How the impact of a giant asteroid changed the face of the moon

A new study of craters on the Moon tends to link the asymmetry between the two lunar faces to a massive meteorite impact at the South Pole level, following the hypothesis that the latter would have stimulated volcanic processes at the level. From its visible side facing the Earth.

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[EN VIDÉO] Grille photographing the far side of the moon
The first images taken by the GRAIL lunar probe show the hidden side of the moon. The dark side of the moon is the side of the earth we never see. © NASA

The moon constantly shows the same face to the Earth: it’s locked with effect Extension (Ha rotation period equal to her Revolution period), and thus contains a file hidden face, invisible from Earth. The differences between the two aspects have been highlighted from the first Overview From the face hidden by the Soviet probe Luna 3, in 1959: Scientists then discovered a rough surface filled with craters (called ridges), in contrast to the visible face, which was riddled with craters lunar seas – wide plains basalt Dark and smooth in appearance.

Do different chemical compositions explain the lunar asymmetry?

But the elements that characterize the two lunar faces are not only geomorphological: missions after Luna 3 have also revealed significant differences in the geochemical composition between the two faces. The visible side houses a abnormality The chemical structure is called Procellarum KREEP Terrane (PKT), which is characterized by high concentrations of potassium (K), rare earth (REE), in phosphorous (P) as well as other elements that produce the heat Like the thorium (y). This anomaly is widely represented in and around the periphery storms (Oceanus Procellarum) and is universally present on the visible side, but appears more distracting on the hidden side.

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Scientists currently agree to point the finger at the chemical anomaly PKT to explain the split between the two lunar faces: Characterized by high concentrations of heat-producing elements, this anomaly may have the potential to trigger late volcanic processes in the visible plane. Satellite face. But the mechanism in the origin of this anomaly remains unknown: high concentrations of incompatible elements (which tend to concentrate in the liquid phase during microcrystallization from one the silhouette), the elements characterizing KREEP have finally crystallized, and thus have formed in a relatively uniform way the upper layer of moon coat ; New study It seems to indicate that the impact of the meteorite in the origin of the formation Antarctica Basin – Aitken (the second largest impact basin in the Solar System, located at the south pole of the Moon, a small part of which is represented on the visible face – 2500 km in diameter and 12 km in depth) could preferentially redistribute the constituent elements of KREEP at the level of the visible face: in Indeed, it appears that the formation of the Antarctic-Aitken basin tentatively coincides with the formation of the last lunar seas.

A massive meteor impact would redistribute chemical elements around the moon

Then the team of researchers produced digital models to simulate the effect Problems From the heat generated by such an effect on the interior of the Moon, and on a possible redistribution of the KREEP elements. Their simulations have proven to be quite crucial: for any simulated impact scenario (from direct and violent to low impact). Speed and low angle), the amount of mobilized KREEP elements varies, but consistently results in high concentrations of these elements around the distal side, consistent with the observed KPT anomaly. Thus, scientists support the idea that a meteorite impact at the origin of the Antarctic-Aitken basin would have made it possible to drill material down to coat upper, lower Shell.

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Dating arguments for various lunar structures on the visible side seem to support this idea: the calculation of craters in the PKT region indicates that this formation would have been later to form the Antarctic impact basin, and the older basaltic seas would date back to -4.3 billion years (about 200 million years after the Aitken impact) ; Their model already shows a thermal asymmetry between the two faces over more than 600 million years after the impact, due to the asymmetry in the chemical composition and the stimulation of volcanic processes on the visible face.

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