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We know why the two sides of the moon are so different

A new study of craters on the moon’s surface tends to associate the asymmetry between the two lunar faces with a massive meteorite impact at the South Pole level, hypothesizing that the latter would thus have motivated the volcanic processes at the level from the visible side.

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[EN VIDÉO] GRAIL films the other side of the moon
First pictures taken by 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 locked by the effect of tide (hair rotation period: equals his period of revolution), and therefore has a dark side, invisible from Earth. The differences between the two faces came to light from the start Overview of the face hidden by the Soviet probe Luna 3, in 1959: the scientists then discovered a rough and heavily cratered surface (called the highlands), in contrast to the visible face that is dotted with lunar seas – wide plains basalt dark and smooth in appearance.

Could different chemical compositions explain the lunar asymmetry?

But the elements that distinguish the two lunar planes are not just geomorphological: missions after Luna 3 also revealed significant differences in geochemical composition between the two planes. The visible side houses a anomaly of chemical composition called Procellarum CREEP Terrane (PKT), characterized by high concentrations of potassium (Family rare earth (REE), phosphorus (P) and other carbon-producing elements. heat like the thorium (E). This anomaly is ubiquitous on and around the ocean of stormOceanus Procellarum) and is found globally on the visible side, but seems much more scattered on the hidden side.

Scientists currently agree to point the finger at the chemical anomaly PKT to explain the dichotomy between the two lunar faces: This anomaly, characterized by high concentrations of heat-producing elements, is said to have had the ability to delay late volcanic processes. motivation at the level of the visible. side of our satellite. But the mechanism underlying this anomaly remained unknown: also high concentrations of incompatible elements (which tend to concentrate in the phase liquid when fractional crystallization of a magma), the elements characteristic of the KREEP would therefore have crystallized last and thus would in a relatively uniform way the upper layer of the moon jacketa new study seems to indicate that the meteorite impact was at the origin of the formation of the South Pole-Aitken Basin (second largest impact basin in the solar systemlocated at the south pole of the moon, and a small part of which is represented on the visible plane – 2500 kilometers in diameter for 12 kilometers depth) could preferentially redistribute the elements that make up the KREEP at the level of the visible plane : indeed , the formation of the South Pole-Aitken Basin seems to temporarily correspond to the formation of the last lunar seas.

A Colossal Meteorite Impact Would Have Redistributed Chemical Elements Around the Moon

The team of researchers then produced digital models to simulate the effect of unclear from heat generated by such an impact on the Moon’s interior, and on a possible redistribution of KREEP elements. Their simulations were largely convincing: for each simulated impact scenario (from a direct and violent impact to a low-velocity, low-angle impact), the amount of mobilized KREEP elements varies, but systematically results in high concentrations. , consistent with the observed KPT deviations. Thus, scientists support the idea that the meteorite impact at the origin of the South Pole-Aitken Basin would have made it possible to conduct excavations matter in the coat top, bottom crust

And the dating arguments of various lunar structures on the visible side seem to support this idea: Counting craters in the PKT zone indicates that this formation would be posterior to the formation of the South Pole impact basin, dating the oldest basalt seas. at -4.3 billion years (about 200 million years after the Aitken impact); their model indeed shows a thermal asymmetry between the two planes more than 600 million years after the impact, caused by the asymmetry in chemical composition and motivational volcanic processes on the visible plane.

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