So the moon got two such different faces

A new study of craters on the Moon’s surface tends to link the asymmetry between the two lunar faces to a massive meteorite impact at the level of its south pole, following the hypothesis that the latter would have motivated volcanic processes on the plane on its visible side , facing the earth.

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First images from the GRAIL lunar probes showing the hidden side of the moon. The dark side of the moon is the one we never see from Earth. ©NASA

The moon constantly presents the same face to the earth: it is blocked by the action of tide (its period of rotation is equal to its period of revolution) and therefore has a dark side, invisible from earth. The differences between the two faces were brought to light from the start overview of the face obscured by the Soviet Luna 3 probe in 1959: Scientists then discovered a rough and heavily cratered surface (the so-called highlands), in contrast to the visible face, which is strewn lunar seas – wide plains basalt dark and smooth in appearance.

Would different chemical compositions explain the lunar asymmetry?

But the elements that differentiate the two lunar surfaces aren’t just geomorphological: missions to Luna 3 also revealed significant differences in geochemical composition between the two surfaces. The visible side houses a anomaly called the chemical composition Procellarum KREEP Terrane (PKT), characterized by high concentrations of potassium (Relationship Rare earth (REE), in phosphorus (P) as well as other heat-generating elements such as thorium (Th). This anomaly is widespread on and around the ocean storms (Oceanus Procellarum) and is found globally on the visible side, but appears to be much more scattered on the hidden side.

Scientists currently agree to point the finger at the PKT chemical anomaly to explain the dichotomy between the two lunar surfaces: Characterized by high concentrations of heat-generating elements, this anomaly may have had the ability to predict late volcanic processes on the moon Level of the visible motivate side of our satellite. But the mechanism underlying this anomaly still remained unknown: Also high concentrations of incompatible elements (which tend to concentrate in the phase fluid When fractional crystallization of a magma), consequently the elements characterizing the KREEP would have crystallized last and would have formed the upper layer of the relatively evenly moon coat ; a new study seems to indicate that the meteorite impact is at the origin of the formation of the South Pole-Aitken Basin (second largest impact basin of the solar systemlocated at the lunar south pole, and a small part of which is shown on the visible side – 2,500 kilometers in diameter at 12 kilometers deep) may have redistributed the elements that make up the KREEP preferentially at the level of the visible side : Indeed, the formation of the South Pole-Aitken Basin to correspond in time to the formation of the last lunar seas.

A colossal meteor impact would have redistributed chemical elements around the moon

The research team then produced digital models to simulate the effect vague the heat generated by such an impact inside the moon and a possible redistribution of KREEP elements. Their simulations proved to be largely conclusive: for each impact scenario simulated (ranging from a direct and severe impact to a minor impact). speed and low angle), the amount of mobilized KREEP elements varies but consistently results in high concentrations of these elements on the other side, consistent with the observed KPT anomalies. Scientists therefore support the idea that the meteorite impact at the origin of the South Pole-Aitken Basin would have made excavation possible matter in the a coat up down crust.

And the dating arguments of various lunar structures on the visible side seem to support this idea: the count of craters in the PKT zone suggests that this formation would lie after the formation of the South Pole impact basin and the oldest basaltic seas would on – 4.3 billion years (about 200 million years after the Aitken impact); Indeed, their model shows a thermal asymmetry between the two sides for more than 600 million years after impact, caused by the asymmetry in the chemical composition and the motivation of volcanic processes on the visible side.

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