NASA's OSIRIS-REx mission delivered Bennu asteroid samples to Earth in September 2023, revealing a chemical fingerprint that challenges theories about how the Solar System formed 4.5 billion years ago.
The 120 grams of material landed in the Utah desert on September 23, 2023. Researchers at ETH Zurich, including Professor Maria Schönbächler, analyzed isotopes of iron, titanium, and chromium in the samples. The analysis, published in Science Advances, found that Bennu shares strikingly similar isotopic signatures with asteroid Ryugu and rare meteorites called CI meteorites.
This chemical kinship suggests these objects formed from the same reservoir of ancient cosmic dust. Their composition differs substantially from other known asteroids, meteorite groups, and planets. The findings overturn earlier assumptions that primitive asteroids like Bennu originated in the outer Solar System late in its development.
Instead, researchers propose that Bennu, Ryugu, and CI meteorites accumulated near the water-ice line, the boundary where temperatures were cold enough for water vapor to freeze. Material from both inner and outer Solar System regions mixed there, with water ice helping particles stick together and grow into larger bodies.
"Bennu is a hybrid: the material does not clearly match either the inner or the outer Solar System," Schönbächler said.
Jupiter played a central role in this process. The gas giant formed within about one million years of the Sun's birth. As it rapidly gained mass, its gravitational influence acted as a barrier, preventing most larger particles from crossing its orbital region while allowing finer dust to circulate and mix extensively near the water-ice line.
This thorough mixing of fine particles preserved a chemical composition in Bennu that closely resembles the overall elemental mixture found in the Sun. Bennu's water-bearing minerals and organic compounds make it valuable for understanding how early Earth obtained chemical ingredients necessary for life.
"Bennu may offer our best glimpse of the original mix of chemical elements from which the terrestrial planets were ultimately built," Schönbächler stated.