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

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.

Bennu, with its intricate composition, is a cosmic tapestry woven from materials collected over billions of years. The asteroid is thought to be a remnant of a substantially larger parent asteroid that fragmented following a catastrophic collision with another asteroid. This parent body, a mix of diverse materials from various locations within and beyond the solar system, accreted billions of years ago, embodying a time when the solar system was still in its formative stages.

The revelations from the new studies significantly enhance our understanding of Bennu’s origins and composition. The results confirm that Bennu is not merely a random collection of space debris; rather, it harbors fragments of stardust, remnants from stars that existed long before our sun began to shine. Analyzing these samples has provided scientists with an unprecedented opportunity to glimpse the early solar system and the processes that shaped it.

Jessica Barnes, an associate professor at the University of Arizona’s Lunar and Planetary Laboratory and a co-lead author of one of the studies, highlighted the significance of this work. The details unearthed from Bennu challenge assumptions made previously and emphasize the need for meticulous analysis that can only be achieved through sample return missions such as OSIRIS-REx. She expressed enthusiasm about the capacity to make claims about an asteroid that had caught the attention of researchers for decades.

The complexity of Bennu’s composition reveals that its parent asteroid likely formed in the distant outskirts of the solar system, possibly beyond the giant planets Jupiter and Saturn. The study postulates that this asteroid was fractured by an incoming impact with another celestial body, leading to the scattering and eventual combination of fragments that coalesced into what we now recognize as Bennu. This provides a glimpse into the dynamic processes of our solar system’s formation and evolution.

Among the most compelling discoveries was the abundant presence of stardust within Bennu’s samples. Using the advanced capabilities of the NanoSIMS instrument, scientists were able to investigate the isotopic compositions of minute particles, revealing isotopes that hint at origins far preceding the formation of our solar system. This ancient cosmic material has traveled through time and space, ultimately becoming part of the building blocks from which planets, including Earth, were formed.

Researchers also found organic materials that display anomalous isotopic signatures indicative of a formation process that likely occurred in interstellar space. This discovery, coupled with the existence of materials formed closer to the sun, paints a picture of a complex environment where various organics intermingle, suggesting a rich chemical landscape that facilitated the emergence of life’s precursors.

This significant exploration is further broadened when comparing Bennu’s samples to those from Ryugu, another asteroid explored by the Japanese Hayabusa 2 mission. The similarities and differences in composition could unveil insights into the varying conditions within the early solar system. This comparative analysis is crucial in understanding the compositional diversity of asteroids and offers tantalizing clues about the conditions that prevailed in different regions during the solar system’s formation.

The transformations that Bennu’s parent asteroid underwent before it became Bennu are equally intriguing. The studies indicate that various minerals in the parent body interacted with water over extended periods, highlighting hydrothermal processes that took place in the asteroid’s early history. These interactions have contributed to the chemistry seen in Bennu today, showing that even asteroids, often considered inert, may have undergone dynamic geological and chemical changes.

In the wake of these discoveries, scientists are beginning to clarify how the interactions of minerals and water took place on the asteroid. The presence of water, likely resulting from icy materials accreted from the outer solar system, interacted with silicate minerals under conditions that are surprisingly temperate—around room temperature. This raises significant questions about the habitability of distant worlds and how asteroids may play a role in the delivery of water and organic materials essential for life.

As the studies unfold, a third paper focused on the impacts that Bennu has experienced throughout its life. Evidence of micrometeorite bombardment and solar wind interactions indicates that Bennu is subjected to rapid “space weathering,” a phenomenon occurring because the asteroid lacks an atmosphere. These weathering effects not only affect the asteroid’s surface but also offer further insight into the dynamic and often violent processes that shape celestial bodies in the vacuum of space.

The research on Bennu highlights the critical importance of sample return missions. While meteorites that land on Earth provide valuable information, they undergo intense atmospheric reactions that can obscure their original characteristics. The pristine samples collected by OSIRIS-REx offer a unique and uncontaminated glimpse into the asteroids of our solar system, shedding light on mysteries that terrestrial specimens cannot reveal.

As we continue to analyze the information gleaned from Bennu’s samples, the implications extend far beyond our immediate understanding of this asteroid. It raises profound questions about the origins of life on Earth and the potential for life elsewhere in the universe. By piecing together the history exemplified by Bennu and its parent asteroid, scientists are embarking on a quest that could alter our perception of astrobiology and the evolution of life beyond our planet.

The work surrounding Bennu is a testament to humanity’s comprehensive journey of exploration and understanding of the universe. It is a reminder of the interconnectedness of life, stellar evolution, and the profound mysteries that the cosmos holds for those daring enough to seek them.

Subject of Research: Asteroid Bennu and its implications for planetary science and astrobiology.
Article Title: Unraveling the Secrets of Asteroid Bennu: Insights from OSIRIS-REx
News Publication Date: October 2023
Web References: NASA’s OSIRIS-REx Mission
References: Nature Astronomy; Nature Geoscience
Image Credits: Credit: Chris Richards, University of Arizona

Keywords

Asteroid, Bennu, OSIRIS-REx, stardust, organic materials, hydrothermal processes, space weathering, sample return mission, planetary science, astrobiology.