OSIRIS-REx samples reveal all five DNA building blocks on asteroid Bennu

In October 2023, a capsule descended through Earth's atmosphere at 27,000 miles per hour, its parachute deployed over the Utah desert. Inside was 4.29 ounces of material collected from an asteroid 200 million miles away — a sample that would reshape what we know about the chemical origins of life itself.

The asteroid Bennu, visited by NASA's OSIRIS-REx spacecraft, has yielded something scientists had never before confirmed on an extraterrestrial rock: all five nucleobases that form the backbone of DNA and RNA. Alongside these molecular building blocks, researchers found minerals rich in carbon, sulfur, phosphorus, and other elements essential to life as we understand it. The minerals resembled those left behind in dried lake beds on Earth, except these had been sitting undisturbed since the solar system's birth 4.6 billion years ago. Two independent research teams published their findings on January 29 in Nature Astronomy, each examining different portions of the Bennu sample under powerful microscopes.

The first team, led by researchers including Tim McCoy, a meteorite curator at the Smithsonian's National Museum of Natural History, used scanning electron microscopy to study the sample's surface at a resolution of one-hundredth the width of a human hair. They identified sodium carbonate — the same mineral that forms when water evaporates from lakes on Earth — embedded with eleven different minerals that serve as precursors for organic compounds. What struck the researchers was how these minerals differed subtly from their terrestrial counterparts: Bennu's versions were rich in phosphorus but poor in boron, the opposite of what typically appears in Earth's ancient lake beds. This suggested that Bennu's parent body had experienced its own distinct chemistry, one that might have been conducive to life's emergence.

The second study, conducted by Japanese scientists, examined a separate piece of the sample and confirmed the presence of all five nucleobases: adenine, guanine, cytosine, thymine, and uracil. These are the letters of life's alphabet, the units that link together with ribose and phosphate to form the double helix structures that carry genetic instructions in every living thing on Earth. A previous mission to the asteroid Ryugu in 2023 had found only uracil among the nucleobases; Bennu's sample was the first to reveal the complete set on a distant asteroid.

Bennu itself carries a certain notoriety. It is classified as potentially hazardous, with a one-in-2,700 chance of striking Earth in the year 2182 — the highest probability of impact among all known space objects. But for scientists, the asteroid's real significance lies in what it preserves. As a carbon-rich body, Bennu likely contains primordial molecules that existed when life first took hold on Earth, offering a window into the chemical conditions of the early solar system.

The OSIRIS-REx mission, which launched in September 2016, spent nearly two years orbiting Bennu before attempting its delicate landing. The spacecraft used a nitrogen burst from its Touch-and-Go Sample Acquisition Mechanism to make contact with the asteroid's surface without sinking into it, capturing the sample in the process. When the capsule returned to Earth, the sample was immediately placed in a clean room to prevent any contamination from terrestrial sources.

Yet the discovery raises as many questions as it answers. The presence of these chemical ingredients does not necessarily mean life emerged on Bennu or its parent body. McCoy acknowledged this uncertainty directly: the asteroid's environment may have been too harsh for these compounds to evolve into the complex organic structures that life requires. "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," he said. The researchers plan to reexamine meteorites already in their collections, searching for similar salts and compounds that might reveal how widespread this chemistry was across the early solar system. They also suspect that similar brine-based chemistry might exist on other bodies — the dwarf planet Ceres and Saturn's moon Enceladus among them — suggesting that the ingredients for life may be far more common in the cosmos than previously thought.