Made of Star-stuff: Asteroid Analyses Shed Light on Solar System’s Origins

The OSIRIS-REx sample return capsule’s landing in September 2023 may have kicked off Asteroid Autumn, but scientists in the Astromaterials Research and Exploration Science Division (ARES) at NASA’s Johnson Space Center had already spent the spring and summer analyzing another rocky body. Their focus? Tiny fragments from asteroid Ryugu – a carbonaceous cousin of asteroid Bennu.  

The sample was collected by JAXA’s (Japan Aerospace Exploration Agency) Hayabusa2 spacecraft, which launched on its multi-year mission in December 2014. The spacecraft arrived at Ryugu in June 2018 and spent the next 17 months orbiting and observing the asteroid. During that time, the spacecraft fired an impactor into the asteroid, touched down twice inside the crater created by the impact, and collected a sample weighing 5.4 grams – about as much as five paperclips. When Hayabusa2 delivered its sample canister to southern Australia in December 2020, it marked only the second time that part of an asteroid was collected in space and brought back to Earth.

Ryugu was chosen as the target for the Hayabusa2 mission because it was a C-type asteroid, believed to be the source of the CI chondrites that have rich deposits of water and organic materials. “This is a very important asteroid class, because their composition is thought to reflect the primordial composition for the whole solar system, because they formed around the same time,” explained Dr. Michael Zolensky, ARES planetary scientist. He also noted that because the sample was contained within a protective capsule, it was not affected by Earth’s atmosphere, organic contamination, and liquid water. The CI class meteorites that scientists had previously studied were not pristine, having fallen to Earth unprotected. “That means we get a better peek at what the primordial dust was like.”

When a tiny portion of the Ryugu sample first arrived at ARES, the division’s Ryugu science team got a very quick, initial look at the fragments to try to identify the materials they contained. “Based on the spectroscopy of the asteroid, JAXA had a good idea of what the samples might be like, but it turned out to be partly wrong,” Zolensky said. “That was pretty cool. For scientists, it’s always fun to find something unexpected.”

ARES team members got a more detailed look at the Ryugu fragments when JAXA started formally loaning portions of the sample to research teams around the globe. In November 2021, NASA received about 10 percent of the total sample collected by Hayabusa2. The materials were housed in a state-of-the-art cleanroom dedicated to Ryugu research and stored in a glovebox filled with dry nitrogen gas to preserve the sample. These handling procedures were similar to those implemented for the Bennu sample returned by OSIRIS-REx.

Dr. Ann Nguyen, planetary scientist and manager of the NanoSIMS 50L ion probe laboratory within ARES, said the team had to come up with new ways to preserve the sample due to its size and fragility. “We got these tiny fragments – about a millimeter each,” she said. “They were so small and delicate. It was surprising how fragile they were compared to the astromaterials samples that ARES already had.”

Nguyen explained that the Ryugu science team was divided into different working groups, each with a distinct research area. She was part of the chemistry team that examined the sample’s elemental and isotopic composition. Her work focused on identifying presolar grains in the sample – that is, grains of dust that came together around stars billions of years ago, before our Sun formed. “These grains were some of the building blocks of our solar system,” Nguyen said. “We look at the contents and the types of these grains to understand what kinds of dust seeded our solar system and how our solar system’s composition came to be.”

Nguyen said the team expected to find certain presolar grains in the sample based on previous observations of Ryugu and meteorites from a similar asteroid class that were already in ARES’ collection. “When we mapped the sample, the majority of the material was what I expected to see,” she said, “but then we saw these little clasts that were completely different than the rest of the sample, and different than what we had ever seen before in terms of their rich presolar grain and primordial organic contents.”

In a paper published in Science Advances in July, Nguyen and her team concluded that Ryugu’s presolar grains came from outside the solar system because their isotopic makeup could only be produced by nuclear reactions inside stars older than our Sun. The team hypothesized that Ryugu formed farther from the Sun than it is today and captured the clasts that may have formed beyond Neptune. “The abundance of presolar material in the clasts tells us that the clasts are truly unique and new specimens in our collection. And it tells us that explosions of distant, dying carbon-rich stars likely contributed more stardust to our solar system than previously thought,” Nguyen said.

Zolensky’s research supported a similar conclusion. He was part of a joint ARES-University of Texas Austin team that found microscopic droplets of liquid water trapped in the minerals that formed within Ryugu 4.5 billion years ago. “These are samples of water from our early solar system,” he said. “The same type of water that would have landed on Earth when it first formed and helped make the first living things on our planet.” Not only did the team find many organic molecules in those water droplets, but they also found large amounts of carbon dioxide. “The only way you get large amounts of carbon dioxide in fluids like this is if the asteroid originally formed way out beyond the Sun, where carbon dioxide is a solid,” Zolensky said. “This is consistent with Ann’s findings, and also the organics research that our colleagues at Goddard are working on, because those organics also showed evidence of having formed in a very cold environment.”

Similar to NASA’s plan for sharing the material collected by OSIRIS-REx, JAXA is issuing periodic calls for proposals to study the Ryugu sample. Nguyen said the ARES team received its second loan in April 2023, with a sample that included the same fragments analyzed in 2022 and two new pieces. This has allowed her team and others to continue their analyses.  

Nguyen believes the research to date – and yet to come – proves the tremendous value of sample return. “We can’t see these kinds of things telescopically. We need to go and get the sample and bring it back to the lab,” she said. “Look at everything we’ve learned with just 5 grams from Ryugu. What can we learn with 60 grams or more from Bennu?”

Zolensky highlighted the value of preserving and continuing to loan the samples to next-generation scientists, as well. “Every time a mission like this happens, new scientists appear with new techniques to conduct new analyses,” he said. “That not only makes the science better, but it also increases the science return from the mission.”