Bennu is an unusually accessible target for science. The asteroid completes one orbit of the Sun every 1.2 years and passes within roughly 300,000 kilometres of Earth every six years, a close approach that made it an ideal destination for a sample-return mission. NASA seized this opportunity with its OSIRIS-REx probe, which in a spectacular operation touched down on the asteroid’s surface and collected material directly from it. In 2023, the sample container descended into the Utah desert carrying around 120 grams of Bennu’s rocky payload, the largest amount of asteroid material ever returned to Earth. From that haul, a small but extraordinarily precious portion made its way to Switzerland, where Maria Schönbächler, Professor of Isotope Geochemistry at ETH Zurich, received half a gram for analysis. Her laboratory began working on the samples immediately, and the completed investigations have now yielded results that reach far beyond Bennu itself, offering new insight into how the entire Solar System took shape.

The key to the discovery lies in isotopes, atoms of the same element that differ slightly in mass because their nuclei contain different numbers of neutrons. The ETH team measured isotopes of three elements: iron, titanium and chromium. Together, these isotopic ratios create a distinctive chemical fingerprint that allows researchers to determine where a body’s raw material originated and, to some extent, how old it is. Because isotopic ratios are inherited from the cloud of dust and gas that formed the Solar System and are altered only by specific nuclear and chemical processes, they act like immutable birth certificates written into the fabric of rocks. For Bennu, that fingerprint turned out to be unlike anything scientists expected.

The measurements revealed that titanium and iron are uniformly distributed throughout the Bennu material, a sign of a remarkably well-mixed source. More striking still, the analyses showed that Bennu has close relatives scattered across the Solar System. The asteroid Ryugu, sampled by Japan’s Hayabusa2 mission, and the so-called CI meteorites, a rare class of primitive, carbon-rich rocky bodies occasionally found on Earth, all share a similar isotopic fingerprint with Bennu. This shared signature indicates that all three bodies formed from the same reservoir of cosmic dust. At the same time, the group differs significantly in isotopic composition from other known asteroids, meteorite groups and the planets, marking Bennu and its siblings out as members of a chemically distinct family with a very specific place of origin.

That place of origin is precisely where the new study overturns long-held assumptions. Until now, scientists had assumed that asteroids such as Bennu formed in the outer regions of the Solar System, possibly in the same environment where comets formed, and that they accreted relatively late in the Solar System’s evolution. The new isotope data contradict both ideas. Instead, the most likely scenario is that the birthplace of Bennu, Ryugu and the CI meteorites lay close to the water-ice line, the boundary marking the point where water vapour freezes. Around 4.5 billion years ago, as the Solar System was still taking shape, this location was a dynamic mixing zone where material from the inner and outer regions met and mingled. The ice present there acted as a kind of glue, binding the finest dust particles together into larger aggregates that would eventually grow into asteroid-sized bodies.

Bennu, in other words, is a hybrid. As Schönbächler explains, the material does not clearly match either the inner or the outer Solar System; it bears characteristics of both regions, having formed in a specific zone where flows of matter from both sides converged. This hybrid character explains several long-standing puzzles about Bennu’s composition, including why its material is so rich in water. In the vicinity of the water-ice line, ice evaporated as temperatures fluctuated, and some of the resulting water vapour condensed again in exactly the region where Bennu’s parent body formed, soaking the accumulating dust with hydrated minerals. The result is an asteroid whose substance carries the chemical memory of a boundary environment that no longer exists in the modern Solar System.

The ETH researchers and their co-authors attribute a central role in this story to Jupiter. The gas giant formed remarkably early, within roughly one million years of the Sun’s birth from a collapsing cloud of dust and gas, driven by gravitational forces within the swirling disc of material that surrounded the young star. Because it grew so rapidly, Jupiter acted as a bridge pillar within that disc: its growing bulk blocked most coarse material from crossing its orbit, while fine dust from various regions of the disc flowed around the giant planet and mixed evenly in the transition zone near the water-ice boundary. The precursors of Bennu, Ryugu and the CI meteorites subsequently accreted in this sheltered region, built almost entirely from finely intermixed dust rather than from the larger pebbles and boulders that Jupiter filtered out.

