Asteroid Bennu Samples Point to a Birthplace Where Inner and Outer Solar System Met

Dust that NASA brought back from the asteroid Bennu is changing the picture of where such ancient rocks were born. According to ScienceDaily, which summarized work from ETH Zurich, a new chemical analysis suggests Bennu formed from a blend of material from both the inner and outer Solar System, and that the young planet Jupiter helped control the mix.

The study, published in Science Advances, challenges the long-held idea that primitive asteroids like Bennu came together far out in the Solar System, near the region where comets formed.

The numbers behind the Bennu findings

• 4.5 billion years: roughly when the Solar System formed, and the era the samples help researchers understand.
• 120 grams: the approximate amount of asteroid material in the capsule that landed in the Utah desert on Sept. 23, 2023, after NASA's OSIRIS-REx mission collected it from Bennu's surface.
• Half a gram: the portion received by the laboratory of Maria Schönbächler, a professor of isotope geochemistry at ETH Zurich, whose team led the analysis.
• Three elements: iron, titanium and chromium, whose isotopes the team measured to build a chemical fingerprint of the asteroid.
• One million years: about how soon after the Sun's birth Jupiter began to form, according to the researchers' scenario.
• 1.2 years: the time Bennu takes to orbit the Sun once. It passes within roughly 300,000 kilometers (186,000 miles) of Earth about every six years.

What the chemical fingerprint shows

Isotopes are versions of the same element with different numbers of neutrons, and therefore slightly different masses. Measuring their relative amounts gives scientists a signature that can hint at where a material's ingredients came from and, in some cases, when they formed.

The team found that iron and titanium are spread remarkably evenly through the Bennu material. They also found a close match between Bennu, the asteroid Ryugu and a rare group of primitive meteorites called CI meteorites. These are carbon-rich leftovers from the early Solar System that have changed relatively little over billions of years, and they are very uncommon among meteorites found on Earth.

According to the report, the three share strikingly similar isotopic fingerprints, which suggests they grew from the same reservoir of ancient cosmic dust. Their signatures differ substantially from those of other known asteroids, meteorite groups and planets.

A hybrid born near the water-ice line

Based on that match, the researchers propose that the material for Bennu, Ryugu and the CI meteorites gathered near the water-ice line. This is the boundary in the disk around the young Sun where it was cold enough for water vapor to freeze. Dust from both the inner and outer regions may have mixed there, and ice would have helped the smallest grains stick together and build larger bodies.

“Bennu is a hybrid,” Schönbächler said, explaining that its material does not clearly match either region. The study also questions an earlier assumption that Bennu's original material assembled relatively late in the Solar System's history.

How Jupiter may have shaped the mix

The team's scenario gives Jupiter a central role. As the giant planet grew quickly, its gravity reshaped how material was spread through the disk of gas, dust and ice around the Sun. The researchers suggest it acted somewhat like a barrier that kept most larger particles from crossing its orbital region while letting finer dust drift past it.

That fine dust, coming from different parts of the disk, could then mix thoroughly near the water-ice line. In this picture, the parent bodies of Bennu and Ryugu, along with the objects that produced CI meteorites, formed from that well-mixed material. Jupiter may therefore have influenced both where these asteroids formed and which ingredients ended up inside them.

Why it matters

For readers, the result is a reminder of what a sample-return mission can do. A few grams of dust, studied in laboratories around the world, can test theories about how planets and asteroids took shape. It also points to a more connected early Solar System than the older picture of separate inner and outer zones, with the largest planet acting as a gatekeeper for the material around it.

The finding is a proposed scenario drawn from one set of measurements. It rests on a single research group's analysis and may be tested further as other teams continue studying their portions of the Bennu samples.

Around Prague, based on reporting by ScienceDaily