Asteroid Bennu — the target of NASA's OSIRIS-REx sample return mission, led by the University of Arizona — is a mixture of materials from throughout, and even beyond, our solar system. Over the past few billion years, its unique and varied contents have been transformed by interactions with water and the harsh space environment.
These details come from a trio of newly published papers based on analysis of Bennu samples delivered to Earth by OSIRIS-REx in 2023. The OSIRIS-REx sample analysis campaign is coordinated by the U of A's Lunar and Planetary Laboratory (LPL) and involves scientists from around the world. LPL researchers contributed to all three studies and led two of them.
"This is work you just can't do with telescopes," said Jessica Barnes, associate professor at the U of A's Lunar and Planetary Laboratory and co-lead author on one of the publications. "It's super exciting that we're finally able to say these things about an asteroid that we've been dreaming of going to for so long and eventually brought back samples from."
Bennu is made of fragments from a larger "parent" asteroid that broke up after it collided with another asteroid, likely in the asteroid belt between the orbits of Mars and Jupiter. The parent asteroid consisted of material with diverse origins — near the sun, far from the sun, and from other stars — that coalesced more than 4 billion years ago as our solar system was forming. These findings are the subject of the first paper, published in Nature Astronomy and jointly led by Barnes and Ann Nguyen with the Astromaterials Research and Exploration Science Division at NASA's Johnson Space Center in Houston.
"Bennu's parent asteroid may have formed in the outer parts of the solar system, possibly beyond the giant planets, Jupiter and Saturn," Barnes said. "We think this parent body was struck by an incoming asteroid and smashed apart. Then the fragments re-assembled and this might have repeated several times."
By looking at the samples returned by the OSIRIS-REx spacecraft, Barnes and her colleagues were able to get the most comprehensive snapshot of its history to date. Among the findings was an abundance of stardust, material that existed before our solar system formed, Barnes said. The discovery of these most ancient materials was made possible, in part, by the NanoSIMS instrument at the U of A's Kuiper-Arizona Laboratory for Astromaterials Analysis , which can reveal a sample's isotopes — variants of chemical elements — at nanometer scales. The tiny grains of stardust are identifiable by their unusual isotopic makeup compared to materials formed in the solar system.
"Those are pieces of stardust from other stars that are long dead, and these pieces were incorporated into the cloud of gas and dust from which our solar system formed," Barnes said. "In addition, we found organic material that's highly anomalous in their isotopes and that was probably formed in interstellar space, and we have solids that formed closer to the sun, and for the first time, we show that all these materials are present in Bennu."
The chemical and isotopic similarities between samples from Bennu and a similar asteroid, Ryugu, which was sampled by the Japanese Hayabusa 2 mission in 2019, and the most chemically primitive meteorites found on Earth suggest their parent asteroids may have formed in a shared region of the early solar system. Yet the differences researchers are observing in the Bennu samples may indicate that the starting materials in this region changed over time or were not as well-mixed as some scientists have thought.
The analyses show that some of the materials in the parent asteroid survived various chemical processes involving heat and water and even the energetic collision that resulted in the formation of Bennu. Nevertheless, most of the materials were transformed by hydrothermal processes, as reported in the second paper, published in Nature Geoscience. In fact, that study found, minerals in the parent asteroid likely formed, dissolved and reformed over time due to interactions with water.
"We think that Bennu's parent asteroid accreted a lot of icy material from the outer solar system, which melted over time," said Tom Zega, director of the Kuiper-Arizona Laboratory who co-led the study with Tim McCoy, curator of meteorites at the Smithsonian.
The team found evidence that silicate minerals would have reacted with the resultant liquid water at relatively low temperatures of about 25 degrees Celsius, or room temperature. That heat could have either lingered from the accretion process itself, when Bennu's parent asteroid first formed, or was generated by impacts later in its history, possibly in combination with the decay of radioactive elements deep inside it. The trapped heat could have melted the ice inside the asteroid, according to Zega.
"Now you have a liquid in contact with a solid and heat — everything you need to start doing chemistry," he said. "The water reacted with the minerals and formed what we see today: samples in which 80% of minerals contain water in their interior, created billions of years ago when the solar system was still forming."
The transformation of Bennu's materials did not end there. The third paper, also published in Nature Geoscience, reports microscopic craters and tiny splashes of once-molten rock on the surfaces of Bennu particles — signs that the asteroid has been peppered by micrometeorite impacts. These impacts, together with the effects of solar wind, are known as "space weathering" and occur because Bennu does not have an atmosphere to protect it. This weathering is happening a lot faster than conventional wisdom would have it, according to the study, which was led by Lindsay Keller at NASA Johnson and Michelle Thompson at Purdue University.
As the leftover materials from planetary formation 4.5 billion years ago, asteroids provide a record of the solar system's history. But many of these remnants may be different from what meteorites recovered on Earth would suggest, Zega said, because different types of meteors (fragments of asteroids) may burn up in the atmosphere and never make it to the ground.
"And those that do make it to the ground can react with Earth's atmosphere, particularly if the meteorite is not recovered quickly after it falls," he added, "which is why sample return missions such as OSIRIS-REx are critical."
