Laboratory analysis of dust returned from asteroid Bennu is reshaping ideas about where primitive asteroids formed in the early Solar System, according to a study led by researchers at ETH Zurich and published in Science Advances.
Mixed origin revealed by isotope fingerprints
The team measured isotopic ratios of three metals — iron, titanium and chromium — in a small portion of the material brought back to Earth by NASA's OSIRIS‑REx mission. Roughly 120 grams of Bennu material landed in Utah on 23 September 2023; ETH Zurich's laboratory received about 0.5 grams for this work, the study reported.
By comparing the relative abundances of isotopes (variants of elements with differing neutron counts), the researchers constructed a chemical fingerprint for Bennu. That fingerprint, they found, does not match the long-held expectation that very primitive asteroids accreted exclusively in the distant, comet‑forming regions of the Solar System. Instead, the isotopic signature indicates Bennu assembled from a mixture of materials originating in both the inner and outer parts of the protoplanetary disc.
Jupiter's early growth as a controlling influence
The analysis supports a scenario in which Jupiter began assembling within about one million years of the Sun's birth. In the researchers' interpretation, the giant planet's rapid growth helped regulate the movements of material through the young Solar System and thus the composition of small bodies that formed thereafter.
The study also highlights a close isotopic match between Bennu and two other rare classes of primitive material: Ryugu (the target of Japan's Hayabusa2 mission) and CI carbonaceous meteorites. CI meteorites are rich in carbon and are considered among the least altered samples of early Solar System matter that reach Earth.
| Parameter | Value |
|---|---|
| Mass returned to Earth | 120 grams |
| Mass analysed by ETH Zurich | 0.5 grams |
| Key isotopes measured | Iron, titanium, chromium |
| Estimated start of Jupiter formation | ~1 million years after Sun's birth (researchers' scenario) |
| Bennu orbital period | 1.2 years |
- Isotopic similarity links Bennu to Ryugu and CI meteorites, suggesting shared ingredients.
- Jupiter's timing may have been crucial in mixing inner and outer disc materials.
- Sample return value: even half a gram of curated material can yield decisive constraints on planetary formation models.
The measurements rely on the principle that isotopes preserve information about the provenance and processing history of material. Because isotopes of the same element have slightly different masses, their relative abundances can serve as a geochemical signature that distinguishes distinct reservoirs in the early Solar System.
In practical terms, the finding challenges the simplified view that primitive, carbon‑rich asteroids formed only at large heliocentric distances. Instead, the data point to a more complex early disc in which materials from different regions were mixed and later accreted into bodies such as Bennu.
For planetary scientists, that complexity has implications for models of planet formation and for how volatile and organic compounds—key to questions about the origin of water and prebiotic material—were distributed across the nascent Solar System.
While the ETH Zurich study provides a significant new constraint, it is one piece in a rapidly expanding dataset derived from returned samples and meteorite analyses. Continued high‑precision isotopic work on Bennu samples and comparisons with material from other missions will be essential to refine timing and transport processes in early Solar System history.
The results underscore the scientific value of sample‑return missions: tiny curated amounts of extraterrestrial material can deliver robust, measurable constraints on the conditions and dynamics that shaped the planets.
