Samples collected from the 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 previous 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 an orbit around the Sun every 1.2 years and comes relatively close to Earth about every six years, coming within about 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 material from the asteroid landed in the Utah desert. Subsequently, scientists around the world received small portions of the precious cargo for detailed study. Among them was Maria Schönbächler, professor of isotopic geochemistry at ETH Zurich, whose laboratory received half a gram of Bennu material. His 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, researchers at ETH Zurich 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 the chemical fingerprint of a material. These patterns provide clues to 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. He also discovered 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 rare among meteorites recovered on Earth. The Bennu, Ryugu and CI meteorites share strikingly similar isotopic fingerprints, suggesting that their parent materials come from the same deposit 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 early stages of the Solar System. Scientists Discover Bennu’s Unexpected Birthplace For years, researchers generally believed that early asteroids like Bennu originated in the outer Solar System, perhaps in the same distant regions where comets formed. They also suspected that Bennu’s parent material was gathered relatively late in the development of the Solar System. The new findings challenge both assumptions. Instead, the researchers propose that the material that became the 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 and form ice. About 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 from the inner and outer regions mixes near this boundary. The water ice helped the smaller particles stick together, allowing them to gradually build up into larger bodies. “Bennu is a hybrid: the material does not clearly correspond to either the inner or outer Solar System,” says Schönbächler. Its unusual chemical composition suggests that Bennu formed from a combination of materials originating from both regions and not from a single, isolated source. Jupiter may have played a crucial role in the formation of Bennu. The researchers also developed a scenario explaining how this mixing may have occurred, with Jupiter emerging as a key player. Jupiter began to form extraordinarily early in the history of the Solar System, about a million years after the birth of the Sun. 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, forming larger objects through a process called accretion. Over time, some of these growing bodies became asteroids and planets. As Jupiter rapidly increased its mass, its immense gravitational influence began to reshape the distribution of material throughout the disk. The researchers suggest that the growing gas giant acted as a structural barrier, preventing most larger particles from crossing its orbital region while allowing finer dust to move around it. This fine material, coming from different parts of the disk, could mix widely near the ice-water line. According to the proposed scenario, the parent bodies of Bennu and Ryugu, along with the objects that produced the CI meteorites, formed from this completely mixed material. Therefore, Jupiter may have helped determine not only where these ancient asteroids formed but also what ingredients were incorporated into them. Ancient dust and ice reveal ingredients that built Earth The proposed formation scenario could also explain two of Bennu’s most intriguing features: its abundance of water-bearing material and its chemical resemblance to the Sun. Near the boundary between water and ice, ice could evaporate as temperatures changed, releasing water vapor that would then condense again in colder 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 small particles could circulate and mix efficiently throughout the disk, they retained a chemical composition that closely resembles the general elemental mix found in the Sun. The process is somewhat similar to the way fine household dust spreads and settles on many different surfaces rather than staying in one place. This extensive mixing could explain why the Bennu material preserves such a representative sample of the ingredients present during the earliest history of the Solar System. “Bennu may offer our best view of the original mixture of chemical elements from which the terrestrial planets were eventually built,” says Schönbächler. Bennu is considered an exceptionally primitive asteroid because its material formed about 4.5 billion years ago and has undergone relatively little chemical alteration since then. That makes their samples particularly valuable for reconstructing the conditions that existed before Earth and other rocky planets finished forming. By studying the asteroid’s chemistry, scientists can better understand how the Solar System developed and what processes determined the composition of its planets. Bennu’s water minerals and organic compounds also make it important for investigating how Earth obtained some of the chemical ingredients necessary for life. Although these findings do not establish exactly how these materials arrived on our planet, they provide valuable evidence about where these ingredients originated and how they were distributed throughout the young Solar System. A new mission to the Mars Moon could reveal more Despite the new discoveries, several questions remain unanswered. “We now wonder whether other asteroids have the same isotopic signature as Bennu and Ryugu,” continues Schönbächler. 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. Once the analysis of Bennu is complete, Schönbächler hopes to have another opportunity to examine material from the early Solar System. The Japanese space agency JAXA is preparing a sample return mission to Phobos, one of the two moons of Mars. 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 analyze in my laboratory,” he says. Schönbächler plans to ask JAXA for access to the samples. However, the scientific community will have 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.
