Imagine scooping dirt from your backyard and finding, hidden in those grains, all the molecular pieces needed to build DNA.

That's essentially what happened when scientists opened containers holding pristine material from asteroid Bennu. The samples, retrieved by NASA's OSIRIS-REx mission and delivered to Earth in 2023, turned out to be astonishingly rich in volatile compounds—especially nitrogen-bearing molecules critical to biology. Among them: ammonia in quantities higher than almost any meteorite ever studied, 33 different amino acids including 14 used by life on Earth, and all five nucleobases that form the rungs of DNA and RNA's twisted ladders.

The findings challenge long-held assumptions about where Earth's prebiotic chemistry came from and what conditions existed in the early solar system.

A Pristine Sample from Deep Time

Bennu is a rubble-pile asteroid—a collection of fragments from a larger parent body that was catastrophically destroyed millions of years ago. The OSIRIS-REx spacecraft collected 121.6 grams of regolith from as deep as half a meter beneath the surface, capturing material that had been shielded from cosmic radiation and space weathering. Unlike meteorites, which are contaminated the moment they hit Earth's atmosphere and soil, Bennu's samples arrived under controlled conditions, sealed and protected.

Early analysis confirmed what remote sensing had suggested: Bennu is carbon-rich, with between 4.5 and 4.7 percent carbon by weight and 0.23 to 0.25 percent nitrogen. That's higher than samples from asteroid Ryugu, returned by Japan's Hayabusa2 mission, and higher than most carbonaceous chondrite meteorites.

But the real surprise was what that nitrogen was doing.

Ammonia Everywhere

When researchers extracted Bennu samples in hot water, they found ammonia concentrations around 13,600 nanomoles per gram—twelve times higher than the Murchison meteorite, seventy-five times higher than Ryugu. Only two meteorites have shown comparable ammonia levels: the CI1 chondrite Orgueil and the CR2 Graves Nunataks 95229.

Ammonia is a simple molecule—one nitrogen atom bonded to three hydrogens—but its presence in such quantities tells a story. It likely didn't survive as free ammonia. Instead, it was locked away as salts or bound to clay minerals and organic matter, preserving it over billions of years. When the samples were heated or dissolved, the ammonia was released.

The ammonia also carried an isotopic signature. Its nitrogen-15 was enriched by 180 parts per thousand relative to Earth's atmosphere, well outside the range of terrestrial organic matter. That enrichment points to an origin in the cold outer reaches of the solar system—either in a molecular cloud before the Sun formed, or in the frigid outer protoplanetary disk where ammonia ice could condense.

Bennu's parent body, it seems, either formed out there and migrated inward, or it accreted ices that drifted in from the cold outer regions through a process called pebble drift.

A Molecular Census

Using a combination of mass spectrometry techniques, researchers catalogued an extraordinary chemical diversity. Fourier-transform ion cyclotron resonance mass spectrometry identified around 16,000 molecular formulas in methanol extracts, with a continuum of sizes and oxidation states. The samples contained polycyclic aromatic hydrocarbons, alkylated aromatics, and molecules with up to seven nitrogen atoms each.

Amino acid analysis revealed 33 identified amino acids, plus additional unidentified aliphatic amino acids with six and seven carbon atoms. Glycine, the simplest amino acid, dominated at 44 nanomoles per gram. The sample also contained beta-alanine, isovaline, norvaline, and others rarely or never used by biology. Fourteen of the twenty standard protein amino acids were present, including alanine, valine, serine, and glutamic acid.

Critically, every chiral non-protein amino acid that could be measured—isovaline, norvaline, beta-amino-n-butyric acid, beta-aminoisobutyric acid, 3-aminopentanoic acid—was racemic. Equal amounts of left-handed and right-handed forms. No bias toward the left-handed chirality that defines terrestrial biology.

This was unexpected. Some carbonaceous chondrites show left-handed amino acid excesses that are thought to have influenced life's handedness. But Bennu's samples, like Ryugu's and some lithologies of the Tagish Lake meteorite, show no such preference. The finding complicates the hypothesis that life's left-handedness was seeded by molecules delivered from space.

All Five Nucleobases

Perhaps most striking, researchers identified all five canonical nucleobases: adenine, guanine, cytosine, thymine, and uracil. These molecules form the genetic alphabet. In Bennu, they appeared alongside eighteen other nitrogen-containing heterocycles—ring-shaped organic molecules with nitrogen atoms embedded in their structures.

The total abundance was modest, around 5 nanomoles per gram, but five to ten times higher than in Ryugu or Orgueil. The ratio of purines to pyrimidines was unusually low—0.55—opposite the pattern seen in the Murchison and Orgueil meteorites. This elevated abundance of pyrimidines may reflect differences in formation pathways or parent body chemistry.

