, including the presence of water and organic matter, can shed light on how planets and asteroids formed and how these materials vital to life may have arrived on Earth. Scientists also hoped to classify Ryugu in more detail and understand its internal structure and possible evolution. Researchers also wondered about the asteroid's resource potential.
Scientists working with the samples have already learned a lot. They found that the asteroid is rich in organic matter, supporting the idea that asteroids may have brought these materials to Earth. Ryugu contains water-bearing minerals, which is evidence that there was more water or water ice in the past. Scientists have also discovered the effects of space weathering on the asteroid's surface and the solar wind particles trapped in its grains.
Artist's impression of Hayabusa2 collecting samples from the surface of the asteroid Ryugu. Photo credit: Akihiro Ikeshita/JAXA
This new research expanded the wealth of insights provided by the tiny 5.4 gram sample. The researchers analyzed 17 Ryugu particles ranging in size from 1 to ~8 mm. They were particularly interested in gaining a more detailed understanding of the asteroid's history. They wanted to find answers to several specific questions:
- When and where did Ryugu's mother body originate?
- What is the original mineralogy, overall elemental abundance, and chemical composition of the accumulated materials, including their ice content?
- How did these materials evolve through chemical reactions?
- How was Ryugu expelled from his parent?
The APS and its Mossbauer spectroscopy revealed more details about Ryugu, and researchers used impact simulators and other tools to piece together the history of the asteroid and its parent.
The researchers found water inclusions containing carbon dioxide in a certain type of crystal. This is evidence that Ryugu's parent body formed in the outer solar system, where cold temperatures allowed the storage of water ice. APS also identified a high concentration of pyrrhotite in the sample. Pyrrhotite is an iron sulfide that is not found anywhere in meteorite fragments similar to Ryugu. This helps limit the location and temperature of the mother body during its formation. The research team says the parent body formed about 1.8 to 2.9 million years after the solar system began to form.
The outer solar system is dominated by materials that form at low temperatures, and Ryugu's parent body was mostly ice. The parent body formed beyond the H2O and CO2 snow boundaries and possibly beyond Jupiter.
The samples are porous and fine-grained, suggesting that the source material contained melted ice over a long period of time. The researchers say that radioactive heating inside the parent body caused the water ice to melt about three million years ago. Over time, reactions between water and rock slowly changed the asteroid's initial anhydrous mineralogy to a largely water-containing mineralogy.
The material was initially less altered at shallow depths and more watery at deeper depths. After about five million years, all material in the parent body reached its maximum temperature and water transformation continued.
The catastrophic head-on collision that blew up Ryugu's parent occurred about a billion years ago. The diameter of the parent body was about 50 km and the impactor was about 6 km. Ryugu is not a single part of its predecessor. Instead, it is a debris pile asteroid, a collection of debris that was dislodged from its parent body by the impact. Ryugu's material formed at different depths on the opposite side of its host rock from the impact and then coagulated to form Ryugu.
This research helps draw a timeline of Ryugu's parents and Ryugu himself on his long journey across the solar system.
Ryugu itself began its journey as part of a larger body only about two million years after the birth of the solar system. After billions of years as part of its parent body, it was formed after a collision. After a long time, it entered its near-Earth orbit, and in the last blink of an eye, humanity rose up and built a technological civilization. We turned to this messenger from the past and tried it out, and it taught us a lot about the history of our solar system.
Hayabusa 2 is now on an extended mission to visit two more asteroids. In 2026, it will conduct a high-speed flyby of the S-type asteroid 98943 Torifune. In 2031, it will rendezvous with 1998 KY26, a small 30m asteroid that is a fast rotator.
Hayabusa 2 will not sample any of these asteroids, but its observations will add to its already impressive contribution.
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