Asteroid Bennu, the near-Earth space rock NASA sampled with its OSIRIS-REx spacecraft, probably took shape in a zone influenced by a young, still-growing Jupiter, according to a study published Sept. 23 in Science Advances. Researchers at ETH Zurich propose that Jupiter acted as a filter, holding back coarse material while fine dust from different parts of the early solar system flowed around it and mixed.

The idea matters beyond one asteroid. If the team is right, Bennu's material closely reflects the well-mixed dust of the disk that built the rocky planets, including Earth. "Bennu may offer our best glimpse of the original mix of chemical elements," Maria Schönbächler, professor of isotope geochemistry at ETH Zurich, said in a university statement, referring to the ingredients from which the terrestrial planets were ultimately built.

The study also challenges a common assumption. Scientists had generally thought that asteroids like Bennu formed relatively late and far out in the solar system, possibly in the same region as comets. The new isotope data point to an origin near the boundary where water could freeze into ice.

A Giant Planet as a Dust Filter

Jupiter formed very early, within about 1 million years of the Sun's birth, according to the ETH Zurich team. Around the young Sun circled a disk of dust and gas, including water ice, in which planets and asteroids grew by gradually collecting material.

Because Jupiter grew so quickly, the researchers propose, it acted like a bridge pillar within the disk. It blocked most coarse material, while fine dust from different regions flowed around it and mixed evenly in a transition zone near the water-ice line. The precursors of Bennu, the asteroid Ryugu, and a rare class of primitive, carbon-rich meteorites called CI chondrites then formed in that region.

The scenario fits two features of the data. Fine dust mixes more thoroughly than coarse clumps, which is consistent with the even distribution of iron and titanium isotopes the team measured in Bennu. It also helps explain why Bennu's material is chemically so similar to the Sun, since the fine dust circling the young Sun was thoroughly mixed. The ETH team compared it to household dust, which eventually ends up everywhere.

The Snow Line and Why Bennu Is a Hybrid

The water-ice line, often called the snow line, marks the distance from the young sun beyond which water vapor froze. About 4.5 billion years ago, material from the inner and outer solar system mixed near this boundary, and ice acted as a glue that bound the finest dust particles together.

"Bennu is a hybrid," Schönbächler said, explaining that its material does not clearly match either the inner or the outer solar system. It carries characteristics of both regions because it formed where material flows from both mixed.

The location also offers an explanation for Bennu's water. According to the researchers, ice in the vicinity evaporated, and some of that water vapor condensed again in the region where Bennu formed. Bennu is rich in water and organic material, which is one reason its samples help scientists study how the young Earth acquired the building blocks of life.

Half a Gram of Asteroid, Three Isotope Fingerprints

OSIRIS-REx collected rock and dust from Bennu's surface in October 2020 and returned it to Earth on Sept. 24, 2023, according to NASA Goddard. The capsule landed in the Utah desert carrying about 120 grams (about 4.2 ounces) of material, and Schönbächler's lab received half a gram for analysis.

The team measured isotopes of iron, titanium, and chromium. Isotopes are forms of the same element that differ slightly in mass, and together they act like a chemical fingerprint showing where material came from. Bennu's fingerprint closely matches that of Ryugu, which Japan's Hayabusa2 mission sampled, and the CI chondrites, indicating that all three formed from the same reservoir of cosmic dust. It differs clearly from other known asteroids, meteorite groups, and planets.

Strong Evidence, but Not the Final Word

The isotope measurements are laboratory data published in a peer-reviewed journal. The Jupiter scenario, however, is the team's proposed interpretation of those measurements, not a directly observed event, and it rests on one laboratory's analysis of a small portion of the returned sample.

The researchers acknowledge open questions. Schönbächler said the team now wants to know whether other asteroids "have the same isotopic signature as Bennu and Ryugu." The ETH team also said it remains unclear how much the young Jupiter contributed to the fact that only fine dust particles clumped together.

A broader puzzle remains. Bennu and Ryugu, the two near-Earth asteroids visited by successful sample-return missions, appear to share an origin, and Space.com noted that it is not yet known whether that is coincidence or a clue to where near-Earth asteroids come from.

Bennu is watched for another reason as well. NASA Goddard lists it as a potentially hazardous object with a 1-in-2,700 chance of striking Earth between 2175 and 2199. That is a low probability for a window roughly 150 years away, and the new study does not change it. Understanding what Bennu is made of, however, also helps scientists understand asteroids that could approach Earth.

More samples should help test the idea. China's Tianwen-2 mission reached the near-Earth asteroid Kamo'oalewa this summer and aims to bring a sample back next year, Space.com reported. Japan's Martian Moons eXploration mission plans to return material from the Martian moon Phobos to Earth in 2031, and Schönbächler said she hopes to analyze that material in her lab.

What Readers Want to Know

What did the new Bennu study find?

ETH Zurich researchers found that iron and titanium isotopes are evenly distributed in Bennu and that its isotopic fingerprint matches Ryugu and CI chondrite meteorites. They concluded that Bennu's parent body most likely formed near the solar system's water-ice line.

What role did Jupiter play?

The researchers propose that young Jupiter blocked coarse material while fine dust flowed around it and mixed near the snow line, where Bennu's parent body formed. This is a proposed model that fits the data, not a directly observed event.

What is the snow line?

It is the distance from the young Sun beyond which water vapor froze into ice. Near this boundary, material from the inner and outer solar system mixed, and ice helped bind fine dust together.

Why do scientists call Bennu a hybrid?

Its material does not clearly match either the inner or outer solar system. It carries characteristics of both because it formed where their material flows combined.

How much of the sample did the researchers study?

ETH Zurich received half a gram of the roughly 120 grams of Bennu material that OSIRIS-REx returned to Earth in September 2023.

Is Bennu a threat to Earth?

NASA Goddard lists Bennu as potentially hazardous, with a 1-in-2,700 chance of impact between 2175 and 2199. The new study does not change that estimate.

What comes next?

Scientists want to know whether other asteroids share Bennu's isotopic signature. Sample returns from China's Tianwen-2 mission and Japan's planned Phobos mission could help answer that question.