New sample analyses by researchers at ETH Zurich point to a surprising origin for the asteroid Bennu: rather than forming far out in the Solar System, Bennu's parent body probably emerged in a special transition zone near the water-ice line, where temperatures were cold enough for water to freeze, at a time when Jupiter itself was still growing.
In brief
ETH researchers analysed material collected by NASA on the asteroid Bennu and determined its chemical signature using various isotopes.
Asteroid Bennu consists of a mixture of dust grains and aggregates from the inner and outer Solar System, originally hold together by water ice.
It is thought to have formed near the water-ice line, where there was sufficient water ice and material flows mixed extensively.
The asteroid Bennu orbits the Sun once every 1.2 years and approaches Earth every six years, passing within around 300,000 kilometres. NASA took advantage of this to collect material from the celestial body. In a spectacular operation in 2023, the US space agency collected samples from the surface of the asteroid Bennu by its OSIRIS-REx probe.
On 23 September, the sample container landed in the Utah desert, carrying around 120 grams of material from Bennu. From there, a small but precious portion made its way to ETH Zurich: Maria Schönbächler, Professor of Isotope Geochemistry, received half a gram for analysis. The ETH researcher's laboratory began analysing the samples immediately.
The investigations have now been completed, and the results have just been published in the Science Advances journal. They not only reveal the chemical fingerprint of the Bennu minerals but also provide new insights into how our Solar System formed.
Fingerprint and close relatives
The ETH researchers analysed isotopes of iron, titanium and chromium. Isotopes are atoms of the same element that differ slightly in mass. Together, they create a distinctive fingerprint that the researchers can use to determine the origin and, to some extent, the age of the asteroid.
These measurements show that titanium and iron are uniformly distributed throughout Bennu. They also reveal that Bennu has some close relatives: the asteroid Ryugu and the so-called CI meteorites, a class of primitive, carbon-rich rocky bodies that are found only very rarely on Earth. All three celestial bodies share a similar isotopic fingerprint, indicating that they formed from the same reservoir of cosmic dust. They also differ significantly in their isotopic composition from other known asteroids, meteorite groups and planets.
Where and how did Bennu form?
To date, scientists had assumed that asteroids such as Bennu formed in the outer regions of the Solar System, possibly in the same place where comets formed. While they opined that Bennu was formed relatively late in the evolution of our Solar System, the new data contradicts both ideas.
The most likely scenario is that the birthplace of Bennu, Ryugu and the CI meteorites was close to the water-ice line. This boundary marks the point where water vapour freezes. Here, 4.5 billion years ago, as Solar System was still taking shape, materials from its inner and outer region mixed. The ice, in turn, acted as a 'glue', binding the finest dust particles together.
"Bennu is a hybrid: the material does not clearly match either the inner or the outer Solar System," says Schönbächler. It bears characteristics of both regions - and was formed in a specific zone where material flows from both regions mixed.
