University of Tokyo: Piecing together history of solar system from asteroid rubble

In December 2020, Japan’s Hayabusa2 spacecraft succeeded in returning samples of rubble collected from near-Earth asteroid Ryugu, wowing people around the world. It marked the second time the Japanese researchers brought back, ahead of others, samples from an asteroid for scientific research. The accomplishment follows the first-ever sample return mission the previous Hayabusa spacecraft achieved in 2010.

Asteroids are small rocky bodies orbiting the sun. They are believed to retain numerous characteristics of the early solar system, which was born 4.56 billion years ago, because they did not undergo the dramatic transformation needed to grow into planets.

So scientists are hoping the samples Hayabusa2 collected from its two landings will provide significant clues to the origin of materials used to create ocean and life on Earth, as well as to the evolution of the solar system.

UTokyo Professors Seiji Sugita and Shogo Tachibana, both at the Graduate School of Science, are among the mission’s key scientists. They also had, from early on in their life, pure and profound curiosity about the formation of Earth and the solar system. UTokyo FOCUS recently caught up with them, as the yearlong initial analysis of the Ryugu samples started last month.

The life of rocks

Sugita, a native of Shizuoka Prefecture in central Japan, says that a story on “the life of a rock” he heard at a local science club when he was in the sixth grade sparked his interest in rocks and has kept him captivated ever since. The club was run by the area’s high school geology teacher, who organized field activities every weekend.

“The life of a rock” refers to a series of changes a rock undergoes over a long period of time. First, magma, or molten rock, erupts from a volcano and hardens into rock when cooled. The rock joins the river and breaks down into sand, piles up to become sandstone, enters the earth and transforms into metamorphic rock, then comes back up to the earth and breaks down into sand again.

The geology teacher would show Sugita a piece of sandstone found in nature and tell the future scientist, “The ground is formed through such a perpetual process; if you look closely at this rock, you can see tiny pieces of basalt. Just by looking at this rock, you can see (the process).”

“I thought, ‘Is that really possible?’” recalled Sugita. “I had thought that the ground we were standing on was just a flat surface, but I was fascinated by the fact that there was indeed such a story, there was evidence, and that it was for real. Looking back, I now realize that he was teaching us the basics of the global cycle of matter in simple terms.”

Sugita went on to study at UTokyo, where he earned a master’s degree for his thesis on numerical simulations of the evolution of lunar topology over time. He earned a Ph.D. at Brown University in the United States, for his research on the high-speed collisions of planetary bodies using the Vertical Gun Range at NASA’s Ames Research Center in the state of California. He joined the Hayabusa2 project in 2011 after returning to Japan and joining UTokyo’s faculty. He was involved in the development of the Optical Navigation Camera (ONC), a vital instrument that served as the “eyes” of the spacecraft.

“The basic design of the camera on Hayabusa2 is almost the same as the one on the first Hayabusa spacecraft,” Sugita said. “But the asteroid we were going to survey was very different, so we reselected the detailed specifications of which wavelengths to focus on.” Before Hayabusa2 arrived at Ryugu, researchers knew that it was a C-type (carbonaceous) asteroid, a type known for being rich in carbon, and that it was a blackish star with very low reflectance, or power to reflect the light it receives. “Between the spacecraft’s launch and before its arrival (at Ryugu), we made all kinds of observations to check the camera’s performance and thoroughly investigated its distribution of sensitivity. That was the most difficult part,” he recalled.

The ONC consists of three cameras: a telephoto camera, which divides light into seven color bands, plus two wide-angle cameras with different fields of view. Sugita and his team of researchers observed and recorded images of the moon, Mars and other stars by manipulating ONC remotely while Hayabusa2 was on its way to Ryugu. The researchers then compared the spectral data of the images with existing data from previous observations of these celestial bodies.

This process, called calibration, is necessary to guarantee the accuracy of the camera. The team had to start delivering best-quality observation data as soon as possible in order to choose the landing sites on Ryugu. The quality of observations hinges greatly on calibration, so members continued the work from just after the launch in December 2014 until the day before the arrival over Ryugu in June 2018, he said.

ONC played a crucial role in navigating Hayabusa2 to Ryugu, but it also contributed to scientific observations after the spacecraft reached the asteroid.

When the diamond-shaped and craggy Ryugu was first captured by camera at close range in June 2018, it surprised Sugita, as well as many researchers. But Sugita’s own excitement as a scientist arose from a more “maniacal” curiosity.

“It was the first time I saw a crater with a raised rim on an asteroid,” he recalled. “Normally the surface of an asteroid is flat, because when the asteroid is hit by a celestial body, the debris, or ejecta, escapes the asteroid gravity and does not accumulate nearby.

“The fact that the rims are raised is evidence that debris accumulated and remained there. If we can prove that these are craters with ejecta deposits around it, we can analyze it in the same way we analyze craters on the moon. We can use crater chronology, which shows there are many craters on the old surface and few craters on the new surface. I realized that this is a celestial body whose history we can track.”

