Scientists working with samples collected by NASA’s OSIRIS-REx mission during its visit to the asteroid Bennu have uncovered a cosmic time capsule, containing a rich variety of materials from our solar system and beyond.
Led by researchers at the University of Arizona, the international team studied how Bennu’s materials transformed over billions of years under harsh space conditions and through interactions with water. Their findings were recently published in three new papers examining the samples returned in 2023.
Asteroid Bennu
“This is work you just can’t do with telescopes,” said Jessica Barnes, co-lead author on one of the papers and associate professor at the U of A’s Lunar and Planetary Laboratory. “It’s super exciting that we’re finally able to say these things about an asteroid that we’ve been dreaming of going to for so long and eventually brought back samples from.”
Bennu itself once belonged to a much larger asteroid that likely broke apart after a collision in the asteroid belt between Mars and Jupiter. Scientists believe that the original asteroid formed more than four billion years ago, possibly beyond the giant planets, before entering our solar system.
“Bennu’s parent asteroid may have formed in the outer parts of the solar system, possibly beyond the giant planets, Jupiter and Saturn,” Barnes said. “We think this parent body was struck by an incoming asteroid and smashed apart. Then the fragments re-assembled and this might have repeated several times.”
Exploring The Samples
The team’s analysis revealed traces of stardust older than our solar system. Using the NanoSIMS instrument at the Kuiper-Arizona Laboratory for Astromaterials Analysis, researchers were able to observe isotopic variations at nanometer scales—essential for studying such ancient grains.
“Those are pieces of stardust from other stars that are long dead, and these pieces were incorporated into the cloud of gas and dust from which our solar system formed,” Barnes explained. “In addition, we found organic material that’s highly anomalous in their isotopes and that was probably formed in interstellar space, and we have solids that formed closer to the sun, and for the first time, we show that all these materials are present in Bennu.”
Comparison To Known Asteroids
The Bennu samples also share similarities with material collected from the Ryugu asteroid by Japan’s Hayabusa2 mission in 2019, as well as with certain chemically primitive meteorites that fell to Earth. Each of these samples points to a common region of origin. However, Bennu’s analysis suggests that over time, the materials in that region may have shifted or been less evenly mixed than previously thought.
Researchers also found evidence that Bennu’s minerals underwent extensive chemical alteration through interactions with heat, water, and physical stress. Among these processes, hydrothermal activity was the most influential, as minerals repeatedly formed, dissolved, and reformed in contact with water.
The study shows that Bennu’s silicates required only room-temperature water to produce the observed reactions. Even in the frigid vacuum of space, residual heat from accretion, later impacts, or energy released during radioactive decay could have been enough to melt water. If trapped inside the asteroid rather than on its surface, liquid water could have persisted for long periods.
“Now you have a liquid in contact with a solid and heat — everything you need to start doing chemistry,” Barnes said. “The water reacted with the minerals and formed what we see today: samples in which 80% of minerals contain water in their interior, created billions of years ago when the solar system was still forming.”
Space Weather and Atmospheric Challenges
Another factor shaping Bennu’s evolution was “space weather”—a combination of solar wind and micrometeorite impacts. Researchers found a microscopic crater and tiny beads of previously melted rock on the surface of its particles, evidence of repeated micro-impacts over time. Surprisingly, this weathering process appeared to progress faster than expected.
Asteroids serve as time capsules of the solar system’s 4.5 billion-year history, preserving material left over from planet formation. Unlike meteorites that fall to Earth and are quickly altered by the atmosphere—or even destroyed entirely before reaching the surface—asteroid samples remain pristine.
“And those that do make it to the ground can react with Earth’s atmosphere, particularly if the meteorite is not recovered quickly after it falls,” Barnes added, “which is why sample return missions such as OSIRIS-REx are critical.”
Together, the new papers provide significant new insights into the mysterious history of our solar system, as well as the interstellar materials that shaped it.
Two of the recent papers, “Composition of Asteroid Bennu Transformed by Aqueous Alteration” and “Space Weathering Effects in Bennu Asteroid Samples,” were published in Nature Geoscience on August 22, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.