Asymmetry of the Moon associated with radioactive isotopes in its mantle
The key role in these processes was played by the KREEP rocks, consisting mainly of potassium, rare earth elements and phosphorus
Planetologists have found that the differences between the visible and reverse sides of the moon may be due to the fact that the radioactive elements are not uniformly distributed over its mantle. The results of the study were published by the journal Nature Geoscience. “On the surface of the moon, there are traces of geological processes that took place in the early days of the solar system. In particular, on its visible side, there are regions with a uniquely high concentration of uranium and thorium. If we find out how this excess, then we will understand how the Moon arose and what conditions reigned on the Earth at that time, “said one of the authors of the study, a geologist from Tokyo Institute of Technology (Japan) Mathieu Lanoville.
Unlike the relatively “flat” visible side, the back of the moon is covered with many hills, crevices, and craters. Measurements of the attraction force of our satellite show that its visible half is heavier than the invisible one, and the craters on the surface of the latter are noticeably deeper, and the lunar crust is relatively thinner.
Astronomers have long been trying to understand what this is connected with. In particular, some planetologists believe that almost immediately after the formation of the moon, one or more “embryos” of planets fell onto its surface. This can explain both the unusual isotopic composition of the moon and the differences between the hemispheres of the satellite.
Lanoville and his colleagues found a simpler and more plausible explanation for these differences. They calculated how the mantle of the newborn Moon was arranged immediately after the collision of its ancestor, Theia, with the Earth.
Planetologists suggest that traces of the first stages of moon formation can be found in special rocks on its surface, which geologists call KREEP. These minerals are especially rich in potassium (K), rare earth elements (REE), and phosphorus (P). Dozens of fragments of these rocks, which formed from 4.3 to 3.9 billion years ago, brought Apollo expeditions to Earth.
The main unusual feature of KREEP deposits is that these rocks contain about 700 times more uranium and thorium than in primitive chondrite asteroids, which are composed of the primary matter of the solar system. This prompted geologists to the idea that radioactive substances and the heat generated by them played an important role in the formation of the moon and geological processes in its bowels.
Guided by this idea, Lanoville and his team tried to reproduce in experiments the process of formation of KREEP and all lunar rocks as a whole. For this, scientists created their artificial analog from the lava samples of two terrestrial volcanoes.
The experiments showed that the KREEP clusters in the mantle really had to play an important role in the formation of the Moon’s bowels, warming them and maintaining them in a molten state much longer than geologists had previously expected. Considering that there are many more of these rocks on the visible side of our satellite, we can conclude that it froze later than the opposite hemisphere of the moon.
Such differences in the behavior of the mantle and the crust of the moon, according to researchers, explain why the appearance of its hemispheres is so very different. In particular, their calculations indicate that the bowels of the visible side of the moon produced about 4 to 13 times more magma than the other half. As a result, vast lunar “seas” formed – traces of gigantic outpourings of lava, as well as a thicker crust that is not on the other side of the moon, the authors conclude.