The Bennu asteroid sample suggests an ocean world origin

Asteroid Bennu sample: A 4-panel composite showing different samples of gray rock.
View larger. | A small portion of the Bennu asteroid sample returned by NASA’s OSIRIS-REx mission, shown in microscope images. Above left: A dark Bennu particle, about a millimeter long, with a bright phosphate outer crust. The other three panels show progressively enlarged images of a fragment of the particle splitting along a bright vein containing phosphate, captured by a scanning electron microscope. Image via Lauretta & Connolly et al. (2024) Meteoritics & Planetary Science, doi:10.1111/maps.14227 / NASA.

Originally published on June 26, 2024 at Editing by EarthSky.

  • A type of rock reflection found on mid-ocean ridges was found through analysis of the Bennu asteroid sample returned by NASA’s OSIRIS-REx mission in September 2023.
  • Magnesium-sodium phosphate found in the sample hints that the asteroid may have broken off from a small, ancient, primitive oceanic world.
  • Japan’s Hayabusa2 mission also delivered an asteroid sample back to Earth, in 2020. It came from the Ryugu asteroid and discovered a similar phosphate.

Scientists have eagerly awaited the opportunity to dig into the pristine 4.3-ounce (121.6-gram) sample of asteroid Bennu collected by NASA’s OSIRIS-REx spacecraft since it was delivered to Earth last fall. They hoped the material would hold secrets of the solar system’s past and the prebiotic chemistry that might have led to life on Earth. An early analysis of the Bennu sample shows that this excitement was warranted. A type of rock in the sample hints that the asteroid may have been torn apart by a small, primitive, oceanic world long gone.

The OSIRIS-REx sample analysis team found that Bennu contains the original ingredients that formed our solar system. Asteroid dust is rich in carbon and nitrogen, as well as organic compounds, all of which are essential ingredients for life as we know it. The sample also contains magnesium-sodium phosphate, which was a surprise to the research team because it was not seen in remote sensing data collected by the spacecraft on Bennu. Its presence suggests an origin of the oceanic world.

The journal Meteoritics & Planetary Science published this new work on June 26, 2024.

OSIRIS-REx is the first US mission to collect a sample from an asteroid. It launched on September 8, 2016. The spacecraft traveled to the near-Earth asteroid Bennu and collected a sample of rocks and dust from the surface. He delivered the champion to Earth on September 24, 2023.

The spacecraft is currently on its way to the infamous asteroid Apophis, which it will reach in April 2029, just as the asteroid is passing by Earth.

Asteroid Bennu sample reveals phosphate surprise

Analysis of the Bennu sample revealed intriguing insights into the asteroid’s composition. Dominated by clay minerals, particularly serpentine, the sample reflects the type of rock found at mid-ocean ridges on Earth, where material from the mantle, the layer beneath the Earth’s crust, meets water.

This interaction not only results in the formation of clay; It also forms a variety of minerals such as carbonates, iron oxides, and iron sulfides. But the most unexpected discovery is the presence of soluble phosphates in water. These compounds are components of the biochemistry for all life known on Earth today.

While a similar phosphate was found in the asteroid Ryugu sample delivered by JAXA’s (Japan Aerospace Exploration Agency) Hayabusa2 mission in 2020, the magnesium-sodium phosphate discovered in the Bennu sample stands out for its purity—that is, the absence of of other materials in the mineral – and its grain size, unprecedented in any meteorite sample.

The finding of magnesium-sodium phosphates in the Bennu sample raises questions about the geochemical processes that concentrated these elements and provides valuable clues about Bennu’s historical conditions. Dante Lauretta, co-lead author of the paper and principal investigator for OSIRIS-REx at the University of Arizona, Tucson, said:

The presence and condition of phosphates, along with other elements and compounds in Bennu, suggest an aqueous past for the asteroid. Bennu could potentially have once been part of a wetter world. However, this hypothesis requires further investigation.

Jason Dworkin, a co-author of the paper and OSIRIS-REx project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said:

OSIRIS-REx gave us exactly what we hoped for: a large sample of pristine, nitrogen- and carbon-rich asteroids from a once-wet world.

Two men in white suits holding a silver canister between them, in a dark area with white mist billowing from the floor.
Technicians handling the precious sample of asteroid Bennu shortly after the sample was returned in 2023. The sample was headed to a newly constructed clean room at the Johnson Space Center in Houston. Image via NASA.

From a new solar system

Despite its possible history of interaction with water, Bennu remains a chemically primitive asteroid, with elemental dimensions closely resembling those of our sun. Lauretta said:

The sample we returned is the largest reservoir of unchanged asteroid material on Earth right now.

This composition provides a glimpse into the early days of our solar system, over 4.5 billion years ago. These rocks have maintained their original state, neither melting nor solidifying since their inception, affirming their ancient origins.

Suggestions for the building blocks of life

The team has confirmed that the asteroid is rich in carbon and nitrogen. These elements are essential to understanding the environments where Bennu’s materials arose and the chemical processes that transformed simple elements into complex molecules, potentially laying the foundations for life on Earth. Lauretta said:

These findings underscore the importance of collecting and studying material from asteroids like Bennu—especially the low-density material that would normally burn up upon entering Earth’s atmosphere.

This material holds the key to unraveling the complex processes of solar system formation and the prebiotic chemistry that may have contributed to the emergence of life on Earth.

What is expected next?

Dozens of other laboratories in the United States and around the world will receive portions of the Bennu sample from NASA’s Johnson Space Center in Houston in the coming months. And many more scientific papers describing the Bennu sample analyzes are expected in the coming years by the OSIRIS-REx Sample Analysis Team. Harold Connolly, co-lead author of the paper and sample scientist of the OSIRIS-REx mission at Rowan University in Glassboro, New Jersey, said:

The Bennu specimens are extremely beautiful extraterrestrial rocks. Each week, analysis by the OSIRIS-REx sample analysis team provides new and sometimes surprising findings that are helping to place important constraints on the origin and evolution of Earth-like planets.

Graphic showing the arcing path of the capsule from space through the atmosphere to the landing with 6 points marked.
View larger. | This was the path of the asteroid sample after OSIRIS-REx dropped it on Sunday, September 24, 2023. The capsule re-entered Earth’s atmosphere south of San Francisco. The sampler then flew over central California and Nevada as it headed for the landing site in the Utah desert southwest of Salt Lake City. Image from NASA.

Conclusion: NASA’s OSIRIS-REx mission sent a sample of asteroid Bennu to Earth in late 2023. Analysis of the sample shows a type of rocky reflection found on terrestrial mid-ocean ridges.

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