Scientists Discover Traces of Salt Water and Building Blocks of Life in NASA’s Samples From the Asteroid Bennu

In a triumph for NASA’s first asteroid sample return mission, new findings suggest that tiny bits of rock retrieved from the asteroid Bennu hold lingering traces of ancient salt water. The discovery hints that life-friendly chemistry could be far more common in space than astronomers previously thought.

After a seven-year voyage, NASA’s OSIRIS-REx spacecraft dropped a capsule of about 120 grams of precious asteroid fragments into the Utah desert in September 2023. Scientists from the Smithsonian’s National Museum of Natural History, in collaboration with an international team of researchers, soon began the painstaking process of analyzing these grains under specialized microscopes. Astronomers hope that Bennu—which, at about 4.5 billion years old, offers a glimpse into the solar system’s early days—might reveal hints how organic materials first arrived on Earth and laid the groundwork for life to evolve.

A new study, published today in Nature, describes a stunning lineup of minerals revealed in the samples—most notably sodium carbonates, which commonly occur on Earth as “soda ash” or in dried-up lake beds. These minerals had never been observed in any meteorite or asteroid sample before.

“I actually saw the samples when they were first opened,” co-lead author Tim McCoy, curator of meteorites at the National Museum of Natural History, tells Smithsonian magazine. “We had expectations they would have a lot of clays, carbonates, sulfides and iron oxides—and we found all of those.”

But the surprising presence of sodium carbonates indicates that Bennu’s parent asteroid once had pockets of liquid water. When that water evaporated, it likely left behind salty, brine-like residues rich in life-supporting elements, such as phosphorus. If those components then rained down on early Earth via meteorite strikes, they could have helped seed the planet with the raw ingredients for life.

“We were super excited to find this—I’ve been studying meteorites for 35 years and had never seen this mineral before,” McCoy adds. “That was really the breakthrough that told us we were looking at an ancient evaporite sequence.”

These components offer hints to how precursors of life might have existed on asteroids. “Phosphates can help to make sugars. Clays can help to make things like nucleotides that build RNA and DNA,” McCoy says. “Sodium brines are really essential to both ... speeding up those reactions and to help release those molecules after they form.”

In a second study, published today in Nature Astronomy, McCoy and other scientists examining Bennu’s components found even more crucial components of life. They discovered amino acids—the building blocks of proteins—in the sample, as well as the five main nucleobases that make up RNA and DNA. These findings echo what researchers have long suspected about asteroids—that they can ferry essential organic molecules across the solar system, potentially delivering them to planets, where life eventually takes hold.

According to Michael Ackerson, a research geologist at the National Museum of Natural History who was not involved in the Bennu studies, this discovery represents a “tremendous leap forward in our understanding of the origins of life.”

“The key mineralogical and chemical material needed for life’s emergence on Earth was being delivered to our planet early in its history,” Ackerson says to Smithsonian magazine. “Effectively, Bennu’s brines created a nursery for the development of complex organic molecules that were subsequently delivered to a nascent Earth.”

And Bennu isn’t alone. The Nature study points out that the dwarf planet Ceres and Saturn’s icy moon Enceladus have also shown evidence of sodium carbonate brines in recent years. As missions to these and other celestial candidates for life advance, investigators will be on the lookout for the same minerals identified in Bennu’s samples.

“Together we have made huge progress in understanding how asteroids like Bennu evolved, and how they may have helped make the Earth habitable,” Sara Russell, a cosmic mineralogist at the Natural History Museum in London and co-lead author of the Nature study, says in a statement.

Still, exactly how these raw components can produce life remains an open question.

“Just like a batch of cookies, you can have the ingredients, but without time and temperature, you’re never going to get a cookie,” McCoy says. “We don’t know how much time and what temperature we need to actually get these elements to react to make something that we would call life.”

Though Earth’s soda lakes are somewhat chemically similar to Bennu’s ancient brines, the asteroid has a couple of clear differences: an abundance of phosphorus and a lack of boron. On our home planet, phosphorus is relatively scarce—and while boron often shows up in Earth’s evaporated lakes, it’s virtually absent in meteorites. Therefore, while Bennu may share the same watery heritage with Earth, its chemical makeup follows a different recipe.

As these samples undergo further analysis, McCoy says he and his team are particularly focused on studying the tiny traces of water locked within the minerals. By measuring the ratio of different hydrogen isotopes in that water, they hope to compare it to the oceans on Earth, potentially answering longstanding questions about where Earth’s water came from.

Ella Jeffries | READ MORE

Ella Jeffries is an editorial intern with Smithsonian magazine.