Bennu Samples Point to Jupiter's Role in Shaping the Early Solar System
Tiny samples of asteroid Bennu suggest the asteroid formed where material from the inner and outer Solar System mixed, and that Jupiter helped shape the dust that built it. Researchers at ETH Zurich report the result in Science Advances.
NASA's OSIRIS-REx mission collected material from Bennu's surface and delivered it to Earth in 2023: on September 23, 2023, a capsule with about 120 grams landed in the Utah desert. Maria Schönbächler, professor of isotope geochemistry at ETH Zurich, received half a gram of it.
A chemical fingerprint of three elements
The team measured isotopes of iron, titanium and chromium. Isotopes are forms of the same element with different numbers of neutrons, and their relative amounts work as a fingerprint that hints at where material came from. Iron and titanium turned out to be distributed remarkably evenly through Bennu's material.
The fingerprint also revealed a close relationship between Bennu, the asteroid Ryugu and CI meteorites, a rare group of carbon-rich, little-altered remnants of the early Solar System. All three share strikingly similar isotopic signatures, which differ substantially from those of other known asteroids, meteorite groups and planets. This suggests their original material came from the same reservoir of ancient cosmic dust.
A different birthplace than expected
For years researchers believed primitive asteroids like Bennu originated in the outer Solar System, possibly where comets formed, and that their material came together relatively late. The new findings challenge both assumptions. The researchers propose that the parent material of Bennu, Ryugu and CI meteorites accumulated near the water-ice line, the boundary in the disk around the young Sun where it was cold enough for water vapour to freeze.
How Jupiter may have mattered
Jupiter began forming within about a million years of the Sun's birth. As it grew, its gravity may have acted like a barrier, stopping most larger particles from crossing its orbital region while letting fine dust pass. That fine dust, coming from different parts of the disk, could mix extensively near the water-ice line. The scenario could also explain why Bennu is rich in water-bearing material and chemically resembles the Sun, since tiny particles circulate and mix efficiently. "Bennu may offer our best glimpse of the original mix of chemical elements from which the terrestrial planets were ultimately built," Schönbächler says.
The findings do not establish exactly how water-bearing minerals and organic compounds reached Earth, but they show where such ingredients originated. Open questions remain, including whether other asteroids share the Bennu–Ryugu signature. Schönbächler plans to apply for samples from JAXA's Phobos sample-return mission, scheduled to launch at the end of October 2026, though its capsule is not expected back until 2031.
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