Asteroid Bennu: A Rubble Pile Containing Organic Compounds

A relic of the early Solar System

A preserved witness of the first billion years

Asteroid Bennu is a carbonaceous asteroid (carbon-rich asteroid containing primitive organic compounds) of type B, originating from the first millions of years of Solar System formation. Its age is estimated at about 4.5 billion years, making it a virtually unchanged witness of the physico-chemical conditions prevailing in the solar nebula. Bennu itself is a second-generation rubble pile, formed less than 65 million years ago from the debris of an older parent body, now gone, which experienced most of the aqueous alteration described in this article.

With an average diameter of about 490 m, Bennu has a so-called "rubble pile" structure, where self-gravity is weak, on the order of \( g \approx 10^{-5} \, \text{m·s}^{-2} \). This low gravity allows the preservation of fragile materials, including hydrated minerals and volatile organic compounds.

The OSIRIS-REx mission

A direct sampling at the heart of the asteroid

NASA's OSIRIS-REx spacecraft, launched in 2016, enabled the direct sampling of Bennu, returned to Earth on September 24, 2023, in the Utah desert. Approximately 121.6 g of regolith were recovered and then curated under high-purity nitrogen at NASA's Johnson Space Center. This mission continues the exploration work aimed at linking cosmic chemistry to the origin of life, under the leadership of principal investigator Dante S. Lauretta of the University of Arizona.

Spectroscopic and isotopic analysis of the samples revealed a carbon content of 4.54% by mass (of which about 90% in organic form) and a nitrogen content of 0.23 to 0.25% by mass: values higher than those measured on samples from asteroid Ryugu or most known carbonaceous meteorites.

Organic molecules and primitive water

Phyllosilicates that tell the story of water

Bennu contains hydrated phyllosilicates (minerals that have incorporated water into their crystal structure) (about 80% by volume of the sample), evidence that its parent body interacted with liquid water very early in Solar System history. This water is not free, but chemically bound, indicating reactions at low temperature.

More than 16,000 distinct molecular formulas, composed of carbon, hydrogen, nitrogen, oxygen, sulfur and magnesium, were identified in the samples. Among them, about thirty amino acids were detected, including 14 of the 20 proteinogenic amino acids used by terrestrial biology, with glycine being the most abundant. All five canonical nucleobases of DNA and RNA (adenine, guanine, cytosine, thymine and uracil) were also identified, alongside about ten other related nitrogenous heterocyclic compounds.

N.B.:
Remarkably, the chiral amino acids detected in the Bennu samples occur in nearly equal proportions between their left- and right-handed forms (racemic), in contrast to the strong left-handed excess characteristic of terrestrial biochemistry. This absence of chiral bias confirms that these are indeed abiotic molecules, not contaminated by Earth's biosphere, and raises the open question of the origin of life's homochirality on Earth.

A saline parent world: the discovery of an ancient evaporated brine

A complete salt sequence revealed in 2025

In January 2025, two independent studies published in Nature and Nature Astronomy transformed our understanding of Bennu's parent body. The team led by Timothy J. McCoy (Smithsonian) and Sara S. Russell (Natural History Museum, London) identified, in the sample grains, a genuine evaporite sequence: sodium phosphates, hydrated sodium carbonates (trona, gaylussite/pirssonite), sodium sulfate (thenardite), halite and sylvite (NaCl and KCl), as well as sodium fluoride. This mineral suite, comparable to that of Searles Lake in California, attests to the progressive evaporation of an alkaline brine on the surface or in the fractures of the parent body, with evaporation degrees estimated between 45 and 70%.

These sodium salts, extremely fragile and sensitive to atmospheric humidity, would not have survived prolonged exposure to terrestrial air: their preservation was only possible thanks to the sample return under a controlled nitrogen atmosphere, illustrating the irreplaceable contribution of sample-return missions compared to the study of meteorites that naturally fall to Earth. Similar brines are suspected to persist today in the subsurface of Ceres and Enceladus, making Bennu a valuable analogue for the study of these ocean worlds.

