When NASA’s OSIRIS-REx spacecraft delivered pristine samples from the asteroid Bennu back to Earth in 2023, scientists knew they were receiving one of the most chemically untouched collections of material ever recovered from the Solar System.
Since then, the returned grains have steadily transformed researchers’ understanding of how asteroids formed, how water altered primitive worlds, and even how the ingredients for life may have spread through the early Solar System.
Now, a new study, published in Nature Communications, has uncovered another surprise hidden inside those microscopic particles. Embedded within Bennu are tiny mineral fragments that formed during the Solar System’s earliest moments—less than 100,000 years after its birth—and they may reveal that the asteroid’s ancient parent body formed much farther from the Sun than scientists once suspected.
The research suggests that Bennu inherited some of the oldest surviving solids ever created, while simultaneously preserving evidence that Jupiter’s formation may have acted as an enormous cosmic filter, separating different populations of dust before planets had even fully formed.
“Our observations of phyllosilicates replacing olivine and interstitial phyllosilicates between olivine grains within AOAs… collectively indicate that these refractory inclusions experienced aqueous alteration on Bennu’s parent planetesimal,” researchers write. “Therefore, they were incorporated into the parent planetesimal before the onset of aqueous alteration, during which phyllosilicates and magnetite formed.”
“These mineralogical, isotopic, and chronological similarities among refractory inclusions in Bennu samples, Ryugu samples, and CI chondrites provide evidence that their parent planetesimal(s) accreted a common suite, and likely similar relative abundances, of high-temperature refractory inclusions and their fragments,” researchers write.
The research concentrates on “refractory inclusions”—tiny mineral-rich objects known as calcium-aluminum-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs). These objects are widely considered the oldest solid materials to condense from the hot gas surrounding the newborn Sun.
Long before planets existed, the infant Solar System consisted of a rotating disk of gas coupled with dust known as the protoplanetary disk. Temperatures near the young Sun exceeded 2,000 degrees Fahrenheit (roughly 1,100 degrees Celsius), allowing only the most heat-resistant minerals to crystallize.
These refractory inclusions were among the very first solids to emerge from that environment before eventually being transported throughout the growing Solar System.
Although scientists have found such inclusions in meteorites for decades, Bennu offered an unprecedented opportunity to examine them in material collected directly from an asteroid rather than from meteorites that survived passage through Earth’s atmosphere.
To investigate, researchers analyzed polished sections prepared from 12 Bennu particles using scanning electron microscopy, transmission electron microscopy, oxygen isotope measurements, and radioactive aluminum-magnesium dating techniques.
The team identified six amoeboid olivine aggregates and three calcium-aluminum-rich inclusions, all remarkably small—none exceeding roughly 100 micrometers across, or about the thickness of a human hair. This turned out to be one of the study’s most important discoveries.
The mineral chemistry and oxygen isotopes showed these inclusions formed in an extremely hot, oxygen-16-rich environment during the Solar System’s earliest stages. Radioactive dating using aluminum-26 indicated at least one inclusion formed essentially at the very beginning of Solar System history, within roughly the first 100,000 years after the earliest known solids condensed.
In other words, Bennu contains fragments that have survived for approximately 4.567 billion years with surprisingly little alteration. Yet, those ancient relics also preserve evidence that they later encountered liquid water.
Several inclusions show minerals partially replaced by phyllosilicates—water-bearing clay minerals—while magnetite and other alteration products reveal that water once circulated through Bennu’s ancient parent body after these primordial objects had already become incorporated into it.
For scientists, that sequence helps establish a timeline. First, the earliest high-temperature solids formed close to the newborn Sun. Later, they were transported outward through the protoplanetary disk. Only after they became part of Bennu’s larger parent asteroid did liquid water alter portions of the rock.
Researchers found distinct similarities between Bennu’s inclusions and those previously identified in samples returned from Japan’s Hayabusa2 mission to asteroid Ryugu, as well as in rare CI carbonaceous chondrite meteorites found on Earth.
The oxygen isotopes, crystal chemistry, and formation ages all closely match, suggesting these seemingly distant objects inherited material from the same population of ancient Solar System building blocks.
That mutual heritage points toward a surprisingly interconnected early Solar System, where material formed close to the Sun was efficiently transported across enormous distances before eventually becoming incorporated into multiple asteroid populations.
However, the absence of something proved just as revealing as what scientists found.
Unlike many carbonaceous meteorites, Bennu contains no large refractory inclusions larger than about one-tenth of a millimeter. Other meteorites frequently preserve inclusions ranging from submillimeter sizes up to several centimeters across.
According to the researchers, the missing population may reflect the increasing influence of the Solar System’s first giant planet.
As proto-Jupiter accumulated mass, its gravity likely created a pressure bump within the gas disk encircling the young Sun. Such pressure bumps act as natural traffic jams for dust, trapping larger particles while allowing much smaller grains to continue drifting through the disk.
Under this scenario, larger refractory inclusions became concentrated closer to Jupiter’s orbit. At the same time, only the smallest particles were able to migrate farther outward into the distant regions where Bennu’s parent body eventually formed.
If correct, Bennu’s microscopic inclusions effectively record the influence of Jupiter before the giant planet had even fully matured.
The hypothesis also helps explain why Bennu, Ryugu, and CI meteorites all possess nearly identical populations of tiny refractory inclusions while lacking the larger varieties commonly found elsewhere.
Rather than showing different source materials, the distinction may instead reflect where—and perhaps when—their parent bodies assembled within the developing protoplanetary disk.
The findings similarly reinforce earlier studies suggesting Bennu is among the Solar System’s most chemically primitive objects. Because large refractory inclusions contain unusually high concentrations of certain elements, their absence helps preserve Bennu’s overall composition close to that of the earliest solar nebula from which the planets formed.
That makes the asteroid an unusually valuable time capsule. Every new microscopic grain examined inside the OSIRIS-REx collection provides another insight into processes that occurred billions of years before Earth existed as a habitable world.
Rather than simply representing rubble from a small asteroid, Bennu appears to preserve a remarkably faithful record of how the Solar System sorted, mixed, transported, and assembled its earliest building materials.
The study also demonstrates why sample-return missions have grown increasingly important to planetary science. Laboratory instruments on Earth can perform measurements impossible for spacecraft operating millions of miles away, letting researchers reconstruct events that occurred within the Solar System’s first fractions of a million years.
“We conclude that Bennu’s parent planetesimal, like those of Ryugu and CI chondrites, accreted small refractory inclusions that persisted in the outer Solar System beyond the influence of the proto-Jupiter-induced pressure bump,” researchers write. “Taken together, these findings highlight the need to integrate Bennu samples into comparative studies of early Solar System solids to better constrain the diversity and possible genetic links among planetesimal formation pathways,” researchers write.
The study, “Refractory inclusions in Bennu samples indicative of outer Solar System accretion,” appeared in Nature Communications.
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com
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