Where asteroid Bennu formed has puzzled planetary scientists since NASA's OSIRIS-REx probe returned around 120 grams of its surface material to Earth. Analyses led by Maria Schönbächler, Professor of Isotope Geochemistry at ETH Zurich, now suggest the asteroid's parent body took shape near the Solar System's water-ice line roughly 4.5 billion years ago, not far out in the Solar System as previously assumed. The findings appear in Science Advances.
Working with about half a gram of the returned sample, the team measured isotopes of iron, titanium and chromium, which act as a chemical fingerprint of an object's birthplace. Bennu turns out to be a blend of dust grains from both the inner and outer Solar System, originally bound together by water ice, and shares its isotopic signature with the asteroid Ryugu and the rare CI meteorites, pointing to a common reservoir of cosmic dust.
The scenario hinges on Jupiter, which formed within about a million years of the Sun and grew quickly enough to act as a barrier, letting fine dust from both regions mix in a narrow transition zone. Because Bennu is rich in water and organic material and has changed little since the Solar System's birth, it preserves clues to the chemical mix that built the terrestrial planets, and perhaps to how Earth acquired the ingredients for life.