Samples collected from asteroid Bennu by NASA are revealing surprising new details about how our Solar System formed approximately 4.5 billion years ago. A new chemical analysis challenges earlier theories about the asteroid’s birthplace and suggests that Jupiter played an important role in shaping the materials from which Bennu and other ancient objects formed.
Bennu completes one orbit around the Sun every 1.2 years and makes a relatively close approach to Earth about every six years, coming within roughly 300,000 kilometers (186,000 miles). NASA’s OSIRIS-REx mission traveled to the asteroid, collected material from its surface, and successfully delivered the samples to Earth in 2023.
On September 23, 2023, a capsule carrying approximately 120 grams of asteroid material landed in the Utah desert. Scientists around the world subsequently received small portions of the precious cargo for detailed study.
Among them was Maria Schönbächler, Professor of Isotope Geochemistry at ETH Zurich, whose laboratory received half a gram of Bennu material. Her team immediately began examining the samples for clues about the asteroid’s history.
The research, now published in Science Advances, has uncovered a distinctive chemical signature that not only helps identify Bennu’s origins but also provides new evidence about the processes that shaped the early Solar System.
Bennu’s Chemical Fingerprint Reveals Its Cosmic Relatives
To investigate Bennu’s origins, the ETH Zurich researchers examined isotopes of three elements: iron, titanium, and chromium.
Isotopes are different forms of the same chemical element that contain varying numbers of neutrons, giving them slightly different masses. By measuring the relative amounts of these isotopes, scientists can identify a material’s chemical fingerprint. These patterns provide clues about where an asteroid’s ingredients originated and, in some cases, when they formed.
The analysis revealed that iron and titanium are distributed remarkably evenly throughout Bennu’s material. It also uncovered a close chemical relationship between Bennu, the asteroid Ryugu, and a rare group of primitive meteorites known as CI meteorites.
CI meteorites are carbon-rich remnants of the early Solar System that have undergone relatively little alteration over billions of years. They are exceptionally uncommon among meteorites recovered on Earth.
Bennu, Ryugu, and CI meteorites share strikingly similar isotopic fingerprints, suggesting that their original materials came from the same reservoir of ancient cosmic dust. Their chemical signatures also differ substantially from those of other known asteroids, meteorite groups, and planets.
These similarities offer an important clue about where these objects formed and what conditions existed during the Solar System’s earliest stages.
Scientists Discover Bennu’s Unexpected Birthplace
For years, researchers generally believed that primitive asteroids like Bennu originated in the outer Solar System, perhaps in the same distant regions where comets formed. They also suspected that Bennu’s original material came together relatively late in the Solar System’s development.
The new findings challenge both assumptions.
Instead, the researchers propose that the material that became Bennu, Ryugu, and CI meteorites accumulated near a boundary known as the water-ice line.
This boundary marks the region in the disk surrounding the young Sun where temperatures were low enough for water vapor to freeze into ice. Around 4.5 billion years ago, it may have been a particularly important meeting place for material traveling from different parts of the developing Solar System.
Dust originating in both the inner and outer regions mixed near this boundary. Water ice helped the smallest particles stick together, allowing them to gradually accumulate into larger bodies.
“Bennu is a hybrid: the material does not clearly match either the inner or the outer Solar System,” says Schönbächler.
Its unusual chemical composition suggests that Bennu formed from a combination of materials originating in both regions rather than from a single, isolated source.
Jupiter May Have Played a Crucial Role in Bennu’s Formation
The researchers also developed a scenario explaining how this mixing could have occurred, with Jupiter emerging as a key player.
Jupiter began forming extraordinarily early in Solar System history, within approximately one million years of the Sun’s birth. At that time, the young Sun was surrounded by a rotating disk of gas, dust, and ice.
Within this disk, countless small particles gradually collided and stuck together, building larger objects through a process called accretion. Over time, some of these growing bodies became asteroids and planets.
As Jupiter rapidly increased in mass, its immense gravitational influence began reshaping the distribution of material throughout the disk.
The researchers suggest that the growing gas giant acted somewhat like a structural barrier, preventing most larger particles from crossing its orbital region while still allowing finer dust to move around it.
This fine material, originating from different parts of the disk, could then mix extensively near the water-ice line.
According to the proposed scenario, the parent bodies of Bennu and Ryugu, along with the objects that produced CI meteorites, formed from this thoroughly mixed material.
Jupiter may therefore have helped determine not only where these ancient asteroids formed but also which ingredients became incorporated into them.
Ancient Dust and Ice Reveal the Ingredients That Built Earth
The proposed formation scenario could also explain two of Bennu’s most intriguing characteristics: its abundance of water-bearing material and its chemical resemblance to the Sun.
Near the water-ice boundary, ice could evaporate as temperatures changed, releasing water vapor that later condensed again in cooler regions. This process would have helped concentrate water in the material from which Bennu formed.
Meanwhile, Jupiter’s influence may have limited the movement of larger particles, leaving the asteroid’s parent material dominated by fine dust.
Because these tiny particles could circulate and mix efficiently throughout the disk, they preserved a chemical composition that closely resembles the overall elemental mixture found in the Sun.
The process is somewhat similar to the way fine household dust spreads and settles across many different surfaces rather than remaining in one location.
This extensive mixing could explain why Bennu’s material retains such a representative sample of the ingredients present during the Solar System’s earliest history.
“Bennu may offer our best glimpse of the original mix of chemical elements from which the terrestrial planets were ultimately built,” as Schönbächler states.
Bennu is considered an exceptionally primitive asteroid because its material formed approximately 4.5 billion years ago and has experienced relatively little chemical alteration since then.
That makes its samples particularly valuable for reconstructing the conditions that existed before Earth and the other rocky planets finished forming.
By studying the asteroid’s chemistry, scientists can better understand how the Solar System developed and which processes determined the composition of its planets.
Bennu’s water-bearing minerals and organic compounds also make it important for investigating how early Earth obtained some of the chemical ingredients necessary for life.
Although these findings do not establish exactly how those materials reached our planet, they provide valuable evidence about where such ingredients originated and how they were distributed throughout the young Solar System.
A New Mission to Mars’ Moon Could Reveal More
Despite the new discoveries, several questions remain unanswered.
“We are now wondering whether other asteroids have the same isotopic signature as Bennu and Ryugu,” Schönbächler goes on to state.
Researchers also want to determine more precisely how Jupiter influenced the movement of dust and whether its gravitational effects were responsible for favoring the accumulation of particularly fine particles.
Additional asteroid samples could help scientists test the proposed formation scenario and establish whether Bennu’s chemical signature is shared by other ancient bodies.
With the Bennu analysis now complete, Schönbächler is looking ahead to another opportunity to examine material from the early Solar System.
Japan’s space agency, JAXA, is preparing a sample-return mission to Phobos, one of Mars’ two moons. The mission is scheduled to launch at the end of October 2026, with the goal of collecting material from the Martian moon and bringing it back to Earth.
“It would be exciting to also obtain this material to analyse it in my laboratory,” she says.
Schönbächler plans to apply to JAXA for access to the samples. However, the scientific community will need to be patient. The capsule carrying material from Phobos is not expected to return to Earth until 2031.
Those future samples could provide another important piece of the puzzle, helping researchers determine how different regions of the early Solar System exchanged material and ultimately produced the planets, moons, and asteroids we observe today.
