Asteroid Bennu Samples Contain Building Blocks of Life

By Jeffrey Kluger
January 29, 2025 2:48 PM EST

There’s certainly nothing living on the asteroid Bennu, an airless, 1,614-ft. rubble pile orbiting the sun about 40.2 million miles from Earth. But that doesn’t mean that Bennu hasn’t all at once become one of the most biologically interesting objects in the solar system.

Samples of Bennu were brought back to Earth by NASA’s OSIRIS-REx spacecraft in 2023. Now, a pair of newly published papers reveal that the samples contain precursors to life that formed in a watery environment—a watery environment very similar to the one that prevailed on Earth before life emerged up to four billion years ago.

One of the papers, published in the journal Nature, found traces of brine that were likely left behind when salty water that could have given rise to organic compounds evaporated. The traces are chemically similar to the makeup of Searles Lake, an ancient, dry lake bed in the Mojave Desert in California. The other paper, in the journal Nature Astronomy, was even more tantalizing, reporting the discovery of all five nucleobases that make up DNA and RNA—adenine, guanine, cytosine, thymine, and uracil—in the Bennu samples. The findings suggest either that the chemistry emerged de novo on both the asteroid and on Earth, or that other, similar asteroids may have brought us the raw ingredients of life when they bombarded our planet billions of years ago.

“This all supports the theory that asteroids like Bennu were among the sources that delivered water and chemical building blocks for life to Earth before life started here,” said Nicky Fox, associate administrator for NASA’s Science Mission Directorate, at a press conference just after both papers were released. “So this really is a groundbreaking scientific discovery.”

“We now know from Bennu that the raw ingredients for life were combining in really interesting and complex ways on Bennu’s parent body,” said Tim McCoy, the curator of meteorites at the Smithsonian’s National Museum of Natural History and the lead author of the Nature paper, in a statement that accompanied the release of the study. “We have discovered that next step on a pathway to life.”

Bennu is both an old and a new asteroid. Like all of the millions of other objects in the asteroid belt, it formed 4.5 billion years ago when our solar system was just accreting. But its loose, rubble-rich structure suggests that it was once part of a much larger body, breaking away from it as a result of an impact with another object a few tens of millions of years ago.

Bennu was attractive as a sample return target for a number of reasons. First, its close proximity to Earth made it relatively easy to reach. Its slow rotation meant that it wouldn’t have flung off the loose surface material that the spacecraft would be trying to collect. And its carbon-rich composition meant the possibility of discovering organics. After launching in 2016, OSIRIS-REx reached Bennu in 2020, gathered up 121.6 grams (4.3 oz.) of material—about the weight of a bar of soap—and carried it back to Earth, where it parachuted through the atmosphere and was collected after it landed in the Department of Defense’s Utah Test and Training Range southwest of Salt Lake City.

From the desert, the samples were transferred to NASA’s Johnson Space Center (JSC) in Houston where they are stored and studied in airtight glove boxes filled with inert nitrogen, to prevent their contamination with Earthly atmosphere or water. JSC, in turn, loaned some of the samples to the Smithsonian, where they were studied by the authors of the Nature paper under similar conditions with a scanning electron microscope, which was able to discriminate bits of asteroid less than a micron across—or one hundredth the width of a human hair. It was that analysis that revealed the chemical richness of the Bennu material.

Most significantly, the researchers discovered sodium carbonate, which is often found in dry, salty lake beds on Earth. In addition, they found sodium-rich phosphates, sulfides, chlorides, and fluorides, which are also common in evaporated terrestrial lakes. Bennu’s parent body would not have been large enough to hang onto an atmosphere and thus would not have had surface lakes. But that doesn’t mean it could not have harbored bodies of water. 

“On this ancestral asteroid, four and a half billion years ago, there was something like a muddy surface that had pockets of fluid or veins of fluid, perhaps only a few feet wide, under the surface,” said McCoy at the press conference. “It was within those cracks that the evaporation occurred. Water was lost to the surface and these minerals were left behind.”

What’s happening on Bennu is likely happening elsewhere. The largest object in the asteroid belt, the dwarf planet Ceres, is an icy body that measures 592 miles across. About 25% of its mass, NASA estimates, is made up of water. Saturn’s moon Enceladus, with a 313-mile diameter, regularly emits icy geysers, produced when oceans beneath its crust are squeezed by Saturn’s gravity. The Cassini spacecraft, which orbited Saturn from 2004 to 2017, conducted chemical analyses of the plumes and detected sodium carbonate in them as well.

“So we know this mineral occurs not just in the ancient history of Bennu’s parent body,” said McCoy. “The same minerals are forming today on icy moons of the outer solar system.”

The findings in the Nature Astronomy paper were significant not just for the discovery of the five RNA and DNA precursors, but for the presence of abundant levels of ammonia, which is a key chemical building block for life. 

“The surprising thing was the high concentrations of ammonia that we found, about 230 parts per million,” said NASA’s Daniel Glavin, a co-investigator on the OSIRIS-REx team. “To put that in perspective, this is about 100 times more than the natural levels of ammonia that you find in soils on the Earth. The high levels of ammonia suggest that this stuff formed in the colder regions of the solar system, far from the sun, since ammonia is volatile. So we had to really have formed this stuff in a cold environment where the ammonia ice would have been stable.” That, in turn, suggests that the RNA and DNA cycle could be playing out on a wide range of other icy bodies far from Earth.

One question left unanswered by the new studies is why life pulled up short on Bennu and its parent body—why it stopped at precursors. Was it the temperature, the absence of atmosphere, the lack of other essential chemical components? The two new papers don’t—and can’t—yet say. 

“We can ask ourselves: if Bennu was such a [hospitable] environment, with rich alkaline mineral [and] brine, why didn't life form on Bennu?” said NASA’s Jason Dworkin, project scientist for OSIRIS-REx. “What did Bennu not have that the Earth did have? This is a future area of study for astrobiologists to ponder.”

Those future astrobiologists will get that opportunity. Just 25% of the Bennu sample was used in the current research. The remainder, like the half-century old Apollo lunar samples, will be preserved in their pristine state for further investigation.  “Perhaps scientists not yet born using methods not yet invented can make discoveries we haven't imagined,” said Dworkin. “Sample return is a gift that keeps on giving.”