Bennu is a near-Earth asteroid about 500 metres across and is widely regarded as a surviving fragment of the Solar System’s earliest building materials. NASA’s OSIRIS-REx mission collected samples from its surface in 2020 and delivered them to Earth in 2023. Laboratory studies of those samples have identified abundant carbon-bearing compounds, hydrated minerals, phosphate and other components that record interactions between rock and water. The new research focuses on a particularly distinctive component: refractory stardust. These microscopic particles, sometimes called presolar grains, condensed in the outflows of dying stars or in the debris surrounding stellar explosions. Because they formed before the Sun, they carry isotopic signatures unlike those produced by ordinary Solar System processes.

“Refractory” describes a material that remains stable at relatively high temperatures compared with more volatile substances. Presolar refractory grains can include silicon carbide, oxide and silicate minerals, depending on the type of stellar environment in which they formed. Their survival in meteorites has allowed scientists to reconstruct aspects of stellar evolution using laboratory measurements of isotopes such as carbon, nitrogen, oxygen and silicon. The grains are commonly identified through anomalous isotopic ratios: instead of matching the average composition of the Solar System, they preserve fingerprints inherited from their parent stars. Yet survival is not guaranteed. Once a grain is exposed to chemically reactive fluids, its surface can dissolve, become coated, exchange elements with the surrounding solution or be replaced by new minerals.

The study’s central chemical setting is an alkali brine, a concentrated water-based solution rich in alkaline elements and ions. On Earth, brines can form when water dissolves salts from rocks and later becomes concentrated through evaporation. Inside an asteroid, similar fluids may develop when ice melts and reacts with minerals. Alkali-rich solutions can reach high pH values, meaning they contain relatively large concentrations of hydroxide ions. Such fluids can attack silicate structures by breaking bonds between silicon and oxygen, mobilizing elements into solution and precipitating secondary minerals. The precise reaction depends on temperature, fluid composition, grain size, porosity and the duration of contact, but even modest alteration over geological timescales can transform the chemical and isotopic record of a microscopic particle.

Bennu’s minerals indicate that liquid water once circulated through its parent body, probably after radioactive heating melted internal ice. The asteroid itself is thought to have formed from fragments of a larger carbonaceous body that experienced this aqueous alteration before being disrupted by impacts. In that setting, fluids could have moved through pores and fractures, reacting with soluble salts and silicate minerals as they travelled. The result would not have been a single uniform chemical event. Instead, different regions and grains could have encountered fluids with changing pH, salinity and oxidation state. A refractory presolar grain located near an active fluid pathway might therefore have been altered much more extensively than a similar grain sealed inside a relatively dry mineral aggregate.

The word “gradual” in the study’s title is crucial. Destruction of stardust in Bennu’s brines would not necessarily resemble a sudden dissolution event. It could proceed through a sequence of surface reactions. A thin outer layer might first lose mobile elements, while the interior retains its original isotopic composition. Continued exposure could roughen the grain, generate pits and cracks, or form a chemically modified rim. Secondary minerals might then grow over the altered surface, physically isolating parts of the grain while also recording the composition of the fluid. At a sufficiently advanced stage, the original particle could be partly or entirely replaced. This stepwise progression matters because researchers examining returned samples may find not only pristine presolar grains but also damaged survivors and mineralogical traces of grains that no longer remain.

Such alteration creates a challenge for interpreting the abundance of stardust in Bennu. If fewer presolar grains are found than expected from the composition of primitive meteorites, the difference may not mean that Bennu formed from material poor in stardust. Some of the grains could have been chemically erased after accretion. Conversely, grains that remain may represent the most resistant mineral types or the portions that were protected from fluid flow. Any estimate of the original presolar inventory must therefore account for selective destruction. The researchers’ focus on alkali brines provides a mechanism by which a body can begin with an ancient stellar record and later preserve only a filtered, incomplete version of it.

The findings also connect Bennu to a broader question in planetary science: how much of an asteroid’s apparent chemical primitiveness is genuinely primordial, and how much is the product of later alteration? Primitive asteroids are not untouched time capsules. They can preserve ancient solids while simultaneously hosting reactions that modify those solids. Water can create new minerals, redistribute elements and alter organic compounds without completely erasing the original parent material. In Bennu, this dual history is especially important because returned samples permit analysis at scales from whole grains to nanometre-thick reaction layers. Electron microscopy, spectroscopy and isotope measurements can reveal whether a particle formed around another star, identify chemical zoning caused by brine exposure and distinguish original material from alteration products.

The work has implications beyond Bennu. Asteroids that contain hydrated minerals and salts may have experienced comparable episodes of internal water circulation. The chemistry of those fluids would have influenced which organic molecules survived, which minerals formed and how efficiently ancient interstellar or circumstellar material was preserved. Understanding brine-driven destruction is also relevant to the interpretation of samples from other carbon-rich bodies, including material delivered by meteorites and future returned-sample missions. Bennu’s microscopic grains are therefore more than isolated curiosities: they are probes of stellar history, asteroid geology and the chemical evolution of the Solar System. By showing that even refractory stardust can be gradually consumed by alkaline fluids, the research underscores a fundamental lesson of planetary science—ancient evidence may endure for billions of years, but it survives only through a continual contest between resistance and alteration.

Isotopes are atoms of the same element that contain different numbers of neutrons. They behave almost identically in ordinary chemical reactions, but their subtle mass differences and radioactive histories make them powerful tracers of cosmic events. Some isotopic patterns were created in ancient stars before the Sun existed, while others emerged from radioactive decay or nuclear reactions in the young Solar System. Because these signatures can survive for billions of years, scientists use them as fingerprints to determine where planetary material formed, how it moved through the protoplanetary disk, and whether apparently similar objects actually came from distinct reservoirs.

