How did lunar craters form? This issue has led to a long discussion between supporters of two hypotheses on the origin of lunar craters: volcanic and meteorite.
According to the volcanic hypothesis, which was put forward by the German astronomer Johann Schröter in the 80s of the 18th century, craters arose as a result of grandiose eruptions on the lunar surface. In 1824, his compatriot Franz von Gruutuisen proposed a meteorite theory that explained the formation of craters by the fall of meteorites.
Only 113 years later, in 1937, a Russian student Kirill Stanyukovich (future doctor of science and professor) proved that when meteorites strike at cosmic velocities, an explosion occurs, as a result of which not only a meteorite is melted, but also some of the rocks at the site of the impact. The explosive theory of Stanyukovich was developed in 1947-1960. by himself, and then by other researchers.
Flights to the Moon starting in 1964 with American Ranger series spacecraft, the discovery of craters on Mars and Mercury, and then on planetary satellites and asteroids, summed up the final result in this discussion, which lasted more than 100 years. Meteorite theory is now generally accepted.
The expeditions of American astronauts to the moon (1969-1972), the landing of the Soviet automatic stations “Luna-16, -20 and -24” (1970-1976), which delivered lunar soil to Earth, led to the emergence of a new science – lunar mineralogy. Lunar minerals fell into the hands of specialists. This allowed mineralogists to compare their structure and composition with terrestrial minerals and meteorites.
First of all, the age of lunar rocks was determined by the content of radioactive isotopes. The oldest of them, as shown by the study by the uranium-lead method, formed 4.46 billion years ago. Similar results were obtained using the strontium method. But almost the same (4.6 billion years) is the age of the oldest rocks of the Earth and meteorites. So, it was then, about 4.5 billion years ago, that the Solar system formed, including the Earth, the Moon and those bodies whose fragments arrive to us in the form of meteorites. Analysis of lunar minerals made it possible to understand what the differences between the continents and the seas on the moon are. It turned out that the seas are covered with volcanic rocks, mainly basalts. They have a rounded shape, a flat surface, the relative youth of which is indicated not only by radioactive analysis, but also by a relatively small number of craters formed by impacts of large meteorites. All this shows that the “seas” are the result of grandiose lava outflows from the bowels of the moon caused by impacts of small asteroids on its surface.
Radioactive analysis showed that most of the Moon Seas (Sea of Vapors, Sea of Clarity, Sea of Tranquility, Ocean of Storms) were formed 4 billion years ago. Somewhat younger than the Sea of Rains: 3.87 billion years have passed since its inception. Probably, during this period the remnants of the bodies from which the Earth and the Moon were formed fell on the Moon.
Under the crust is a mantle in which, like the earth, you can distinguish the upper, middle and lower. The thickness of the upper mantle is about 250 km, and the average is about 500 km, and its border with the lower mantle is located at a depth of about 1000 km. Up to this level, the shear wave velocities are almost constant, and this means that the subsoil material is in a solid state, representing a powerful and relatively cold lithosphere in which seismic vibrations do not decay for a long time. At the boundary with the lower mantle, temperatures approach melting temperatures, hence the strong absorption of seismic waves. This region represents the lunar asthenosphere.
In the center, apparently, is a small liquid core with a radius of less than 350 kilometers, through which transverse waves do not pass. The core may be iron-sulfite or iron; in the latter case, it should be less, which is better consistent with the estimates of the density distribution over depth. Its mass probably does not exceed 2% of the mass of the entire moon. The temperature in the core depends on its composition and, apparently, is in the range 1300 – 1900 K. The lower boundary corresponds to the assumption that the heavy fraction of the lunar protestation is enriched in sulfur, mainly in the form of sulfides, and the formation of a core from the Fe – FeS eutectic with a melting point (slightly dependent pressure) of about 1300 K. With the upper boundary, the assumption about the enrichment of the proton material of the Moon with light metals (Mg, Ca, Na, Al), which, together with silicon and oxygen, are part of the most important rock-forming minerals of basic and ult main species – pyroxenes and olivines. The latter assumption is also favored by the low content of iron and nickel in the moon, as indicated by its low average area.
Long-term studies have shown that the main lunar rocks are:
1) marine basalts, more or less rich in iron and titanium;
2) continental basalts, rich in stone, rare earth elements and phosphorus;
3) aluminum continental basalts – a possible result of impact melting;
4) igneous rocks