Life's building blocks found in Bennu asteroid sample

A new analysis of samples from the asteroid Bennu, NASA’s first asteroid sample captured in space and delivered to Earth, reveals that evaporated water left a briny broth where salts and minerals allowed the elemental ingredients of life to intermingle and create more complex structures. The discovery suggests that extra-terrestrial brines provided a crucial setting for the development of organic compounds.

In a paper published in the journal Nature, scientists at the Smithsonian’s National Museum of Natural History describe a sequence of evaporated minerals that date back to the early formation of the solar system. The assortment of minerals includes compounds that have never been observed in other extra-terrestrial samples.

“We now know from Bennu that the raw ingredients of life were combining in really interesting and complex ways on Bennu’s parent body,” said Tim McCoy, the museum’s curator of meteorites and co-lead author on the new paper. “We have discovered that next step on a pathway to life.”

Bennu’s parent asteroid, which formed around 4.5 billion years ago, seems to have been home to pockets of liquid water. The new findings indicate that water evaporated and left behind brines that resemble the salty crusts of dry lakebeds on Earth.

A historic mission

Bennu has long intrigued researchers due to its near-Earth orbit and carbon-rich composition. Scientists posited that the asteroid contained traces of water and organic molecules and theorised that similar asteroids could have brought these materials to a primordial Earth.

In 2020, NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer) spacecraft collected samples from Bennu, becoming the first US space mission to collect a sample from the surface of an asteroid and the only sample collected from a planetary body in nearly 50 years. In September 2023, as OSIRIS-REx soared past Earth, it dropped a capsule containing the Bennu samples. When the capsule touched down in the Utah desert, scientists were on site to retrieve it and protect the samples inside from terrestrial contamination.

“The clues we’re looking for are so minuscule and so easily destroyed or altered from exposure to Earth’s environment,” noted Danny Glavin, a senior sample scientist at NASA’s Goddard Space Flight Center and co-lead author of a second paper in Nature Astronomy. “That’s why some of these new discoveries would not be possible without a sample-return mission, meticulous contamination-control measures, and careful curation and storage of this precious material from Bennu.”

In total, OSIRIS-REx collected around 120 grams of material, which is about the weight of a bar of soap and double the mission-required amount. The invaluable samples were divvied up and sent to researchers around the world to analyse. NASA sent the Smithsonian multiple Bennu samples, which were analysed by McCoy and his colleagues using the museum’s state-of-the-art scanning electron microscope. This allowed the researchers to inspect microscopic features on asteroid fragments less than a micrometre — or 1/100th the width of a human hair — in size.

Jason Dworkin holds up a vial that contains part of the sample from asteroid Bennu delivered to Earth by NASA’s OSIRIS-REx mission in 2023. Dworkin is the mission’s project scientist at NASA’s Goddard Space Flight Center. Image credit: NASA/James Tralie.

The team was surprised to find traces of water-bearing sodium carbonate compounds in the Bennu samples studied at the museum. Commonly known as soda ash or by the mineral name trona, these compounds have never been directly observed in any other asteroid or meteorite. On Earth, sodium carbonates often resemble baking soda and naturally occur in evaporated lakes that were rich in sodium, such as Searles Lake in the Mojave Desert.

The surprising discovery of sodium carbonate prompted McCoy to examine mineral specimens in the museum’s National Mineral Collection that contained the compound. He also reached out to his teammates around the world to see if they had observed anything noteworthy in other Bennu samples. The scientists discovered 11 minerals in total that likely existed in a brine-like environment on Bennu’s parent body, ranging from calcite to halite and sylvite.

Bennu’s brine differs from terrestrial brines due to its mineral makeup. For example, the Bennu samples are rich in phosphorus, which is abundant in meteorites and relatively scarce on Earth. The samples also largely lack boron, which is a common element in hypersaline soda lakes on Earth but extremely rare in meteorites.

The researchers posit that similar brines likely still exist on other extra-terrestrial bodies, including the dwarf planet Ceres and Saturn’s icy moon, Enceladus, where spacecraft have detected sodium carbonate. These brines likely also exist on other asteroids, and McCoy and his colleagues plan to re-examine meteorite specimens in the museum’s collection. While some of the salts observed in the Bennu brine would break down in Earth’s atmosphere, these minerals may leave tell-tale traces on meteorites that past scientists may have missed.

A pathway towards life

While the Bennu brines contain an intriguing suite of minerals and elements, it remains unclear if the local environment was suitable to craft these ingredients into highly complex organic structures.

“We now know we have the basic building blocks to move along this pathway towards life, but we don’t know how far along that pathway this environment could allow things to progress,” McCoy said.

The second study, in Nature Astronomy, offers additional insights into Bennu’s composition. Among the most compelling detections were all five nucleobases that life on Earth uses to store and transmit genetic instructions in more complex terrestrial biomolecules, such as DNA and RNA, including how to arrange amino acids into proteins. The researchers also found xanthine, hypoxanthine and nicotinic acid (vitamin B3).

Scientists also described exceptionally high abundances of ammonia in the Bennu samples. Ammonia is important to biology because it can react with formaldehyde, which also was detected in the samples, to form complex molecules such as amino acids in the right conditions. When amino acids link up into long chains, they make proteins, which go on to power nearly every biological function.

McCoy thinks the new discoveries illustrate the scientific legacy of the OSIRIS-REx mission, as the samples it collected will fuel research for decades. His co-lead author, Sara Russell from the Natural History Museum in London, heartily agrees.

“It’s been an absolute joy to be involved in this amazing mission, and to collaborate with scientists from around the world to attempt to answer one of the biggest questions asked by humanity: how did life begin?” Russell said. “Together we have made huge progress in understanding how asteroids like Bennu evolved, and how they may have helped make the Earth habitable.”

Top image: A mosaic image of asteroid Bennu, composed of 12 PolyCam images collected by the OSIRIS-REx spacecraft from a range of 24 km. Image credit: NASA/Goddard/University of Arizona.