Isotope measurements of samples returned by NASA’s OSIRIS-REx mission reveal that asteroid Bennu shares a distinct chemical fingerprint with asteroid Ryugu and rare CI carbonaceous chondrites. Published in Science Advances, researchers at ETH Zurich argue these bodies formed near the ancient water-ice line rather than the outer Solar System.
Isotopic Clues Challenge Old Solar System Models
When NASA’s OSIRIS-REx spacecraft returned roughly 120 grams of Bennu material to Earth in September 2023, it set off a global race to decode the origins of our stellar neighborhood. Researchers at ETH Zurich received a fraction of this pristine extraterrestrial cargo for high-precision laboratory analysis. The team focused on five distinct particle portions representing varying sample masses and types, measuring specific isotopes of titanium, chromium, and iron.
Isotopes act as atomic signatures because distinct regions of the early Solar System inherited unique mixtures of ancient stellar material. While chromium showed minor variations driven by secondary alteration by liquid water inside Bennu’s parent body, the titanium and iron compositions were remarkably consistent. Those elemental signatures align closely with asteroid Ryugu and rare CI carbonaceous chondrites, indicating that all three celestial bodies incorporated a similar reservoir of early Solar System material.
Bennu as an Isotopic Hybrid
For decades, planetary scientists relied on a clean binary division for primitive meteorites. Noncarbonaceous materials typically pointed back to the inner Solar System, while carbonaceous materials were linked with more distant regions. Bennu complicated that neat categorization from the moment its physical composition was assessed.
The asteroid is carbonaceous, packed with carbon and water-altered minerals. Yet its iron isotope ratios lean much closer to inner Solar System material than many other carbonaceous meteorites do. Isotope geochemist Maria Schönbächler at ETH Zurich highlighted this ambiguity directly:
“Bennu is a hybrid: the material does not clearly match either the inner or the outer Solar System.”
That hybrid nature undermines older models that place Bennu’s parent body far out in the comet-forming zones. Instead, the data point toward a volatile-rich transition zone where dust from both inner and outer disk environments managed to meet and mix.
Jupiter’s Role as a Size-Selective Cosmic Sieve
How did material from two distinct zones merge so smoothly? The new research points to early Jupiter. As the gas giant grew, it likely acted as an effective size-selective filter.
Larger particles were blocked or trapped, while fine dust grains and fragments were allowed to drift across Jupiter’s orbital path. This leaked dust eventually integrated into the material that formed Bennu-like bodies.
Because CI-like materials preserve chemistry unusually close to the average composition of the Solar System, these samples offer researchers a direct window into the fundamental building blocks of Earth and the other terrestrial planets. Even after internal alteration by liquid water, Bennu’s core isotopic record remains an invaluable time capsule of early planetary assembly.
