Bennu asteroid samples reveal Jupiter's role in solar system formation

In October 2023, a NASA spacecraft called OSIRIS-REx descended toward the asteroid Bennu and collected roughly 120 grams of dust and rock from its surface—a feat of precision engineering executed while the asteroid hurtled through space at thousands of miles per hour. Bennu is no distant wanderer; it completes an orbit around the sun every 1.2 years and swings past Earth every six years, coming within about 300,000 kilometers of our planet. That proximity made it an ideal target for sample collection, and when the container touched down in the Utah desert months later, it carried material that scientists had been waiting decades to study.

A half-gram of that precious cargo made its way to ETH Zurich, where Maria Schönbächler, a professor of isotope geochemistry, began the painstaking work of analysis. Her team focused on three elements—iron, titanium, and chromium—measuring the subtle mass differences between their isotopes. These isotopic signatures act like fingerprints, revealing where a material originated and roughly how old it is. What emerged from the data was striking: Bennu's isotopic composition matched that of another asteroid called Ryugu and a rare class of meteorites known as CI meteorites, carbon-rich bodies that fall to Earth only occasionally. All three shared the same cosmic fingerprint, suggesting they had formed from the same reservoir of primordial dust. Yet they differed markedly from every other known asteroid, meteorite group, and planet scientists had measured.

The findings overturned long-held assumptions about where Bennu came from. Researchers had believed that asteroids like Bennu formed in the outer reaches of the solar system, perhaps in the region where comets originated, and that they assembled relatively late in the solar system's history. The isotope data told a different story. Bennu, Ryugu, and the CI meteorites most likely formed near the water-ice line—the boundary where water vapor freezes—roughly 4.5 billion years ago, when the solar system was still coalescing from a disk of dust and gas around the young sun. In that zone, materials from both the inner and outer solar system mixed together, with water ice acting as a binding agent that glued fine dust particles into larger bodies. Bennu, Schönbächler explained, is a hybrid: it bears chemical signatures of both the inner and outer solar system, formed in a region where material from both flowed together.

The key to understanding how this mixing occurred lies with Jupiter. The gas giant formed remarkably quickly, within the first million years after the sun ignited, growing massive enough to reshape the entire disk of material surrounding the young star. As Jupiter swelled, it acted as a kind of barrier, blocking the coarser material in the disk from flowing freely. But fine dust particles—the smallest grains—could flow around the giant planet, mixing evenly in the transition zone near the water-ice boundary. This is where Bennu's precursors took shape. The scenario explains why Bennu is so rich in water: ice in the vicinity evaporated, and some of the water vapor recondensed in the region where the asteroid formed. Jupiter's influence also ensured that Bennu assembled primarily from fine dust, which had been thoroughly mixed throughout the disk—much like household dust that eventually settles everywhere in a home. The result is material so chemically similar to the sun itself that Bennu may represent the original mix of elements from which the terrestrial planets ultimately built themselves.

What makes Bennu extraordinary is its primordial nature. The material in the samples has barely changed since the solar system's birth 4.5 billion years ago, making it a window into conditions that existed when planets were forming. Because the asteroid is also rich in organic compounds and water, it offers clues to how the young Earth acquired the chemical building blocks necessary for life. Schönbächler and her colleagues have published their findings in Science Advances, but questions remain. They are now investigating whether other asteroids carry the same isotopic signature as Bennu and Ryugu, and whether Jupiter's role in concentrating fine dust was as decisive as the current evidence suggests. The work is part of a larger effort to reconstruct the solar system's earliest moments—a story written in the chemistry of ancient rocks.