Bennu asteroid samples point to a surprising origin involving Jupiter

The asteroid Bennu orbits the sun once every 1.2 years and approaches Earth every six years, passing within around 300,000 kilometers. 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 Sept. 23, 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. Her laboratory began analyzing the samples immediately.

The investigations have now been completed, and the results have been published in Science Advances. They reveal the chemical fingerprint of Bennu’s minerals and provide new insights into how our solar system formed.

Fingerprint and close relatives

The ETH researchers analyzed 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 researchers can use to determine the origin and, to some extent, the age of the asteroid.

The 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 found only very rarely on Earth. All three share a similar isotopic fingerprint, indicating that they formed from the same reservoir of cosmic dust. They also differ significantly in isotopic composition from other known asteroids, meteorite groups and planets.

Where and how did Bennu form?

Scientists had assumed that asteroids such as Bennu formed in the outer regions of the solar system, possibly where comets formed. They also thought Bennu formed relatively late in the solar system’s evolution. The new data contradict both ideas.

The most likely scenario is that Bennu, Ryugu and the CI meteorites formed close to the water-ice line. This boundary marks the point where water vapor freezes. Here, 4.5 billion years ago, as the solar system was still taking shape, materials from its inner and outer regions mixed. The ice 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,” Schönbächler says. It bears characteristics of both regions—and formed in a zone where material flows from both regions mixed.

New scenarios

Schönbächler and her co-authors propose a scenario in which Jupiter played a key role in the formation of these celestial bodies. The gas giant formed early, within 1 million years of the sun’s birth from a dust cloud through gravitational forces. A circular disk of dust and gas, including water ice, formed around the young sun. Within the disk, planets and asteroids took shape through accretion.

Because of its rapid growth, Jupiter acted as a “bridge pillar” within the disk: It blocked most coarse material, while fine dust from various regions of the orbiting disk 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 vapor condensed again in the region where Bennu formed. At the same time, Jupiter’s protective influence ensured that Bennu formed mainly from fine dust. This explains why Bennu’s material is so chemically similar to that of the sun: The fine dust orbiting in the disk around the young sun was thoroughly mixed—a bit like dust at home, which 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,” Schönbächler says. 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.

By analyzing Bennu’s geochemistry, the researchers are improving our understanding of how the solar system arose and under what conditions planets formed. Because the asteroid is rich in water and organic material, it also provides clues to 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 says. It also remains unclear to what extent the young Jupiter contributed to the clumping of only fine dust particles. Further research will help clarify the picture.

The analyses of Bennu’s material are complete. Schönbächler 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 analyze it in my laboratory,” she says. She plans to apply to the Japanese space agency JAXA, but she will need patience: The capsule containing the Phobos material is not expected to return to Earth until 2031.

Who’s behind this story?

Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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