A few days agostill active on Mars, took to very strange: on the sandy ground beaten by the Martians is a small flower-shaped mineral. Barely 1 cm wide, the concretion has several arms, connected to the base.
However, this is not the first time Curiosity has photographed this type of training. For scientists, they are the result of a diagenetic process, which means that they are the result of a rearrangement. in situ of, leading to the growth of a new crystal. The formation of such structures is remarkably associated with the presence of saturated sulfate water.
Mineral formation, captured by Mars Curiosity Sun 3396, created from 6 images taken by the Mahli instrument on Mars Curiosity Rover Sun 3396 and 3397. © Nasa / JPL-Caltech / MSSS
A Martian analogue of our sand roses
In this sense, it could be the Martian analog of our roses. The appearance of the formation, in fact, is reminiscent of an evaporitic concretion. On Earth, there are evaporites resulting from deposits that precipitate during the evaporation process of a supersaturated with chlorides or sulphates ( ). The rock salt is the best known evaporite, however or anhydrite are also part of this class of minerals. Therefore, evaporites generally form in shallow basins, subject to strong evaporation and poor hydrological regime, such as lagoons or .
Well-known sand roses are formed under specific conditions. These magnificent rocksorange are crystallizations of in a generally sandy environment. Therefore, sand roses do not form in the same way as chalk in the middle. . Gypsum crystals grow even within an unconsolidated medium (such as sand or sand) ), soaked with a rich in . As evaporation progresses, the groundwater level is concentrated into ions, until saturation allows the crystallization of the minerals. The crystals grow by pushing the sand around them. Thus we find the roses of the sands in the buried under a few meters of sand.
A small mineral flower dug up by erosion
A similar process could be the origin of the so-called fine Martian concretion Black hawthorn salt. Previous studies of similar concretions show that the crystals would have grown inside the soil. But how to explain its presence on the surface today? Simply by the erosion process. In time, the Martian winds burned the rock, until these small concretions came to light. More resistant to erosion by its nature, this concretion is now found on the ground, like a small mineral flower with.
In another Martian site studied by Curiosity, the composition of this type of concretion was analyzed using instruments.and APXS. It seems, at least for this site, that concretions are especially enriched in magnesium (Mg) and in various varieties of (S). The chemical composition would be mainly MgSO4, what differentiates these concretions from the terrestrial gypsum, of CaSO composition4.
This type of concretion therefore gives a lot of information about the past environment of Mars, and in particular about the presence of water in the form of brine in its basement.