[Un article de The Conversation écrit par Nick Timms – Professeur associé, École des sciences de la terre et des planètes, Université Curtin ; Phil Bland – Directeur, Space Program, Université Curtin ; et William Rickard – Professeur associé, Faculté des sciences et de l’ingénierie, Université Curtin]
In October 2020, a robotic spacecraft the size of a van has briefly posed on the surface of Bennu, an asteroid 525 meters wide, located 320 million kilometers from the earth, before redecolling.
This is how the Osiris-Rex mission of NASA was able to collect a precious sample of dust and small stones flush this surface, after spending two years orbit around the asteroid to imagine it. The capsule containing the sample returned to earth in September 2023, in the Utah desert, in the United States. Since then, an international team of which we have been hastened to study some 120 grams of materials taken from Bennu.
Our conclusions are presented in two new articles published on January 29, 2025 in Nature And Nature Astronomy. Our analyzes of Bennu's dust shows that the ancestor of Bennu, a larger asteroid, could have contained salt water – which opens up new perspectives on the chemistry of the beginnings of the solar system.
Intouched vestiges of rocks dating from the dawn of time
Asteroids are the remains of larger bodies dating from the beginning of the history of our solar system, which have been destroyed by collisions with other objects – we speak of “parents” bodies. The fragments that remain today are in orbit around the sun and have a great diversity of forms, sizes and chemical compositions.
The Bennu asteroid was chosen for the Osiris-Rex mission because the remote sensing observations made from the Earth indicated that it was an asteroid type B, that is to say rich in carbon and in Hydrated clay minerals-and may have similarities with the oldest meteorite group found directly on earth, “carbon chondrites”. But unlike meteorite samples found on earth, the samples taken from asteroids have not been physically or chemically changed by the terrestrial atmosphere and biosphere.
These are the untouched witnesses of immemorial time, and that is why they allow to approach essential questions on the evolution of the primitive solar system, the formation of planets and the ingredients of life.
[NDLT : D’autres missions d’exploration d’astéroïdes, en particulier la mission Hayabusa2 qui a ramené en décembre 2020 de la poussière de l’astéroïde Ryugu, visent à répondre à ces questions.]
Another objective of the Osiris-Rex mission is to establish a link between the results obtained from samples taken in the laboratory and those obtained thanks to remote sensing techniques. This allows us to corroborate astronomical asteroid observations in order to improve our studies in the solar system.
Tiny salted mineral crystals
To avoid contamination, the sealed capsule containing the sample was stored and handled in a huge glass box when it returns to earth. Rubber gloves have been introduced into this box by the side so that scientists can handle the samples without touching them directly. It has also been served with nitrogen to prevent humidity and oxygen from the earth's atmosphere from entering it.
When we analyzed the interior of Bennu's dust particles, we were surprised to find tiny salt mineral crystals known as halite (or “gem salt”, based on sodium chloride) and Sylvine or Sylvite (formerly “bitter salt” based on potassium chloride).
This is a revolutionary discovery.
Indeed, halite is extremely rare in meteorites. It was only found in three of the hundreds of thousands of known meteorites on earth. We also know that halite is very soluble. It can deteriorate quickly when exposed to air or water on earth.
Other members of the sample analysis team reported by Osiris-Rex have identified a variety of other saline minerals. These include sodium carbonates, phosphates, sulfates and fluorides.
These minerals can be formed by evaporation of salum, like the deposits which form in the salt lakes of the earth. When we compared these results to the chemical composition of the salted lakes of the earth, an image began to emerge, that of salums evaporating on the parent body of the Bennu asteroid, leaving traces on the spot … of salts.
A variety of organic compounds
This discovery gives a new overview of a hydraulic activity in the early days of our solar system.
But the presence of saline minerals is important for another reason: on earth, these minerals catalyze the formation of organic compounds such as nucleobics and nucleosides, that is to say that they accelerate chemical reactions producing elements of The “primordial soup” which would have enabled the emergence of life on earth.
And in fact, a distinct analysis of the sample of Bennu by our colleagues has identified a wide variety of organic compounds present on the Bennu asteroid.
They recognized 14 of the 20 amino acids found in terrestrial biological processes, but also include several amino acids absent from the biological processes that we know on earth, as well as ammonia and the five nucleobics that constitute RNA and earthly DNA.
Thus, these two new studies show that a brackish environment rich in carbon on the parent body of Bennu was conducive to the assembly of the constituent elements of life – even if no life has been detected.
And now ?
These results could allow researchers to better understand what is happening on frozen bodies far from our solar system, especially on the moon of Saturn called Enceladus and on the planet dwarf Ceres located in the asteroid belt between Mars and Jupiter.
Enceladus and Ceres both have brine oceans below the surface. Could they shelter life?
We continue to study Bennu using the samples collected by Osiris-Rex in 2020. Currently, we look at the chronology of events that would have led to the breakup of the parent body of Bennu and we are looking for proofs of impact recorded by Various minerals in samples.
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