Tiny grains of space rock from asteroid Bennu reveal that the ancient body formed near the water-ice line of the early solar system. Researchers analyzing isotopes in the sample found that Jupiter acted as a cosmic sieve, helping shape the material that built the terrestrial planets 4.5 billion years ago.
Scientists long assumed Bennu originated in the distant outer reaches of the solar system alongside comets, but new analysis shows its ancestral material assembled closer to home. NASA’s OSIRIS-REx spacecraft dropped off a capsule containing 0.26 pounds (120 grams) of regolith from asteroid Bennu in the Utah desert on September 24, 2023, delivering a pristine forensic archive that included 121.6 grams according to a separate weighing.
Planetary scientists received tiny fractions of that material to probe its chemical secrets, including half a gram sent to a laboratory at ETH Zurich. A team led by researchers at ETH Zurich examined subtle variations in isotopes of iron, titanium, and chromium within the returned grains, publishing their findings in Science Advances.
Isotopes Reveal Hybrid Origin
Isotopes act as atomic fingerprints, carrying clues about where and when elements formed in the protoplanetary disk surrounding the young Sun. The team measured these variations across individual particle types and mixed samples.
The uniform distribution of iron and titanium throughout the Bennu samples showed an unusual level of chemical mixing.
“Bennu is a hybrid: the material does not clearly match either the inner or the outer Solar System,”
Maria Schönbächler, professor of isotope geochemistry at ETH Zurich, via Gizmodo
This composition means the asteroid preserves primordial ingredients that closely match the pre-solar nebula and the Sun itself. Because these pristine components escaped being roasted in a fireball, researchers believe Bennu may offer our best glimpse of the original mix of chemical elements from which the terrestrial planets were ultimately built, a sentiment echoed by Schönbächler in a statement where she stated that the asteroid offers our best glimpse of the original mix of chemical elements.
Jupiter Sifts Material Near Snow Line
The researchers point to the powerful gravitational influence of Jupiter to explain how inner and outer solar system materials blended. Within roughly one million years of the Sun’s birth (or about two million years according to some models), the gas giant grew enormous, eventually exceeding 20 Earth masses.
The planet functioned like a barrier or a sieve as it cleared a path through the surrounding disk of gas and dust. Heavy, millimeter-sized molten droplets known as chondrules became trapped in pressure ridges outside Jupiter’s orbit, while finer dust grains coupled tightly to the gas and slipped past the giant planet.
Those grains drifted inward until they piled up near the water-ice line, also known as the snow line, where temperatures dropped enough for water vapor to freeze into solid ice crystals.
- Water-ice acted as a natural glue, binding the fine dust particles together.
- Local pressure gaps and traffic jams of solids encouraged the material to collapse into larger parent bodies.
This localized mixing zone sits roughly where Jupiter is positioned today. It accounts for both the abundance of water-altered minerals found inside Bennu and its striking chemical resemblance to the Sun.
Researchers Debate Parent Body Origin
The ETH Zurich team maintains that fine dust transport across Jupiter’s orbit best explains the uniform isotopic mix. This contrasts with some recent studies that argue for an origin point at the current position of Jupiter or beyond based on volatile chemistry—citing planetary scientist Timothy McCoy placing its parent body at the current position of Jupiter or beyond in Scientific American’s 2025 reporting.
Bennu eventually broke off from its larger carbon-rich parent asteroid roughly 700 million to 2 billion years ago, embarking on a multi-step journey of collisional shoves and gravitational nudges that brought it into a near-Earth orbit. Today, the tiny grains returned by NASA’s OSIRIS-REx mission continue to redefine our understanding of how cosmic debris assembled into habitable worlds.