The mysteries of the Moon

Prior to the beginning of the space age, little was known about the nature of the lunar subsoil and the composition of the rocks composing the lunar surface. Now something has become clear, but still the mysteries of the Moon do not stop disputes about the internal structure and the origin of its magnetic field. True, according to the data on the radio emission of the Moon, many of its researchers, even before 1957, had a good idea of the structure of the finely divided upper cover of our satellite. Direct drilling of the Moon has affected so far only the most superficial layers of the neighboring world. And if it were not for the moon-quakes and the moon seismographs not recording them, we would still not know what is hidden under the outer cover of the moon. With moonquakes, as on Earth, two types of elastic vibrations are formed. Some of them are called surface waves. By the nature of their distribution can be judged in the structure of the lunar cortex and subcortical layer (upper mantle). Others, much more interesting, are called volume waves. They permeate the entire solid body of the Moon. Some of them are longitudinal oscillations of the lunar material, others - transverse. In essence, both are sound waves very low frequencies.

Now something has become clear, but still the mysteries of the Moon do not stop disputes about the internal structure and the origin of its magnetic field

Any heterogeneity in the solid body of the Moon (in particular, the presence of a dense core) immediately affects the seismic waves. They can refract, reflect, lose the energy contained in them, change some of their properties. For all these changes, one can judge what the lunar bowels are. Moonquakes can be natural or artificial. In the first case, the cause is a sharp displacement of the lunar crust under the influence of volcanic processes or impact on the lunar surface of a large meteorite. In the second case, seismic waves are generated either by artificial explosions, or by falling onto the moon of artificial objects (for example, parts of a spacecraft). American researchers of the Moon arranged artificial explosions of small power in the neighboring world. Analysis of the seismograms showed that the regolith (the surface loosened layer of the Moon) extends to a depth of 40-45 m. For deeper exploration, the energy of the explosions was too small. And yet mankind was lucky. In the middle of May, 1972, a large meteorite weighing 600 tons unexpectedly fell onto the Moon. As it was already said, it formed a crater with a diameter of about 100 m, and, most importantly, generated quite strong seismic waves "enlightened" the Moon. The thickness of the lunar crust at the site of the meteorite fall is 60 km. Below a depth of 960 km stretches the mantle, under which the core is located. The result is curious: it turns out that the lunar crust is twice as thick as the terrestrial (under the continents), and the lunar core has an unusually large diameter - about 1500 km. From these facts, far-reaching conclusions can be drawn. If the Moon had in its bowels the same percentage of radioactive elements as in the regolith, then it would have to be completely melted due to radioactive heat. But it is not. Hence, the radioactive elements have accumulated only in the surface layers of the Moon. This could only come about if, during the initial epochs of its existence, the moon was a fiery sphere and heavy radioactive elements, together with the slag, surfaced on its surface. Judging by the seismograms, the lunar crust has the following structure. Under the regolith to a depth of 25 km it consists of basalt. Below is the eclogite - a rock of the same composition as basalt, but more dense. We do not know anything about the composition of the lunar mantle and the core of the Moon, except perhaps because their temperature is quite high and amounts to at least hundreds of degrees. Otherwise, it is difficult to explain the flow of heat that comes from the lunar bowels and is recorded by instruments left on the Moon. The moon has around itself a very weak magnetic field, about a thousand times weaker than the earth's. However, some soil samples taken from the Moon are strongly magnetized. This residual magnetism of the lunar rocks, apparently, was caused by the fact that the Moon once had a strong magnetic field. Why she later "demagnetized", it is difficult to say.