A year ago, scientists got their first look at material collected from nearby asteroid 162173 Ryugu. Now the results of these studies have been revealed, shedding light on the history of our solar system and the long journey of this cosmic wanderer.
At its closest orbit, asteroid 162173 Ryugu is only about 60,000 miles from Earth. That’s only a quarter of the distance to the moon. But according to newly published results from an international team of scientists, this boulder began its cosmic journey more than 4 billion years ago and billions of kilometers away in the outer part of our solar system. It traveled to us through space, absorbing the history of this corner of the universe.
These revelations are just part of the results of a worldwide effort to examine samples from the surface of Ryugu. These asteroid dust grains were carefully collected and transported back to Earth by Hayabusa 2, a mission operated by Japan’s space agency JAXA, and then sent to institutions around the world. Scientists have subjected these tiny fragments to dozens of experiments to unravel their mysteries, to determine what they are made of and how the asteroid they came from might have formed.
“For planetary scientists, this is first-degree information, straight from the solar system, and therefore invaluable.” — Esen Ercan Alp, Distinguished Fellow of the Argonne
The resulting paper, recently published in Science, includes authors from more than 100 institutions in 11 countries. They include the US Department of Energy’s (DOE) Argonne National Laboratory, which houses the Advanced Photon Source (APS), a user facility of the DOE Office of Science. The APS produces ultra-bright X-rays that can be used to determine the chemical and structural composition of samples atom by atom.
Argonne Distinguished Fellow Esen Ercan Alp led the research team at Argonne, which includes physicist and group leader Jiyong Zhao and physicist Michael Hu, and beamline scientist Barbara Lavina from Argonne and the University of Chicago. All are co-authors of the paper.
Alp and his team worked for years to get included in this study. The main contribution of the APS, Alp said, is a specific X-ray technique that he and his team specialize in. It’s called Mössbauer spectroscopy – named after the German physicist Rudolf Mössbauer – and is highly sensitive to minute changes in the chemistry of samples. Using this technique, Alp and his team were able to determine the chemical composition of these fragments particle by particle.
What she and her international colleagues found was surprising, Alp said.
“There is ample evidence that Ryugu began in the outer solar system,” he said. “Asteroids found in the outer reaches of the solar system would have different properties than those found closer to the sun.”
The APS, Alp said, found several pieces of evidence supporting this hypothesis. For one thing, the grains that make up the asteroid are much finer than one would expect if they had formed at higher temperatures. On the other hand, the structure of the fragments is porous, which means that they once contained water and ice. Lower temperatures and ice are much more common in the outer solar system, Alp said.
The Ryugu fragments are very small – ranging from 400 microns, or the size of six human hairs, to 1 millimeter in diameter. However, the X-ray beam used on Beamline 3-ID-B can be focused down to 15 microns. The team was able to take multiple measurements on each of the fragments. They found the same porous, fine-grained structure in all samples.
Using the APS’s finely tuned spectroscopy capabilities, the team was able to measure the extent of oxidation the samples had undergone. This was particularly interesting as the fragments themselves have never been exposed to oxygen – they were delivered in vacuum-sealed containers in pristine condition from their journey through space.
While the APS team found a chemical composition similar to meteorites that have struck Earth – specifically a group of them called CI chondrites, of which only nine are known on the planet – they discovered something that sets the Ryugu fragments apart from others .
Spectroscopic measurements revealed a large quantity of pyrrhotite, an iron sulfide not found anywhere in the dozen meteorite samples the team also examined, courtesy of French collaborators Mathieu Roskoz (National Museum of Natural History) and Pierre Beck (University of Grenoble Alpes) . . This result also helps scientists narrow down the temperature and location of Ryugu’s parent asteroid at the time of its formation.
“Our results and those of other teams show that these asteroid samples are distinct from meteorites, particularly because meteorites have undergone fiery atmosphere entry, weathering, and especially oxidation on Earth,” Hu said. “This is exciting because it’s a very different kind of sample, from far out in the solar system.”
Along with all the data, the paper lays out the multi-billion-year history of 162173 Ryugu. It was once part of a much larger asteroid that formed about 2 million years after the solar system — about 4.5 billion years ago. It was composed of many different materials, including water and carbon dioxide ice, and over the next three million years the ice melted. This resulted in a hydrated interior and a drier surface.
About a billion years ago, another chunk of space rock collided with this asteroid, breaking it up and sending debris flying, and some of those fragments merged into the Ryugu asteroid we know today.
“For planetary scientists, this is first-degree information coming directly from the solar system and is therefore invaluable,” Alp said.
The Argonne team is planning a paper of its own detailing its X-ray techniques and results. But being part of such a large, multinational scientific effort is exciting, they said, and they look forward to being part of future experiments of this nature.
“Participating in such a well-coordinated international research project was an exciting and challenging experience for us,” Zhao said. “With an APS upgrade in the works that will yield even brighter X-rays, we expect to study more materials like this from distant asteroids and planets.”
This project was funded in part by a France and Chicago Collaborating in the Sciences (FACCTS) grant administered by the University of Chicago.
About the Advanced Photon Source
The US Department of Energy Office of Science’s Advanced Photon Source (APS) at Argonne National Laboratory is one of the world’s most productive X-ray light source facilities. The APS makes high-brightness X-rays available to a diverse community of researchers in materials science, chemistry, condensed matter physics, life and environmental sciences, and applied research. These X-rays are ideal for studying materials and biological structures; elementary distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems, from batteries to fuel injectors, all of which form the basis of our nation’s economic, technological, and physical well-being. Each year, more than 5,000 researchers use the APS to produce over 2,000 publications detailing impactful discoveries and solve more vital biological protein structures than users of any other X-ray source research facility. APS scientists and engineers are inventing technologies that are at the heart of advancing accelerator and light source operations. These include the introducers that produce X-rays with extreme brightness valued by researchers, lenses that focus the X-rays down to a few nanometers, instruments that maximize the way the X-rays interact with the samples being examined, and software, which collects and manages the vast amount of data resulting from discovery research at the APS.
This research used resources from the Advanced Photon Source, a US DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract number DE-AC02-06CH11357.
Argonne National Laboratory seeks solutions to pressing national problems in science and technology. Argonne, the nation’s premier national laboratory, conducts pioneering basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of corporations, universities, and federal, state, and local governments to help solve their specific problems, advance America’s scientific leadership, and prepare the nation for a brighter future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the US Department of Energy’s Office of Science.
The Office of Science of the US Department of Energy is the largest single funder of basic science research in the United States, working to address some of the most pressing challenges of our time. For more information, see https://energy.gov/science.
Formation and Evolution of Carbonaceous Asteroid Ryugu: Direct Evidence from Returned Samples
Article publication date
September 22, 2022