This scenario elegantly accounts for another of Bennu’s defining traits: the extraordinary chemical similarity of its material to that of the Sun itself. Because Jupiter’s protective influence ensured that Bennu formed mainly from fine dust, and because fine dust orbiting in the disc around the young Sun was thoroughly mixed, the asteroid’s composition mirrors the average Solar System inventory of elements. Schönbächler compares it to fine dust at home, which simply ends up everywhere over time. That makes Bennu an extraordinarily valuable scientific resource. It is a very primordial asteroid, and its material dates back to the birth of the Solar System around 4.5 billion years ago, having hardly changed since. As Schönbächler notes, Bennu may offer our best glimpse of the original mix of chemical elements from which the terrestrial planets were ultimately built.

The implications extend to the deepest questions about our own origins. By performing precise geochemical analyses of Bennu’s samples, researchers are refining our understanding of how the Solar System arose and under what conditions planets formed. Because the asteroid is rich in water and organic material, it also provides important pieces of the puzzle regarding how the young Earth acquired the building blocks of life, the volatile compounds and carbon-based chemistry that may have been delivered to our planet by primitive bodies like Bennu during the chaotic early era of planetary formation.

Many questions remain open. The team is now wondering whether other asteroids share the same isotopic signature as Bennu and Ryugu, and it is still unclear to what extent the young Jupiter contributed to the fact that only fine dust particles clumped together in the transition zone. Further research will help clarify this picture. Meanwhile, Schönbächler is eagerly awaiting the Japanese sample-return mission to Mars’ moon Phobos, due to launch at the end of October this year, and intends to apply to the Japanese space agency JAXA for material to analyse in her laboratory. Patience will be required, however: the capsule containing the Phobos material is not expected to return to Earth until 2031. When it does, it may allow scientists to test whether the strange hybrid fingerprint of Bennu, forged beside a growing Jupiter at the edge of the ice, was shared more widely across the early Solar System than anyone had imagined.

Subject of Research: Isotopic analysis of OSIRIS-REx samples revealing the formation origin of asteroid Bennu near the Solar System's water-ice line

Article Title: Mystery surrounding the formation of asteroid Bennu solved

Article References: Mystery surrounding the formation of asteroid Bennu solved. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: asteroid Bennu, OSIRIS-REx, isotope geochemistry, water-ice line, Jupiter, Solar System formation, Ryugu, CI meteorites, ETH Zurich, sample return, Science Advances, planet formation

Previous research had already hinted that something like this preferential sorting was taking place, but only in objects that formed between two and four million years after the solar system’s origin. Among carbonaceous chondrites, the primitive stony meteorites that contain organic compounds and water within their silicate minerals, those that formed earlier consistently contained a higher percentage of chondrules and a lower percentage of matrix. This pattern suggested that in the regions where the first planetesimals were coalescing, the icy, volatile-rich dust was already being squeezed out in favor of the heat-forged rocky beads. What was missing was direct evidence from the very earliest epoch, the first million years, when the first generation of solid bodies came together. No undifferentiated bodies from that period survive intact today, leaving a critical gap in the record of how the solar system’s construction began.

Damanveer Grewal, an assistant professor of Earth and planetary sciences in Yale’s Faculty of Arts and Sciences and first author of the study, set out to close that gap with an unconventional approach. Rather than searching for preserved early bodies, which do not exist, he turned to iron meteorites whose parent bodies formed in the outer solar system during that missing first million years. These parent bodies had accumulated so much of the radioactive isotope aluminum-26 that they melted completely, destroying every physical trace of their original chondrule-to-matrix composition. On the surface, that total melting would seem to erase all useful information. But Grewal and his colleagues realized that the chemical fingerprints of the original ingredients would survive the melting, locked into the metallic cores of these differentiated bodies.

