The asteroid Bennu is a carbonaceous asteroid of type B, dating back to the first few million years of the Solar System's formation. Its age is estimated at about 4.5 billion years, making it an almost unchanged witness to the physicochemical conditions in the solar nebula.
With an average diameter of about 490 meters, Bennu has a "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.
NASA's OSIRIS-REx probe, launched in 2016, enabled the direct sampling of Bennu, with samples returned to Earth in 2023. This mission follows the work initiated by Gerald A. Soffen (1926-2000) and other pioneers of planetary exploration, aiming to link cosmic chemistry to the origin of life.
Spectroscopic and isotopic analysis of the samples revealed the presence of carbon, nitrogen, sulfur, and hydrogen, key elements of terrestrial prebiotic chemistry.
Bennu contains hydrated phyllosilicates, evidence that its parent body interacted with liquid water very early in the history of the Solar System. This water is not free but chemically bound, indicating low-temperature reactions below 300 K.
Simple amino acids and nitrogenous bases have also been detected. These molecules are not living organisms, but they are essential precursors for the synthesis of proteins and nucleic acids.
N.B.:
The stability of these molecules in space is favored by the absence of free oxygen and very low temperatures, which limit destructive oxidation reactions.
The study of Bennu suggests that the chemical building blocks of life were already present very early during the formation of the Solar System, long before the appearance of Earth itself. These organic and hydrated mineral compounds formed directly in the solar nebula, through low-temperature physicochemical processes involving ultraviolet radiation, grain collisions, and catalysis on mineral surfaces.
In this context, Bennu is not a late vector of biological material, but a fossil reservoir preserving the initial chemical state of the protoplanetary disk. The amino acids, carbon compounds, and bound water observed today testify to widespread organic chemistry, active from the first few million years, at a time when the terrestrial planets were still accreting embryos.
From an energetic point of view, 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 destroying fragile molecules.
| Compound | Chemical Type | Potential Role | Detection Method |
|---|---|---|---|
| Amorphous carbon | Elemental | Support for organic chemistry | Infrared spectroscopy |
| Phyllosilicates | Hydrated mineral | Trace of primitive water | Mineralogical analysis |
| Simple amino acids | Organic molecule | Glycine (\( \mathrm{NH_2{-}CH_2{-}COOH} \)) was found in the returned samples | Chromatography |
| Carbonates | Mineral salt | Regulation of chemical pH | Mass spectrometry |
Source: NASA, OSIRIS-REx mission; Nature Astronomy, analyses 2023-2024.
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