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

A new study of craters on the lunar surface tends to attribute the asymmetry between the two lunar faces to a massive meteorite impact at its south pole, following the hypothesis that the latter thus motivated volcanic processes at a level on its visible side.

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[EN VIDÉO] GRAIL filming the far side of the Moon
The first images taken by the GRAIL lunar probes show the hidden side of the moon. The dark side of the Moon is the one we never see from Earth. © NASA

Moon constantly appears before the Earth by the same face: it is locked by the effect high tide (her rotation period equals his period of revolution), and therefore has dark side, invisible from Earth. Differences between the two individuals were identified from the outset. review face hidden by the Soviet Luna-3 probe in 1959: scientists then discovered an uneven and heavily cratered surface (called mountainous terrain), in contrast to the visible face, dotted with lunar seas – wide plains basalt dark and smooth looking.

Could different chemical compositions explain the lunar asymmetry?

But the elements that distinguish the two surfaces of the Moon are not just geomorphological: Missions since Luna 3 have also revealed significant differences in geochemical composition between the two sides. On the visible side is anomaly chemical composition is called Procellarum KREEP Terrane (PCT), characterized by high concentrations potassium (K), in rare earth (REE), phosphorus (P) and other carbon-forming elements. heat such as thorium (T). This anomaly is widely represented in and around the ocean. Storms (Ocean Procellarum) and is ubiquitous on the visible side, but appears much more scattered on the hidden side.

Scientists now agree to point the finger at the PKT chemical anomaly to explain the dichotomy between the two lunar facets: characterized by a high concentration of fuel elements, this anomaly could have the ability to motivate late volcanic processes at the level of the visible range. side of our satellite. But the mechanism for the occurrence of this anomaly has so far remained unknown: also the presence of high concentrations of incompatible elements (which tend to concentrate in the phase liquid when fractional crystallization one magma), the elements characterizing KREEP would therefore be the last to crystallize and thus form a relatively uniform top layer moon coat ; new study seems to indicate that a meteorite impact early in formation South Pole-Aitken Basin (second largest impact basin in solar systemlocated at the lunar south pole, and a small part of which is represented on the visible face – 2500 kilometers in diameter by 12 kilometers in depth) could redistribute the elements that make up the CRIEP, mainly at the level of the visible face: indeed, the formation of the South Pole-Aitken basin, according to apparently corresponds in time to the formation of the last lunar seas.

The fall of a colossal meteorite would lead to a redistribution of chemical elements around the moon.

The research team then made digital models to simulate the effect fuzzy heat generated during such an impact on the interior of the Moon, and on the possible redistribution of KREEP elements. Their modeling was largely convincing: for any simulated impact scenario (from a direct and high impact to a low velocity, low angle impact), the number of KREEP elements mobilized varies, but systematically results in high concentrations of these elements around the far side. , in accordance with the observed CPT anomalies. Thus, scientists support the idea that a meteorite impact at the birth of the South Pole-Aitken Basin would allow excavation case in Coat top, bottom crust.

And the dating arguments for various lunar structures on the visible side seem to support this idea: the count of craters in the PKT zone indicates that this formation must have been more recent than the formation of the south pole impact basin, and the oldest basalt seas would be dated to -4. 3 billion years (about 200 million years after the Aitken impact); their model does show a thermal asymmetry between the two sides more than 600 million years after the impact, caused by chemical asymmetries and causing volcanic processes on the visible side.

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