Bennu sample reveals how water flowed through the newly forming asteroid
by Sam JarmanSam Jarman
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A team of US astronomers has carried out one of the deepest analyses to date of a sample from the asteroid Bennu, revealing new details about how water and organic material interacted during the earliest stages of the solar system.
Publishing their results in the journal PNAS, the team, led by Mehmet Yesiltas at Stony Brook University, showed that the asteroid's composition is divided into three distinct chemical domains. At the nanoscale, these domains strongly suggest that water flowed through the newly forming asteroid in restricted channels, leaving some regions largely untouched.
A piece of Bennu
In September 2023, NASA's OSIRIS-REx mission returned only the third-ever sample of asteroid material to Earth. The sample originated from Bennu, a near-Earth object roughly 500 meters wide that is already known to be rich in carbon.
Through deeper analysis of its chemical composition, astronomers hope the sample can help them piece together new clues about the early history of the solar system, when asteroids like Bennu first formed.
In Earth-based geology, important insights often come from detecting subtle variations in chemical composition, many of which are only visible at extremely small scales. Until now, however, Bennu's material had not been studied with such high precision.
Distinct domains
In their study, Yesiltas' team applied a range of cutting-edge analytical techniques, allowing them to probe variations down to scales of just 20 nanometers—around the size of a single large molecule. Using a combination of infrared and Raman spectroscopy, they revealed that the sample's composition is not uniformly distributed, but instead segregated into three distinct domains, with very little overlap.
These include aliphatic-rich regions, dominated by hydrocarbons arranged in open chains; carbonate-rich domains, composed of inorganic minerals rich in calcium and magnesium; and nitrogen-bearing organic-rich domains, which host a distinct class of complex organic molecules. These nitrogen-bearing compounds were likely either inherited from older material that helped form the asteroid, or chemically modified during interactions between fluid and rock.
A glimpse of history
For Yesiltas and colleagues, these results already offer important clues about Bennu's formation, especially the role played by water. Because the three domains are so clearly separated, they suggest that water didn't alter the asteroid uniformly.
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Particularly strong evidence comes from the distribution of organosulfur compounds, which are almost entirely restricted to carbonate-rich regions. This pattern indicates that carbonate minerals likely precipitated from water-rich fluids.
In contrast, both aliphatic and nitrogen-bearing organic compounds are more vulnerable to alteration when exposed to water, implying that these regions must have remained relatively dry during key stages of the asteroid's evolution.
Taken together, the coexistence of these sharply distinct domains suggests that water flowed through Bennu along restricted pathways, altering some domains while preserving the more fragile compounds found in other regions.
Overall, the discovery marks an important step forward in understanding how asteroids formed around the nascent sun. With further analysis, along with comparisons to samples from the asteroid Ryugu (returned to Earth by Japan's Hayabusa2 mission), the researchers now hope that Bennu's history can be reconstructed in even greater detail. In turn, this could provide fresh insights into the processes that shaped the early solar system, and the origins of our own planet.
Written for you by our author Sam Jarman, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.
Publication details
Mehmet Yesiltas et al, Nanoscale infrared spectroscopy reveals complex organic–mineral assemblages in asteroid Bennu, Proceedings of the National Academy of Sciences (2026). DOI: 10.1073/pnas.2601891123
Journal information: Proceedings of the National Academy of Sciences
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