Originally published by NASA on August 22, 2025. Edits by EarthSky.
- Samples brought back from asteroid Bennu by the OSIRIS-Rex spacecraft show dramatic transformations over the asteroid’s lifetime.
- Bennu originated as part of a larger asteroid made up of material from diverse origins ranging near and far from the sun. And it’s from the primordial cloud that preceded the solar system.
- Both heat and water transformed parts of the asteroid after it was formed. However, some particles containing organic matter likely formed in interstellar space.
Asteroid Bennu samples show complex evolution
Samples of Asteroid Bennu were returned to Earth by NASA’s OSIRIS-REx mission in 2020. Overall, they are a mixture of dust that formed in our solar system, organic matter from interstellar space and pre-solar system stardust. What’s more, their unique and varied contents were dramatically transformed over time by interactions with water and exposure to the harsh space environment.
These insights come from a trio of newly published papers based on the analysis of Bennu samples by scientists at NASA and other institutions.
Bennu is made of fragments from a larger parent asteroid destroyed by a collision in the asteroid belt. The asteroid belt lies between the orbits of Mars and Jupiter. One of the papers was published in the peer-reviewed journal Nature Astronomy on August 22, 2025. It was co-led by Jessica Barnes at the University of Arizona, Tucson, and Ann Nguyen of NASA’s Johnson Space Center in Houston. It suggests that Bennu’s ancestor was made up of material near the sun, far from the sun and even beyond our solar system.
Primordial materials found in asteroid Bennu
The analyses show that some of the materials in the parent asteroid escaped various chemical processes driven by heat and water. Plus, it even survived the extremely energetic collision that broke it apart and formed Bennu. Nguyen said:
We traced the origins of these initial materials accumulated by Bennu’s ancestor. We found stardust grains with compositions that predate the solar system, organic matter that likely formed in interstellar space, and high temperature minerals that formed closer to the sun. All of these constituents were transported great distances to the region that Bennu’s parent asteroid formed.
Comparison of asteroid Bennu with asteroid Ryugu
Another asteroid, Ryugu, was sampled by the Japan Aerospace Exploration Agency’s Hayabusa2 mission in 2020. How do the chemical and atomic similarities of samples from Bennu and the asteroid Ryugu compare to primitive meteorites collected on Earth? Studies suggest they might have formed in a similar, distant region of the early solar system. Yet the differences from Ryugu and meteorites that were also in the Bennu samples might indicate that this region changed over time.
Though some original constituents survived, most of Bennu’s materials were transformed by reactions with water. This was reported in the paper co-led by Tom Zega of the University of Arizona and Tim McCoy of the Smithsonian’s National Museum of Natural History in Washington and published in Nature Geoscience on August 22, 2025. In fact, minerals in the parent asteroid likely formed, dissolved and reformed over time. Zega explained:
Bennu’s parent asteroid accumulated ice and dust. Eventually that ice melted, and the resulting liquid reacted with the dust to form what we see today, a sample that is 80% minerals that contain water. We think the parent asteroid accumulated a lot of icy material from the outer solar system, and then all it needed was a little bit of heat to melt the ice and cause liquids to react with solids.
Space weathering
Bennu’s transformation did not end there. The third paper, co-led by Lindsay Keller at NASA Johnson and Michelle Thompson of Purdue University, and published in Nature Geoscience on August 22, 2025, found microscopic craters and tiny splashes of once-molten rock. They are known as impact melts on the sample surfaces and are signs that the asteroid was bombarded by micrometeorites. These impacts, together with the effects of solar wind, are known as space weathering and occurred because Bennu has no atmosphere to protect it. Keller said:
The surface weathering at Bennu is happening a lot faster than conventional wisdom would have it, and the impact melt mechanism appears to dominate, contrary to what we originally thought. Space weathering is an important process that affects all asteroids, and with returned samples, we can tease out the properties controlling it and use the data and extrapolate it to explain the surface and evolution of asteroid bodies that we haven’t visited.
Asteroids are a unique resource for study
As the leftover materials from planetary formation 4.5 billion years ago, asteroids provide a record of the solar system’s history. But as Zega noted, we’re seeing that some of these remnants differ from what has been found in meteorites on Earth. That’s because certain types of asteroids burn up in the atmosphere and never make it to the ground. And as researchers point out, that is why collecting actual asteroid samples is so important. Barnes said:
The samples are really crucial for this work. We could only get the answers we got because of the samples. It’s super exciting that we’re finally able to see these things about an asteroid that we’ve been dreaming of going to for so long.
Bottom line: Asteroid Bennu samples show diverse characteristics that give insight into how, when, and where the asteroid was formed.
Source: The variety and origin of materials accreted by Bennu’s parent asteroid
Source: Mineralogical evidence for hydrothermal alteration of Bennu samples
Additional source: Space weathering effects in Bennu asteroid samples
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