The team identified two independent chemical tracers, both tied specifically to the matrix component. The first is sulfur, which exists in concentrated form within matrix material. The amount of sulfur preserved in the iron meteorites therefore reveals how much fine-grained, volatile-rich dust the original parent body had incorporated before it melted. The second tracer is the oxidation state of iron, which reflects how much water ice and oxidized dust the original body contained. Because matrix is the component that carries water ice and oxidized material, a low oxidation state in the surviving metal indicates that very little icy dust was present when the body assembled. By measuring both tracers in the same set of meteorites, the researchers could reconstruct the original composition of bodies that had otherwise lost all physical memory of their building blocks.

The results were striking. Using the paired tracers, the researchers calculated that matrix made up only 8 to 17 percent of the original bodies sampled by these iron meteorites. That range is lower than the matrix fraction found in any known chondrite, meaning the earliest planetesimals of the outer solar system were more chondrule-dominated than any primitive meteorite ever recovered on Earth. In other words, the first solid bodies ever built in the solar system were constructed from 83 to 92 percent chondrules, with only a small admixture of the icy, volatile-rich dust that dominates objects that formed later. The two tracers, measured independently, converged on the same answer, giving the team confidence that the reconstruction was robust rather than an artifact of any single measurement.

Both tracers independently tell the same story: these early planetesimals were remarkably matrix-poor, Grewal explained. That convergence is what makes the result robust. The finding demonstrates that the assembly process was remarkably selective from the very beginning, sorting heat-forged chondrules into the first generation of solid bodies while excluding most of the cold, volatile-bearing dust. This selectivity implies that physical processes in the young protoplanetary disk, such as aerodynamic sorting of particles by size and density, were already operating efficiently within the first million years, concentrating the millimeter-sized chondrules and winnowing away the finer matrix grains before the first planetesimals accreted.

The discovery also resolves a long-standing puzzle about the meteorite record itself. Chondrules from the earliest epoch are scarce among the meteorites collected on Earth, and the new study explains why. The bodies that incorporated those oldest chondrules were the same bodies that accumulated enough radioactive aluminum-26 to melt completely, and that melting erased the physical evidence of their chondrule-rich composition. The oldest chondrules were not absent from the early solar system; they were simply swallowed by bodies that later transformed beyond recognition, leaving only their chemical ghosts in the iron meteorites that survive today. The scarcity of ancient chondrules in chondrites is thus a consequence of planetary differentiation, not of their original rarity.

Beyond solving that puzzle, the findings carry broader implications for understanding how the planets themselves came to be. Chondrules are the ubiquitous little beads of rock that served as the basic building blocks from which the planets were eventually assembled, and the new work shows that they were already being sorted and incorporated into the first generation of solid bodies from the very start. If the earliest planetesimals were so strongly enriched in chondrules and so depleted in volatile-rich matrix, then the seeds of the planets began their lives chemically dry and rocky, with the water and organic material arriving later or in different proportions than many models had assumed. This has consequences for theories of how Earth acquired its water and for understanding the volatile budgets of the outer solar system’s icy bodies.

The study was co-authored by Zhongtian Zhang of Princeton University and Joanna Drążkowska of the Max Planck Institute for Solar System Research in Germany, and the research was funded by Yale University. For Grewal, the work also carries a sense of deep connection to the deep past. Chondrules are found inside chondrites, the most primitive meteorites in geological collections, and holding one in your hand means holding a fragment of a process that started billions of years ago, a timescale that is hard to wrap your head around. By reading the chemical memory preserved in melted iron cores, the team has recovered a chapter of solar system history that physical evidence alone could never provide, revealing that when the solar system first began to build, it chose fire over ice.

Subject of Research: Geochemical reconstruction of the chondrule-to-matrix composition of the solar system's first planetesimals

Article Title: From the start, the solar system chose fire over ice to build its first bodies

Article References: From the start, the solar system chose fire over ice to build its first bodies. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: chondrules, matrix, planetesimals, iron meteorites, carbonaceous chondrites, solar system formation, aluminum-26, protoplanetary disk, Yale University, Nature Astronomy, oxidation state, volatile-rich dust