The near-Earth asteroid (101955) Bennu has been the focus of research for several years. This asteroid, which is around 500 meters wide and weighs 60 million tons, could come dangerously close to Earth in the distant future, and at the same time it is considered a particularly exciting relic from the early days of the solar system. To learn more about Bennu, NASA sent its OSIRIS-REx spacecraft to the asteroid. This initially collected data from 2018, then took samples from Bennu’s surface in 2020 and brought them back to Earth in 2023.
Mysterious dual nature
The measurement data and initial sample analyzes showed a surprisingly complex composition of the asteroid: Bennu therefore contains bound water, ice and minerals that must have arisen from saline liquids. This suggests that the asteroid must have formed in the cold, outer region of the solar system and its protoplanetary disk. At the same time, however, carbon-containing minerals and silicates were also found in the Bennu samples, the composition of which indicates an origin near the sun.
How can this “dual nature” of the asteroid be explained? Some astronomers suspect that Bennu must have formed relatively late and far out, beyond Jupiter and Saturn. “This model suggests that chondrites of the rare Ivuna type formed in the same region as comets and that they were only transported into the inner solar system later through interactions with Uranus and Neptune,” explain Maria Schönbächler from ETH Zurich and her team. However, some analysis results do not fit this scenario.
View from above into the sample capsule of the OSIRIS-Rex mission. It brought material from the asteroid Bennu back to Earth. — © NASA/Erika Blumenfeld & Joseph Aebersold
Time travel with isotopes
In order to provide more clarity about the origin of Bennu, Schönbächler and her colleagues have now examined isotopes of iron, titanium and chromium in the asteroid samples from Bennu in more detail. The isotope distribution can reveal in which area of the solar cloud the asteroid was once formed and from which material it emerged. “They can therefore provide crucial insights into the formation conditions and region of the asteroid parent bodies,” say the researchers.
The analyzes showed: Bennu differs significantly from other known asteroids in its isotopic composition – but shows great similarity to the asteroid Ryugu and other chondrites of the Ivuna type (CI). “Our isotope data suggest that these CI asteroids were formed in a region of the solar system that contained particles from different areas of the protoplanetary disk,” report Schönbächler and her team. This caused Bennu’s mixing of material from the inner and outer solar systems.
Between the water ice line and young Jupiter
But where was this region? Based on their analyzes and additional models, Schönbächler and her colleagues conclude that Bennu must have formed just outside the water ice line of the young solar system. It was cool enough there for water vapor to condense, freeze and attach to dust grains. “This explains the relatively high water content that we observe in Bennu, Ryugu and CI chrondrites,” say the researchers.
However, another factor was crucial for the unusual material mix of these asteroids: Jupiter. This gas giant was formed a million years after the birth of our sun, earlier than any other planet. As a result, Jupiter’s presence also influenced the flow of particles in the primordial solar cloud: it blocked the passage of coarser material from the outside into the inner solar system and mixed fine particles from both areas through its gravitational influence.
This is what the protoplanetary disk of the solar system may have looked like around 4.5 billion years ago. — © Schönbächler et al./ Science Advances, CC by 4.0
Mixed reservoir
This created a transition zone near Jupiter and the water ice line in which the material of the protoplanetary disk mixed in a characteristic way. The precursors of Bennu, Ryugu and the CI meteorites formed there. “This position allowed them to accumulate material from an inwardly drifting, well-mixed reservoir, while being far enough out to preserve the water- and carbon-rich character of the outer solar system,” the team writes.
The asteroid samples have thus provided valuable insights not only about Bennu and Ryugu, but also about the structure of the early solar system – and thus the conditions under which our Earth was formed.
Source: Maria Schönbächler (Swiss Federal Institute of Technology Zurich) et al., Science Advances, 2026; doi: 10.1126/sciadv.aei9107