Samples returned to Earth by NASA’s OSIRIS-REx mission are offering new clues about how the solar system formed roughly 4.5 billion years ago. Chemical analysis of asteroid Bennu suggests that its building materials originated from both the inner and outer solar system and accumulated near the boundary where water freezes into ice. The findings also indicate that the young Jupiter may have helped mix and redistribute the dust that eventually formed Bennu, the asteroid Ryugu and a rare group of primitive meteorites.
Bennu orbits the Sun once every 1.2 years and passes relatively close to Earth about every six years, coming within roughly 300,000 kilometers (186,000 miles). NASA’s OSIRIS-REx spacecraft traveled to the asteroid, collected material from its surface and successfully delivered the sample to Earth in 2023.
On September 23, 2023, a capsule containing approximately 120 grams of asteroid material landed in the Utah desert. Scientists around the world subsequently received portions of the valuable cargo for detailed study.
Among them was Maria Schönbechler, a professor of isotope geochemistry at ETH Zurich. Her laboratory received half a gram of Bennu material, and her team began examining it for clues about the asteroid’s history. The research has revealed distinctive chemical signatures that help pinpoint Bennu’s origins and provide new evidence about the processes that shaped the early solar system.
Bennu’s chemical fingerprints reveal 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 with different numbers of neutrons and slightly different masses. By measuring the relative amounts of these isotopes, scientists can identify a material’s chemical fingerprint. These patterns can indicate where an asteroid’s components originated and, in some cases, when they formed.
The analysis found that iron and titanium are distributed surprisingly evenly throughout Bennu’s material. It also revealed 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 change over billions of years. They are extremely rare 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 differ significantly from those of other known asteroids, meteorites and planets.
These similarities provide important clues about where the objects formed and the conditions that existed during the solar system’s early stages.
Scientists identify Bennu’s unexpected birthplace
For many years, researchers generally believed that primitive asteroids such as Bennu originated in the outer solar system, perhaps in the distant regions where comets formed. They also suspected that Bennu’s original material assembled relatively late in the solar system’s development.
The new findings challenge both assumptions.
The researchers instead propose that the materials that became Bennu, Ryugu and the CI meteorites accumulated near a boundary known as the water-ice interface.
This boundary marks a region in the disk surrounding the young Sun where temperatures were low enough for water vapor to freeze into ice. About 4.5 billion years ago, it may have been an important location where material moving from different parts of the developing solar system converged.
Dust from the inner and outer regions could have mixed near this boundary. Water ice would have helped the smallest particles stick together, allowing them to gradually build larger clumps.
“Bennu is a hybrid; the material clearly does not match anything inside or outside the solar system,” Schönbechler said.
Bennu’s unusual composition suggests that it formed from a combination of materials from both regions rather than from a single, isolated source.
Jupiter may have helped shape Bennu
The researchers also developed a scenario to explain how this mixing occurred, with Jupiter playing a potentially important role.
Jupiter began forming very early in the solar system’s history, within about 1 million years of the Sun’s birth. At the time, the young Sun was surrounded by a rotating disk of gas, dust and ice.
Within this disk, millions of tiny particles 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’s mass rapidly increased, its powerful gravity began reshaping the distribution of matter throughout the disk.
The researchers suggest that the growing gas giant may have acted as a kind of tectonic barrier. It could have prevented larger particles from crossing its orbital region while allowing finer dust to move around it.
Fine material from different parts of the disk could then have mixed extensively near the water-ice line.
Under this proposed scenario, the parent bodies of Bennu and Ryugu, along with the objects that produced the CI meteorites, formed from this thoroughly mixed material.
Jupiter may therefore have helped determine not only where these ancient asteroids formed, but also which components became part of them.
Ancient dust and ice offer clues to Earth’s origins
The proposed formation scenario could also explain two of Bennu’s notable characteristics: its abundance of water-bearing materials and its chemical similarity to the Sun.
Near the water-ice interface, ice could evaporate as temperatures changed, releasing water vapor that later condensed in colder regions. This process may have concentrated water in the material that formed Bennu.
At the same time, Jupiter’s influence may have limited the movement of larger particles, leaving the asteroid’s source material dominated by fine dust.
These tiny particles could circulate and mix efficiently throughout the disk, preserving a chemical composition that closely resembled the overall mixture of elements found in the Sun.
The process is similar to the way fine household dust spreads and settles on different surfaces instead of remaining in one place.
This extensive mixing may explain why Bennu’s material retains a representative sample of the components present during the solar system’s early history.
“Bennu may offer the best glimpse of the original mix of chemical elements from which terrestrial planets were ultimately constructed,” Schönbechler said.
Bennu is considered a very primitive asteroid because its material formed about 4.5 billion years ago and has undergone relatively few chemical changes since then. Its samples are therefore particularly valuable for reconstructing conditions before Earth and the other rocky planets completed their formation.
Studying asteroid chemistry can help scientists understand how the solar system developed and which processes determined the planets’ compositions.
Bennu’s water-bearing minerals and organic compounds are also relevant to understanding how early Earth acquired some of the chemical components needed for life.
Although the discoveries do not prove exactly how these materials reached Earth, they provide evidence about where such components originated and how they were distributed throughout the young solar system.
A future mission to a Martian moon could provide more evidence
Several questions remain unanswered. “We are now wondering whether other asteroids have the same isotopic signature as Bennu and Ryugu,” Schönbechler said.
The researchers also want to determine more precisely how Jupiter influenced the movement of dust and whether its gravity played a particularly important role in concentrating tiny particles.
Additional asteroid samples could help scientists test these proposed formation scenarios and establish whether Bennu’s chemical signatures are shared by other ancient objects.
Schönbechler is also pursuing other opportunities to study 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 and aims to collect material from Mars’ moons and return it to Earth.
“It would be very exciting to have this material and be able to analyze it in my lab,” she said.
Schönbechler plans to apply to JAXA for access to the samples. The scientific community will have to wait, however: the capsule carrying material from Phobos is not scheduled to return to Earth until 2031.
Those future samples could provide another important piece of the puzzle, helping researchers understand how different regions of the early solar system exchanged material and ultimately produced the planets, moons and asteroids observed today.
Source: www.sciencedaily.com