Primitive dust samples returned from asteroid Bennu reveal that the space rock formed near the ancient water-ice boundary roughly 4.5 billion years ago. Researchers analyzing iron, titanium, and chromium isotopes discovered a hybrid composition that bridges the inner and outer Solar System.
Scientists examining material brought back by NASA’s OSIRIS-REx mission have uncovered fresh details regarding how the primordial building blocks of the planets assembled. The spacecraft visited the near-Earth asteroid between 2018 and 2021 before parachuting a sample of 0.26 pounds (120 grams) of regolith from Bennu down to Earth on Sept. 23, 2023.
That cargo was subsequently shared out among researchers around the globe, with half a gram being received by Schönbächler’s lab. Among these shipments, portions arrived at an isotope geochemistry facility managed by ETH Zurich in Switzerland, where scientists set out to analyze isotopes—atomic variants distinguished by varying neutron counts—specifically focusing on iron, titanium and chromium within the material.
Isotopic Signatures Link Bennu to Ryugu and CI Chondrites
By comparing elemental variations across samples, researchers identified a distinct chemical fingerprint. Analyzing distinct Bennu specimens across two separate laboratories, the team discovered that iron and titanium signatures remained remarkably uniform in every sample, whereas chromium displayed minor fluctuations that remain only partially accounted for.
These isotopic traits closely match those found in asteroid Ryugu, from which Japan’s Hayabusa2 mission has also obtained a sample, as well as a rare breed of meteorite called CI chondrites that also share a similar isotopic composition with Bennu and Ryugu, suggesting a common birthplace. Both Bennu and Ryugu are pieces of larger bodies that at some point were smashed apart.
“Bennu may offer our best glimpse of the original mix of chemical elements from which the terrestrial planets were ultimately built,” said Maria Schönbächler, a professor of isotope geochemistry at ETH Zurich in Switzerland, in a statement.
Maria Schönbächler, isotope geochemist at ETH Zurich
The Snow Line and Jupiter’s Filtering Role
Although previous hypotheses suggested these parent bodies originated far out in the outer solar system alongside comets—potentially developing later than planets due to sluggish evolutionary rates in those distant zones—the latest measurements point instead to an origin situated just beyond the ancient water-ice boundary rather than deep within comet territory. This places Bennu’s parent material in a transition zone between inner and outer Solar System reservoirs. The isotopic uniformity points to an assembly derived from minute dust particles instead of larger, pebbly formations.
Scientists suggest that juvenile Jupiter functioned as a particle-size barrier, holding back bigger debris while permitting fine dust to traverse its orbital path and blend into the constituents that ultimately constructed bodies resembling Bennu. During its growth, the massive gas giant cleared out a clear path within the primordial disk of debris, impeding heavy rocks while allowing microscopic dust to continue mixing.
Heavy, coarser rocks and ice chunks from the outer system were blocked from drifting inward. Fine dust grains were able to cross its orbit and become incorporated into the material that eventually formed Bennu-like bodies. Water-ice that sublimated just inside the snow line supplied water-bearing minerals to the accretion zone.
Primordial Elements Shared With the Sun
Because the incoming dust was fine and well-mixed, the resulting parent body preserved a chemical composition remarkably close to the average composition of the Solar System itself. This hybrid derivation resolves a long-standing scientific conundrum, given that Bennu’s parent asteroid appears to have gathered matter spanning a far wider expanse of the solar system than a single compact body ought to encompass.
The mineral grains and dust comprising Bennu preserve elemental signatures originating from both the inner and outer zones of the solar system rather than being restricted to a solitary area. Its iron and titanium in particular were found to be well mixed throughout, demonstrating that the asteroid inherited traits from both regions before its parent body was eventually smashed apart.