He passionately explained that the characteristics of the crater, combined with findings from other studies, have allowed researchers to come up with a dynamic “story” on the history of Ryugu. The story goes like this: The asteroid is assumed to be a collection of fragments from a broken parent body that measures about 100 kilometers in diameter. The ice inside the original parent body is estimated to have melted into water and reacted with the surrounding rocks to turn into clay minerals. The clay minerals then turned black when they became “baked” or “hard-boiled” due to an event in which they were exposed to high-temperature heat. After that, the star was broken into small pieces by collisions with other celestial bodies, traveled close to Venus or Mercury and then came back, and experienced increases and decreases in rotation speed, resulting in its current color distribution and shape.

“The evidence is probably in the samples (collected by the Hayabusa2 mission),” Sugita said. “It would be a great discovery if we can find a piece confirming such a story.”

The initial analysis of the samples, to be carried out for the next year, is crucial for proving the story. Tachibana, who leads the initial analysis team comprising 269 researchers at 109 universities and research institutions in 14 countries, is responsible for overseeing the entire project.

Opening the “jewel box”

Tachibana was born and raised in Ishikawa Prefecture, situated along the Sea of Japan coast in central Japan. When he was in elementary school, he was drawing pictures of the planets in the solar system, and was amazed at how the colors varied from one planet to another. He wondered what exactly made the colors different, a question that ultimately led to his career as a scientist. After earning his Ph.D. at Osaka University’s Graduate School of Science, where he studied Earth and space science, he continued research at Arizona State University in the U.S., UTokyo and Hokkaido University before joining the faculty of UTokyo’s Graduate School of Science as professor in 2017.

He has been involved in Hayabusa2 since before the launch of the project, as the head of science for the development of the sampler equipment. He also traveled to Australia to collect Hayabusa2’s capsule after its return to Earth, where he was involved in opening the container filled with the samples.

Tachibana emphasizes that although the public attention on Hayabusa2 has tended to focus on the adventurous aspect of the spacecraft’s journey to and from Ryugu, the sample analysis, which is just beginning, is most important for solving the mystery of the formation of Earth and the planets of the solar system. The scientists have just opened the “jewel box,” he said.

“We still don’t really know what Ryugu will tell us,” Tachibana said. “But we do know that it seems to contain water and organic matter, and I think it will give us some information about the origins of the solar system, as well as help answer questions of where Earth’s oceans came from and how the materials for life were transported.”

Six teams

Researchers undertaking the initial analysis are divided into six teams, each of which will analyze a different research target using a variety of techniques.

The chemistry team will examine the chemical characteristics of the samples and use isotope microscopes to analyze isotopes (elements with the same number of protons but slightly different weights due to the difference in the number of neutrons) to identify differences and relationships with the types of meteorites that fall onto Earth.

The stone team will use synchrotron radiation high-energy beams and electron microscopes to observe the internal structure of materials that are about 1 millimeter in diameter or larger.

The sand team will examine particles finer than stones. The surface of Ryugu is thought to have been impacted by plasma from the sun and extremely small meteorites. The team’s researchers will analyze the effects of these and other factors.

The volatile team will analyze volatile materials such as hydrogen, nitrogen, oxygen and rare gases. The IOM (insoluble organic matter) team will try to clarify the structure and distribution of solid organic matter, which is black like coal and has a complex molecular structure.

Then the SOM (soluble organic matter) team will dissolve organic molecules in water or alcohol, and identify the structure, type and number of molecules in the water or alcohol to discover the characteristics of Ryugu.

Each team is international, working closely with members overseas. The teams will keep in touch online with members who are unable to come to Japan due to COVID-19 restrictions.

Sugita also plans to be involved in the sample analysis and has high expectations for the insights to be gained. Although Japan is overwhelmed by the United States in the scale of its space science research budget, Sugita said that Japan has achieved a scientific feat by becoming the first among countries engaged in sample return missions to bring back samples from a C-type asteroid. In that sense, Japan can act as a teacher to the U.S., he said.

He added that he would like students to experience the fun and thrill of working on world-class research.

“Japan is one of the world’s top runners in scientific research, not only in space science but science in general,” he said. “If you join a group like that and work hard, you can engage in high-level exchanges with some of the world’s top scientists. For international students, too, if they come to Japan and do their best, they can have that kind of experience, so I really hope they will do their best.”

Tachibana meanwhile points out the importance of studying subjects other than science for students interested in a career in science.

“If you really want to do research, it is important to study language and social sciences as well, and I don’t think anything you study in school is useless,” he said. “I think it’s especially important for researchers in the field of Earth and planetary science, which is all about trying to understand nature, to have comprehensive skills.

“Also, research is different from study, so you should enjoy it. I think how much fun you can have is an important factor in continuing research.”