Nucleobases can be synthesized from ammonia and formaldehyde, especially under alkaline conditions. Experiments bombarding interstellar ice analogs with radiation preferentially produce pyrimidines over purines. It's possible Bennu inherited its nucleobases, or their precursors, from the cold molecular cloud or outer disk environment.

Nine monocarboxylic acids were identified, dominated by formic acid (4,106 nanomoles per gram) and acetic acid (1,436 nanomoles per gram). Sixteen aliphatic primary amines were present, led by methylamine (914 nanomoles per gram) and ethylamine (121 nanomoles per gram). Untargeted analysis detected roughly 10,000 nitrogen-bearing chemical species overall.

A Cold, Wet Chemistry

Bennu's soluble organic matter suggests formation and alteration at low temperatures, possibly in ammonia-rich fluids. The amino acid distribution—dominated by glycine, with low beta-alanine to glycine ratios—differs from the highly aqueously altered CI1 chondrites and resembles less-altered type-2 chondrites like Tagish Lake. Yet Bennu's mineralogy, rich in phyllosilicates and carbonates, points to extensive water-rock interaction.

This paradox may be explained by Bennu's unique chemical environment. High concentrations of ammonium salts could have created liquid brines at very low temperatures—ammonia-water eutectic mixtures can remain liquid down to 176 Kelvin. Such brines would allow organic chemistry to continue long after the short-lived radioactive isotopes that initially heated the parent body had decayed.

Petrologic data from other Bennu samples show that late-stage fluids precipitated evaporite minerals in sequence: calcium and magnesium carbonates first, then phosphates, sodium carbonates, and finally halides and sulfates. These minerals indicate alkaline pH, high concentrations of dissolved inorganic carbon, and temperatures below 55 degrees Celsius—conditions favorable for complex organic synthesis.

Experiments have shown that glycine, adenine, and guanine can be synthesized from ammonium cyanide kept at 195 Kelvin for 25 years. Eutectic freezing, clay catalysis, and magnesium salts are also employed in laboratory polymerization of nucleotides. Bennu may have hosted similar processes.

Implications for Earth and Beyond

Bennu's volatile-rich nature, nitrogen-15 enrichments, and abundance of isotopically anomalous organic matter all point to an origin in the outer solar system where ammonia ice was stable—beyond Jupiter's current orbit. Dynamical simulations suggest Bennu's secondary parent body formed in the inner asteroid belt between 2.1 and 2.5 astronomical units, breaking apart 730 to 1,550 million years ago. But its ultimate origin may lie much farther out.

One hypothesis is that Bennu's parent body migrated inward during the giant planets' migrations, similar to proposals for CI chondrites and Ryugu. Another is that icy pebbles drifted inward from colder regions and accreted onto bodies forming closer to the Sun, delivering ammonia and other volatiles to what would become the asteroid belt.

B-type asteroids like Bennu, named for their blue spectral slopes, may represent fragments of extinct comets or a continuum of objects ranging from dry planetesimals to volatile-rich icy bodies. There is some evidence of low-temperature aqueous activity in comets—hydrated minerals and carbonates were detected in the ejecta from comet Tempel 1, and the mineral cubanite was found in samples from comet Wild 2. But Bennu's phyllosilicate-dominated mineralogy and meter-scale carbonate veins suggest large-scale hydrothermal activity lasting millions of years, hard to reconcile with a cometary origin.

Alternatively, Bennu may consist of fragments from a Ceres-like primitive icy body. Ceres, the largest object in the asteroid belt, shows evidence of ammonium and carbonate salts, high organic carbon content, and extensive rock-fluid interactions. Bennu's chemistry and mineralogy align well with such a model.

Regardless of origin, asteroids like Bennu could have been critical suppliers of nitrogen-rich volatiles and prebiotic compounds to the early Earth. Ammonia, amino acids, nucleobases, phosphates, and other chemical precursors delivered by impacts may have contributed to the prebiotic inventory that enabled life to emerge.

What Comes Next

The analysis of Bennu samples has only begun. Future work will measure isotopic compositions of individual amino acids, carboxylic acids, and nucleobases to constrain their formation mechanisms. Researchers will explore the chemical precursors to nucleic acids and study how amino acid distributions vary across different Bennu particles. Laboratory experiments simulating ammonia-rich brines at low temperatures will test hypotheses about organic synthesis pathways.

Additional sample return missions—from comets, from Ceres—will provide comparative data to untangle the relationships between volatile-rich asteroids and primitive icy bodies.

For now, Bennu has delivered a chemical time capsule. Its grains preserve a record of processes that occurred in the outer solar system more than 4.5 billion years ago, and they hold molecular echoes of the chemistry that may have sparked life on our planet.

Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.1038/s41550-024-02472-9