The asteroid Bennu orbits the Sun once every 1.2 years and approaches Earth every six years, passing within around 300,000 kilometres. NASA took advantage of this to collect material from the celestial body. In a spectacular operation in 2023, the US space agency collected samples from the surface of the asteroid Bennu by its OSIRIS-REx probe.
On 23 September, the sample container landed in the Utah desert, carrying around 120 grams of material from Bennu. From there, a small but precious portion made its way to ETH Zurich: Maria Schönbächler, Professor of Isotope Geochemistry, received half a gram for analysis. The ETH researcher’s laboratory began analysing the samples immediately.
The investigations have now been completed, and the results have just been published in the Science Advances journal. They not only reveal the chemical fingerprint of the Bennu minerals but also provide new insights into how our Solar System formed.
Fingerprint and close relatives
The ETH researchers analysed isotopes of iron, titanium and chromium. Isotopes are atoms of the same element that differ slightly in mass. Together, they create a distinctive fingerprint that the researchers can use to determine the origin and, to some extent, the age of the asteroid.
These measurements show that titanium and iron are uniformly distributed throughout Bennu. They also reveal that Bennu has some close relatives: the asteroid Ryugu and the so-called CI meteorites, a class of primitive, carbon-rich rocky bodies that are found only very rarely on Earth. All three celestial bodies share a similar isotopic fingerprint, indicating that they formed from the same reservoir of cosmic dust. They also differ significantly in their isotopic composition from other known asteroids, meteorite groups and planets.
Where and how did Bennu form?
To date, scientists had assumed that asteroids such as Bennu formed in the outer regions of the Solar System, possibly in the same place where comets formed. While they opined that Bennu was formed relatively late in the evolution of our Solar System, the new data contradicts both ideas.
The most likely scenario is that the birthplace of Bennu, Ryugu and the CI meteorites was close to the water-ice line. This boundary marks the point where water vapour freezes. Here, 4.5 billion years ago, as Solar System was still taking shape, materials from its inner and outer region mixed. The ice, in turn, acted as a ‘glue’, binding the finest dust particles together.
“Bennu is a hybrid: the material does not clearly match either the inner or the outer Solar System,” says Schönbächler. It bears characteristics of both regions – and was formed in a specific zone where material flows from both regions mixed.
New scenarios
Maria Schönbächler and her co-authors propose the following scenario: they attribute a key role in the formation of these celestial bodies to Jupiter. This gas giant formed at a very early stage, within one million years of our Sun's birth from a dust cloud through gravitational forces. A circular disc of dust and gas, including water ice, formed around the central star, within which planets and asteroids took shape through the process of accretion.
Due to its rapid growth, Jupiter acted as a ‘bridge pillar’ within this disc: it blocked most coarse material, while fine dust from various regions of the orbiting disc flowed around it and mixed evenly in the transition zone near the water-ice boundary. The precursors of Bennu, Ryugu and the CI meteorites subsequently formed in this region.
Solar dust and ice
This scenario also explains why Bennu’s material is rich in water: ice in the vicinity evaporated and some of the water vapour condensed again in the very region where Bennu formed. At the same time, Jupiter’s protective influence ensured that Bennu formed mainly out of fine dust. This explains why Bennu’s material is so chemically similar to that of the Sun: the fine dust orbiting in the disc around the young Sun was thoroughly mixed – a bit like fine dust at home, which simply ends up everywhere over time.
“Bennu may offer our best glimpse of the original mix of chemical elements from which the terrestrial planets were ultimately built,” as Schönbächler states. Bennu is a very primordial asteroid. Its material dates back to the birth of our Solar System around 4.5 billion years ago and has hardly changed since then.
In performing the geochemical analysis of Bennu, the researchers are improving our understanding of how our Solar System arose and under what conditions planets were formed. Due to the fact that 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.
Sampling mission to a Martian moon
“We are now wondering whether other asteroids have the same isotopic signature as Bennu and Ryugu,” Schönbächler goes on to state. It also remains unclear as to what extent the young Jupiter contributed to the fact that only fine dust particles clumped together. Further research will help to clarify this picture.
The analyses of the Bennu material have been completed. The ETH professor is now eagerly awaiting the Japanese sample-return mission to Mars’ moon Phobos, which is due to launch at the end of October this year. “It would be exciting to also obtain this material to analyse it in my laboratory,” she says. And she will be applying to the Japanese space agency JAXA. She will, however, need a good measure of patience: the capsule containing the Phobos material is not expected to return to Earth until the year 2031.
Science Advances
Nucleosynthetic constraints on the origin of Bennu and CI-like asteroids
23-Sep-2026