A new analysis of samples returned from the near-Earth asteroid Bennu has revealed a rich collection of organic molecules, including several chemical building blocks used by life on Earth, as well as the potentially historic detection of the complex amino acid tryptophan.
Discovered in 1999, Bennu is a near-Earth asteroid that passes by our planet every six years. It was the target of NASA’s OSIRIS-REx mission, which aimed to collect samples from the asteroid and deliver them to Earth in September 2023.
Now that samples are safely in labs for examination, this new study, led by Angel Mojarro of NASA’s Goddard Space Flight Center and published in the Proceedings of the National Academy of Sciences, examined tiny fragments of Bennu’s rocky surface. Because these samples were taken directly from the asteroid and sealed before re-entry, they preserve a pristine record of early Solar System chemistry, free from contamination by Earth’s atmosphere and biosphere.
“Our findings expand the evidence that prebiotic organic molecules can form within primitive accreting planetary bodies and could have been delivered via impacts to early Earth and other solar system bodies, potentially contributing to the origins of life,” the researchers wrote in their study.
The team focused on two main types of organic material within the Bennu samples. One is a tough, tar-like “insoluble” organic sample made of large, interconnected carbon-rich structures, similar in some ways to very old coals or kerogen on Earth. The other is a “soluble” sample made up of smaller, more mobile molecules that can be extracted with liquids, such as amino acids and nucleobases, the molecules life uses to build proteins and to store genetic information in DNA and RNA.
To study both, the team used a combination of heating samples to release volatile compounds and a wet-chemistry method that chemically tags small molecules for detection with high-sensitivity mass spectrometry.
Mojarro and his co-authors identified 15 of the 20 standard amino acids used by terrestrial life to assemble proteins, along with all five nucleobases that form the “letters” of DNA and RNA: adenine, guanine, cytosine, thymine, and uracil. Earlier work on other Bennu fragments had already shown that the asteroid carries 14 protein-forming amino acids and the full set of nucleobases, but the new study adds one more amino acid to the list.
In a historical first, a tentative detection of the amino acid tryptophan in the aggregate Bennu sample indicates that a relatively complex amino acid exists in extraterrestrial objects. Tryptophan is one of the 20 amino acids used by life, and on Earth, it plays roles in both protein structure and cellular signaling. In the Bennu samples, it appears at trace levels across multiple subsamples and is absent from blank laboratory controls, so the team argues that it is unlikely to be a contamination artifact, while still stressing that further measurements will be needed to confirm the detection beyond doubt.
If confirmed, its presence would suggest that some fragile organic molecules are missing from meteorites because they do not survive the heating and shock of atmospheric entry, highlighting the importance of sample-return missions for capturing the full range of prebiotic compounds in space.
The study also shows that Bennu is not chemically uniform. OSIRIS-REx returned not only a mixed “aggregate” powder of fine particles, but also three visually distinct stones which correspond to different boulder types seen on the asteroid’s surface. When the team analyzed these stones separately, they found clear differences in both the soluble and insoluble organics for each one.
The different types of rock indicate that Bennu’s parent body experienced multiple, distinct episodes of aqueous alteration in a wet, alkaline, ammonia-rich environment, and that different lithologies record different moments in this history rather than a single, uniform alteration event. In other words, wherever Bennu originally came from, it has had a complex upbringing across multiple environments, which have impacted its chemical makeup.
“Sample return missions from a variety of planetary bodies are accordingly crucial to enabling new discoveries and elucidating products of cosmochemistry,” the authors concluded.
MJ Banias covers space, security, and technology with The Debrief. You can email him at mj@thedebrief.org or follow him on Twitter @mjbanias.