Our research group publishes, today in Nature, the first tangible evidence of the past and lasting existence of environments on the surface of Mars particularly favorable to the spontaneous synthesis of the first molecules of biology, necessary for the emergence of the life.

We have discovered fossil structures that bear witness to repeated and long-lasting drying-wetting cycles of very old sediments from the surface of Mars. This alternative dry-wet mode promotes the concentration and polymerization of simple organic molecules (sugars or amino acids) which may have been contained in the sediments. These processes are a fundamental step towards the synthesis of biological molecules such as nucleic acids (DNA or RNA).

The question that worries scientists is not so much whether life existed on a planet other than Earth, but rather knowing where and how life as we know it on Earth was built.

Since the mid-1980s, biochemists have recognized the RNA world as a fundamental preliminary step on the road to life. RNA would have constituted the original autocatalytic molecule and bearer of genetic information, with enzymatic functions ensured by short RNAs. Proteins would then have supplanted RNA as enzymes due to greater diversity, and DNA replaced RNA as the molecule carrying genetic information due to better stability.

To access the RNA world, which is a complex molecule, it was necessary to construct a polymer-type sequence of ribonucleotides, each consisting of a phosphate group, a sugar (ribose) and a nitrogenous base (adenine For example).

We report in this article unpublished observations transmitted by the rover (or “rover”) Curiosity which, equipped with analytical instruments of the landscapes and the chemistry and mineralogy of the rocks, explores since 2012 the slopes of Mount Sharp inside from the Gale crater.

During “sols” (Martian days) 3154 to 3156 in June 2021, we discovered singular structures, unearthed from the roof of ancient sedimentary layers dated to around 3.7 billion years old.

These structures are rectilinear ridges that appear in relief a few centimeters on the upper surface of sedimentary strata. These wrinkles seen from above are contiguous and are organized according to a perfectly polygonal geometry. They are made up in detail by the alignment of small nodules more or less attached to each other of essentially sulfated rock. A nodule is a small ball that appears in relief in and on the surface of strata.

Fossil desiccation fissures have already been documented on the surface of Mars. But the ones discovered here are clearly different due to three particular “details”:

The polygonal pattern is a Y pattern, forming adjoining “tomette” type hexagons, with angles approaching 120° at the junction points of the slots;

The shrinkage slots here are filled with sulphate minerals (calcium sulphate and magnesium);

These polygonal patterns are observed recurrently over a total thickness of 18 meters of the sedimentary column.

According to various experimental works carried out in terrestrial laboratories on mud tanks, this Y pattern of the junctions of the cracks is characteristic of repeated drying-wetting cycles of the sediment. On first drying, the shrinkage slits arrange themselves in a T, forming a “tile” type pattern with angles of approximately 90° at the joining points. As the experimental wetting-drying cycles progress, the slits “fatigue”, and show typical Y angles of 120° by the end of the 10th cycle.

Sulphates are so-called evaporitic chemical sedimentary rocks, that is to say resulting from the precipitation of brines associated with the evaporation of saline water. Their presence within the shrinkage cracks confirms the interpretation of these in terms of desiccation cracks. The sulphate-bearing nodules are very irregular in morphology and chemical composition, which also suggests several phases of precipitation (drying) – partial dissolution (wetting) of the nodules.

The fact that these polygonal patterns are repeatedly found over an 18-meter thickness of vertical stacking of sedimentary strata indicates that this ancient depositional environment, subject to certainly seasonal climatic cycles of wetting-drying, was maintained. over a period of hundreds of thousands of years.

This potential for polymerization of simple molecules within the sediments showing the polygonal structures takes on a particular meaning knowing that these contain on the one hand clay minerals of the smectite family and on the other hand a significant quantity of organic matter. Smectites are so-called “swelling” clays for which it has been shown experimentally that they have the ability to adsorb and concentrate nucleotides between their constituent sheets.

The SAM instrument (Sample at Mars) also revealed the presence within these same strata of simple organic compounds such as chlorobenzenes, toluenes or even various alkanes. These compounds are probably of meteoritic origin, and their residual quantity can reach about 500 g per m3 of sediment. These molecules could therefore serve as some of the “building blocks” of more complex molecules such as RNA.

In summary, we deduce from our observations, our measurements on Mars, and the various terrestrial concepts and experiments, that the evaporitic basin of Gale constituted a very favorable and durable environment with the development of this process of polymerization of the simple organic molecules in molecules more complex ones necessary for the emergence of life.

Finally, we know that the structures studied here are located in a geological unit of vertical transition from an older formation rich in clays to a more recent formation rich in sulphates, and that this same transition has been detected by orbital way in numerous craters and plains of Mars.

As a result, it now appears that the probability that biotic molecular precursors could have formed and been fossilized on the surface of Mars around 3.7 billion years ago during the Hesperian is no longer negligible.

Unlike the surface of the Earth, that of the planet Mars is not renewed or transformed by plate tectonics. The surface of Mars has thus preserved almost intact very old rocks, including those formed in an environment and climate conducive to the spontaneous construction of biotic molecular precursors.

Our discovery opens new perspectives for research on the origin of life, including (especially) on other planets than ours. It is also able to reconsider the primary objectives of Mars exploration missions and those of sample return in particular.

* Gilles Dromart, professor of geology, Ecole Normale Supérieure de Lyon