Exceptionally abundant ammonia

The same analysis campaign, led by Daniel P. Glavin and Jason P. Dworkin (NASA Goddard), revealed exceptionally high ammonium concentrations (approximately 13.6 µmol per gram in a hot water extract), well above those of classic carbonaceous meteorites. This ammonia, associated with formaldehyde also detected in the sample, is a key precursor for the synthesis of amino acids and nitrogenous heterocyclic compounds via Strecker-type reactions, occurring directly in the parent body's brine.

2025-2026: bio-essential sugars complete the RNA inventory

Ribose, glucose and the third pillar of prebiotic chemistry

A nucleobase, a sugar and a phosphate group: these are the three components required to form a nucleotide, the elementary building block of RNA and DNA. While nucleobases and phosphates had already been identified in Bennu, the third pillar was missing. This has now been remedied by a study published on December 2, 2025 in Nature Geoscience by a Japanese-American team led by Yoshihiro Furukawa (Tohoku University).

Unequivocal detection of ribose and glucose by mass spectrometry

By analyzing an acidic and aqueous extract of the Bennu sample by gas chromatography coupled with tandem mass spectrometry, the researchers detected four aldopentoses — including ribose, the constituent sugar of RNA — as well as two aldohexoses, including glucose, the main energy source of cellular metabolism on Earth. Neither biological contamination nor analytical artifact can explain these detections: the racemic nature of the associated amino acids and the absence of sugars in the procedure blanks confirm an authentic extraterrestrial origin.

The formose reaction: a prebiotic synthesis in alkaline medium

The total abundance of these sugars remains low, on the order of 1% of that of amino acids, and their distribution — larger sugars (hexoses) being more abundant than smaller ones (tetroses) — is explained by a so-called "formose" reaction, where formaldehyde progressively condenses into increasingly complex sugars within the alkaline brines of the parent body, catalyzed by calcium and magnesium ions present in the carbonates. The pH measured in the Bennu extract (8.23), clearly alkaline, is consistent with this chemistry and with the presence of the evaporites described above.

N.B.:
Deoxyribose, the constituent sugar of DNA, was not detected in this sample, probably due to its much higher chemical reactivity than ribose. This difference suggests that ribose, and therefore an RNA-world-type chemistry, would have been more available than DNA in the primitive organic matter of the Solar System.

Bennu and the primordial building blocks of the Solar System

A generalized organic chemistry from the first millions of years

The study of Bennu suggests that life's chemical building blocks were already present very early during the formation of the Solar System, well before Earth's appearance itself. These organic compounds and hydrated minerals formed directly in the solar nebula (cloud of gas and dust that gave birth to the Solar System), under the effect of low-temperature physico-chemical processes involving ultraviolet radiation, grain shocks and catalysis on mineral surfaces, before being reworked within the parent body's brines.

Bennu, a fossil reservoir of the protoplanetary disk

In this context, Bennu is not a late vector of biological matter, but a fossil reservoir preserving the initial chemical state of the protoplanetary disk. The amino acids, nucleobases, sugars, ammonia and bound water observed today bear witness to a generalized organic chemistry, active from the first millions of years, at a time when the terrestrial planets were still only accreting embryos.

Mild energy conditions favorable to preservation

From an energy standpoint, these reactions do not require violent impacts. They are compatible with weakly heated environments, where energy comes from short-lived radioactive decay, local thermal gradients and surface reactions, allowing a progressive complexification of matter without destruction of fragile molecules.

A simultaneous delivery of life's building blocks to early Earth

Given that fragments of carbonaceous asteroids similar to Bennu were likely widely dispersed in the inner Solar System, these three families of life's building blocks — nitrogenous bases, sugars and amino acids — could have been delivered together to early Earth and the other terrestrial planets.

Comparative table of detected compounds