ETH Zurich Solves Mystery of Asteroid Bennu's Origin

The asteroid Bennu orbits the Sun once every 1.2 years and passes within about 300,000 kilometres of Earth every six years. NASA took advantage of one such approach to collect material from the asteroid, retrieving samples from Bennu's surface with its OSIRIS-REx probe in a 2023 operation. The sample container landed in the Utah desert carrying around 120 grams of material, and a small portion eventually reached ETH Zurich, where Maria Schonbachler, Professor of Isotope Geochemistry, received half a gram for analysis.

The results, now published in the journal Science Advances, reveal both the chemical fingerprint of Bennu's minerals and new insight into how the Solar System itself formed.

The ETH team analysed isotopes of iron, titanium and chromium, atoms of the same element that differ slightly in mass, which together create a distinctive fingerprint researchers can use to trace an object's origin and age. The measurements show titanium and iron distributed uniformly throughout Bennu, and reveal that Bennu has close relatives: the asteroid Ryugu and a class of rare, primitive, carbon-rich meteorites known as CI meteorites. All three bodies share a similar isotopic fingerprint, indicating they formed from the same reservoir of cosmic dust, and all three differ significantly from other known asteroids, meteorite groups and planets.

Scientists had previously assumed that asteroids like Bennu formed in the outer Solar System, in roughly the same region as comets, and relatively late in the Solar System's evolution. The new data contradicts both assumptions. The most likely scenario instead places the birthplace of Bennu, Ryugu and the CI meteorites close to the water-ice line, the boundary marking where water vapor freezes. There, some 4.5 billion years ago, as the Solar System was still taking shape, material from its inner and outer regions mixed, with ice acting as a kind of glue binding the finest dust particles together.

Schonbachler describes Bennu as a hybrid whose material does not clearly match either the inner or outer Solar System, but instead bears characteristics of both, having formed in a specific zone where material from both regions mixed.

Schonbachler and her co-authors propose a scenario in which Jupiter played a key role in the formation of these bodies. The gas giant formed unusually early, within about a million years of the Sun's birth from a collapsing dust cloud. A circular disc of dust and gas, including water ice, formed around the young Sun, within which planets and asteroids took shape through accretion. Because Jupiter grew so quickly, it acted as a kind of barrier within this disc, blocking most coarse material while allowing fine dust from different regions of the disc to flow around it and mix evenly in the transition zone near the water-ice boundary. The precursors of Bennu, Ryugu and the CI meteorites are thought to have formed in this same region.

This scenario also explains why Bennu's material is rich in water: ice nearby evaporated, and some of the resulting water vapor condensed again in the same region where Bennu formed. Jupiter's protective effect meanwhile ensured that Bennu formed mainly from fine, thoroughly mixed dust, which also explains why its chemistry closely resembles that of the Sun itself.

Schonbachler notes that Bennu may offer one of the best available glimpses of the original mix of chemical elements from which the terrestrial planets were ultimately built. Bennu is considered an especially primordial asteroid, its material dating back to the Solar System's birth around 4.5 billion years ago and having changed little since. Because the asteroid is rich in water and organic material, the findings also help fill in how the young Earth may have acquired some of the building blocks of life.

Schonbachler says the team is now curious whether other asteroids share the same isotopic signature as Bennu and Ryugu, and how much the young Jupiter contributed to fine dust clumping together in this way, questions further research should help clarify.

With the Bennu analyses complete, Schonbachler is now looking ahead to Japan's planned sample-return mission to the Martian moon Phobos, due to launch at the end of October. She intends to apply to the Japanese space agency JAXA for access to that material as well, though she will need patience: the Phobos sample capsule is not expected to return to Earth until 2031.

CONTACT: https://doi.org/10.1126/sciadv.aei9107