Bennu Samples Rewrite the Asteroid's Birthplace — and Jupiter's Role

The ETH Zurich team instead places Bennu's building material near the water-ice line — the zone in the disk around the young Sun where temperatures dropped low enough for water vapor to freeze. Ice helped the smallest dust grains stick together and gradually build larger bodies.

"Bennu is a hybrid: the material does not clearly match either the inner or the outer Solar System," Schönbächler said. Its composition points to ingredients from both regions, not a single isolated source.

How does Jupiter fit in?

The researchers propose that Jupiter, which began forming within about one million years of the Sun's birth, reshaped the disk as it grew. Its gravity acted like a structural barrier: larger particles could not cross its orbital region, but fine dust moved around it freely. That dust — from different parts of the disk — then mixed thoroughly near the water-ice line, and the parent bodies of Bennu, Ryugu and the CI meteorites formed from that blend.

The scenario also explains two of Bennu's oddities. Ice evaporating and recondensing near the boundary concentrated water in its parent material, accounting for its abundance of water-bearing minerals. And because fine dust circulates and mixes efficiently — much as household dust settles across every surface in a home — Bennu preserves a chemical composition closely resembling the Sun's overall elemental mixture.

"Bennu may offer our best glimpse of the original mix of chemical elements from which the terrestrial planets were ultimately built," Schönbächler said.

Why it matters — and what remains open

Bennu counts as exceptionally primitive: its material formed about 4.5 billion years ago and has changed little since. That makes the samples valuable for reconstructing conditions before Earth and the other rocky planets finished forming, and its water-bearing minerals and organic compounds bear on how early Earth obtained ingredients relevant to life. The findings do not establish how those materials reached Earth — only where such ingredients originated and how they spread through the young Solar System.

Questions remain. "We are now wondering whether other asteroids have the same isotopic signature as Bennu and Ryugu," Schönbächler said. Researchers also want to pin down how Jupiter moved dust and whether its gravity favored the accumulation of especially fine particles.

More samples may soon help. JAXA, Japan's space agency, is preparing a sample-return mission to Phobos, one of Mars' two moons, scheduled to launch at the end of October 2026, with the capsule returning in 2031. Schönbächler plans to apply to JAXA for material. "It would be exciting to also obtain this material to analyse it in my laboratory," she said.