The study focuses on samples from Ryugu and Bennu, two dark, carbon-rich near-Earth asteroids explored by Japan’s Hayabusa2 mission and NASA’s OSIRIS-REx mission. Unlike ordinary meteorites, which have passed through Earth’s atmosphere and often experienced heating, contamination, fragmentation, and prolonged terrestrial weathering, returned asteroid samples are collected, sealed, and analyzed under carefully controlled conditions. That difference is scientifically crucial. Meteorites provide an invaluable but selective archive: only material strong enough to survive ejection from an asteroid, travel through space, enter Earth’s atmosphere, and remain recognizable on the surface becomes part of the collection. Fragile and chemically unusual materials may be missing before scientists ever examine them.

According to the researchers, the Ryugu and Bennu samples retain isotopic variations that are not readily visible in conventional meteorite groups. These differences indicate that the parent materials of the two asteroids, and perhaps the broader region from which they originated, were not chemically uniform. Instead, the early Solar System contained multiple reservoirs with distinct isotopic compositions. Such reservoirs may have formed from dust and grains inherited from different stellar sources or may have developed as material was separated, transported, and mixed within the disk of gas and dust surrounding the newborn Sun.

This result challenges a long-standing assumption behind many models of planetary formation: that the meteorites available on Earth adequately represent the full range of primitive material that existed in the Solar System. Meteorite classification has traditionally relied on recognizable chemical and mineralogical features, and many meteorites do preserve ancient components with remarkable clarity. Yet the new evidence suggests that the surviving meteorite population may be biased toward certain types of parent bodies and geological histories. Processes such as aqueous alteration, thermal metamorphism, impact fragmentation, and atmospheric entry can modify or destroy the very isotopic contrasts that researchers are trying to measure.

The importance of isotopic heterogeneity extends beyond the classification of asteroids. Planetary scientists use differences in isotopic composition to reconstruct the architecture of the protoplanetary disk, including the movement of solids across large distances. If materials formed in different parts of the disk retained distinct isotope signatures, those signatures can reveal whether early planetary building blocks remained isolated or were mixed by powerful transport processes. They can also help test competing explanations for the separation between inner and outer Solar System materials, a division reflected in the compositions of rocky planets, primitive asteroids, and the giant planets’ smaller remnants.

Ryugu and Bennu are especially valuable because they are considered primitive bodies: their bulk compositions and mineral assemblages preserve evidence of processes that occurred before, or during, the earliest stages of planetary assembly. Both asteroids contain carbon-bearing material and minerals that interacted with water inside their parent bodies. Their samples therefore provide a layered record. Some components may predate the Sun, while others formed after the Solar System emerged. Untangling these histories requires high-precision measurements capable of distinguishing inherited nucleosynthetic signatures from changes caused by radioactive decay, chemical alteration, or later processing. The new study’s central message is that the returned samples contain information that can be difficult or impossible to recover from meteorites alone.

The discovery also highlights why sample-return missions have become a turning point in planetary science. Remote observations can determine an asteroid’s color, reflectivity, shape, and surface mineral signals, but they cannot fully resolve microscopic isotope distributions. Laboratory analysis, by contrast, can measure variations at extremely small scales and compare them with reference materials from Earth and meteorite collections. Every grain can be examined for its elemental abundances, mineral structure, organic compounds, and isotopic ratios. Because the samples from Ryugu and Bennu were collected directly from their asteroids and protected from extensive terrestrial exposure, they allow researchers to test whether patterns seen in meteorites are universal or merely the result of sampling bias.

The findings may force scientists to revise how they connect meteorite groups to asteroid families and how they estimate the composition of the material that built the planets. A meteorite that appears representative of a primitive asteroid may instead record only a narrow portion of its parent body’s chemical diversity. Conversely, isotopic signatures preserved in Ryugu and Bennu could help identify links between returned samples, meteorites, and distant populations of asteroids that previously seemed unrelated. Future work will likely compare these measurements with samples from other missions, including material returned from different regions of the Moon and, eventually, Mars. Each new collection could reveal another missing piece of the Solar System’s chemical map.

For now, the study presents Ryugu and Bennu as time capsules of a more varied cosmic environment than the meteorite record alone had suggested. Their isotopic heterogeneity preserves traces of the raw ingredients that existed before planets, moons, and asteroids settled into their modern forms. The result does not make meteorites obsolete; instead, it shows that they are one part of a much larger and more selective archive. By combining meteorite analyses with pristine asteroid samples, scientists can begin to distinguish the original structure of the Solar System from the alterations imposed by billions of years of geological and atmospheric history. What looks like uniformity in Earth’s collections may, in fact, be the residue of a far more diverse beginning.

Subject of Research: Primordial isotopic heterogeneity preserved in samples from the asteroids Ryugu and Bennu, and its significance for understanding the early Solar System.

Article Title: Ryugu and Bennu preserve primordial isotopic heterogeneity absent from the meteorite record.

Article References: Bizzarro, M., Schiller, M., van Kooten, E. et al. “Ryugu and Bennu preserve primordial isotopic heterogeneity absent from the meteorite record.” Nature Communications (2026). https://doi.org/10.1038/s41467-026-76720-z

Image Credits: AI Generated

DOI: 10.1038/s41467-026-76720-z

Keywords: Ryugu, Bennu, asteroid samples, isotopes, primordial Solar System, meteorites, planetary formation, sample-return missions, cosmochemistry, isotopic heterogeneity