Extraterrestrial Sugars Found On The Asteroid Bennu: Where Do They Really Come From?

The asteroid Bennu, discovered in 1999, is closely monitored because we know that it contains primitive materials that could give us valuable information about the formation of our Solar System and the origin of life on Earth. Since the OSIRIS-REx mission organized by NASA brought back 250 grams of its rock two years ago, we have had confirmation: this NEO carries with it traces of water and organic molecules several billion years old. Further analyzes have since been carried out on these samples, and we are now certain that extraterrestrial complex sugars are present on its surface.

To avoid any confusion, let’s clarify the terminology from the outset. Here, ” aliens » obviously does not mean “ organic » or « living “, but that these sugars were simply formed outside the Earth. These molecules were synthesized in space, then preserved in the mineral matrix of Bennu since the earliest ages of the Solar System. On December 2, three studies were published regarding this discovery; two in the review Nature Astronomy (available freely on this page and this one), and another in Nature Geoscience. Together, they confirm what astrobiologists already suspected: Bennu, chemically speaking, is an unusual primitive asteroid.

Before life, chemistry: what does Bennu teach us?

One of the studies (Nature Geoscience) was interested in the prebiotic chemistry of Bennu, that is to say the set of chemical reactions which can produce complex molecules without biological intervention. By analyzing the samples reported by OSIRIS-REx, researchers identified two different sugars : ribose, a sugar with five carbon atoms which forms the backbone of RNA (Ribonucleic acid), and glucose, a sugar with six carbons never before observed in an extraterrestrial material.

These molecules, in themselves, are not biological signatures, but they are irrefutable proof that compounds essential to the functioning of biological systems could be formed in space, in the total absence of any form of life. Life, as we know it today from a chemical point of view, therefore already existed before our planet was formedat least in the form of its fundamental components.

In the same samples, the researchers did not find deoxyribose, a sugar that can be nicknamed “ spine » DNA (Deoxyribonucleic Acid). Close to ribose in its chemical structure, deoxyribose differs in a fundamental way: it is ribose which is the backbone of RNA, unlike DNA ; an extremely important difference.

Finding ribose on Bennu therefore amounts to reinforcing a central theory of astrobiology: the RNA world hypothesis. This would suggest that RNA was, initially, a kind of molecule “ Swiss army knife “. Simple enough to form spontaneously in a prebiotic environment, but versatile enough to support two essential processes for life: retain chemical information and cause reactions around it.

This means that long before DNA appeared, rudimentary chemical systems could already form and evolve. Life on Earth could be, in this respect, the culmination of a chemical process which began billions of years before our planet was born. Let us not forget that other elementary building blocks of life (amino acids, nucleobases and carboxylic acids) have already been found on Bennu: it is therefore, to datethe oldest chemical archive in the Solar System.

Unknown materials and dust older than the Sun

The second study (Nature Astronomy) demonstrated that Bennu also carries other compounds on its surface. The first of these compounds is described as a “ space eraser »: a polymeric substance (similar to plastic) viscous, very rich in nitrogen and oxygen, which does not correspond to any natural material that has ever been observed in space.

According to analyses, it was formed very early, when Bennu was still a young asteroid, and evolved under the effect of space radiation and other chemical reactions, to become, in part, hydrophobic (water resistant). Initially, this polymer was supposed to be soluble or reactive, like many simple organic compounds, but its nature has changed.

The presence of this gum also forces us to reconsider the idea according to which water is only a disruptive element, dissolving and destroying all soluble organic compounds. In this specific case, the evolution of this polymer seems to have preceded, or even accompanied, these aqueous episodes, giving the possibility to this compound to survive where others disappeared.

Since Bennu came from a larger parent body, which experienced a heating phase that allowed water to circulate inside the rocks, most of the organic compounds were eliminated by the heat and compositional changes induced by the water. The fact that this polymer is still present indicates that it acquired properties of resistance to a hostile environment very early on, which is quite exceptional. Indeed, few organic materials can withstand such harsh conditions, which leads us to believe that it belongs to a category that we do not yet know.

The third study (Nature Astronomy) was interested in presolar grains on the surface of Bennu: tiny solid particles formed in the envelope of stars at the end of their life or during supernova explosions, then dispersed in space. These grains, as their name suggests, already existed before our Solar System was born and were thus embedded, as is, in the materials which would form its first solid bodies (primitive asteroids, planetesimals, etc.)

In the samples, the researchers found an exceptional quantity: about six times more than any celestial body studied so far. Such an abundance is the ultimate proof that the genesis of Bennu took place in a region of the proto-solar disk particularly rich in dust from dead stars.

The asteroid, in its young years, was thus bathed in an environment saturated with stellar debriswhich aggregated to form it and give it its current form: an asteroid 500 meters in diameter, weighing approximately 73 billion tonnes. These new analyzes certainly do not help us enormously in understanding how life appearedbut they put us on the path to understanding with what elements it could one day emerge. Sometimes we have to settle for little in astrobiology, because the big question of the origin of life is first resolved by the study of its precursors. In this regard, Bennu is undoubtedly one of our best allies (like the study of Martian geochemical processes), since he is a very talkative witness of a time when the early Universe was already highly active. If life on Earth is truly a “ accident “, we have all the evidence before our eyes showing us that the Universe had prepared it well. Can we really, in this case, speak of an accident? ?