In a paper published, Jan. 29, 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 extraterrestrial 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 the 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.
In 2020, NASA's OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer) spacecraft collected samples from Bennu, becoming the first U.S. 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 - since the Apollo missions. 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.
In total, OSIRIS-REx collected around120 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 loaned to researchers around the world to analyze. This included Sara Russell, a cosmic mineralogist at the Natural History Museum in London and the co-lead author on the new paper with McCoy.
"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."
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.
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 extraterrestrial 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 reexamine 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 telltale traces on meteorites that past scientists may have missed.
"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.
A second study, publishing concurrently in the journal Nature Astronomy Jan. 29, offers additional insights into Bennu's composition. This paper describes multiple protein-building amino acids in the Bennu samples. It also reports the discovery of the five nucleobases that make up RNA and DNA. Some of these compounds have not been observed in meteorites that fall to Earth. Senior scientists Danny Glavin and Jason Dworkin at NASA Goddard Space Flight Center in Greenbelt, Maryland, are lead authors on the Nature Astronomy paper.
The two new studies are among the first published analyses of the Bennu samples. The Nature paper co-led by McCoy and Russell is also a major milestone in the National Museum of Natural History's initiative, Our Unique Planet. As a public - private research partnership, Our Unique Planet investigates what sets Earth apart from its cosmic neighbors by exploring the origins of the planet's oceans and continents as well as how minerals may have served as templates for life.
McCoy thinks the new discoveries illustrate the scientific legacy of the OSIRIS-REx mission as the samples it collected will fuel research for decades. The samples also highlight how much is left to learn about Bennu.
"This is the kind of finding you hope you're going to make on a mission," McCoy said. "We found something we didn't expect, and that's the best reward for any kind of exploration."
In addition to McCoy, Smithsonian-affiliated co-authors included Cari Corrigan, Rob Wardell, Tim Gooding and Tim Rose. This study also included authors affiliated with the University of Arizona; NASA; Goethe University; Curtin University; Cote d'Azur University; the University of California, Berkeley; California State University; Purdue University; the University of Manchester; Hokkaido University; the University of Rochester; Lawrence Berkeley National Laboratory; University of Queensland; Jean Monnet University; the Southwest Research Institute; Open University; the Royal Ontario Museum; the University of Tokyo; Rowan University; and the American Museum of Natural History.
Research Report:An evaporite sequence from ancient brine recorded in Bennu samples
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