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Moon surface

The main types of geological structures on the moon are continents and seas. The dark sea surface occupies more of the visible side of the moon, and is practically absent on the reverse side.

Moon surface
MATERIALS form the upper part of the lunar crust, the composition of which is from anorthosites on the surface to dunites and troctolites at the base of the crust. The thickness of this crust is estimated from a network of seismometers left by the Apollo on the Moon and recording the passage of waves from endogenous and shock moonquakes.

In the center of the visible side, the crust thickness averages 60 km, in the areas of the Nectar and Vostochny seas it increases to 80 – 100 km, and on the reverse side it can reach 100 – 150 km. Gravimetric data obtained by Doppler tracking of the speeds of orbital apparatuses indicate a decrease in the thickness of the crust by about half in areas of the rounded seas of Rains, Clarity, Vostochny, etc. An intermittent horizon with an increased speed of seismic waves is marked at the base of the crust.

SEAS, occupying the remaining 16% of the surface, are dark basaltic plains with relatively rare craters located in regional depressions, usually in the rounded depressions of the moon basins. Everywhere, marine basalts overlap the more ancient continental structures. The difference in the hypsometric levels of the continents and the seas is explained by isostatic compensation, since the density of the basalts of the seas is higher than the density of anorthosites by 0.3 – 0.4 g / cm3. However, in some rounded seas there are such excess masses (masks) that it is necessary to allow the outflow to the surface 20 km of basalt. This estimate seems to be overestimated, since it is usually possible to estimate the power of flooding basalts only by the first kilometers from uncovered relicts to basalt craters in the seas. Most likely, the mascones were created by the sum of the masses of surface basalts and mantle uplifts beneath these seas, i.e., a decrease in the thickness of the crust in these places, which was apparently a consequence of the fall of asteroids with the formation of giant basin basins and subsequent isostatic alignment and associated volcanism. In the arrangement of the dark seas, two bands are outlined: from the Ocean of Storms to the Sea of ​​Clouds and from the Sea of ​​Rains through the Sea of ​​Abundance to the South Sea. The position of these bands is apparently determined by global systems of gaps that facilitated the release of magma, but the fact that the basalts filled the chain of rounded pools in this case is not evidence of the endogenous nature of these pools; basalts with their excess weight only contributed to their preservation in the form of depressions. Lava strata are formed by numerous streams, the thickness of which reaches 20 – 50 m. Judging by their composition and experiments with the melting of lunar samples, the viscosity of the streams was very small, comparable to the viscosity of water, and therefore they spread over great distances, often without forming noticeable edge ledges. In some local depressions, lava lakes with a depth of at least 200 m were formed (the area of ​​the East Sea), in which, with subsequent drainage of lavas, marginal ledges remained, indicating the level of lava standing. In addition to the dark basalt seas, there are undoubtedly more ancient lava fields on the continents with a clarified surface, probably covered by thin covers of emissions from later impact basins and craters. Relatively young impact small craters pierce these covers and bring out dark material, for example, in the area of ​​the Schiller and Vargentin craters, in the vicinity of the Sea of ​​Nectar and others. However, in most cases, these are “ancient” seas (structurally older than the dark sea basalts) cannot be distinguished from facies of fluidized emissions from basins. Thus, a powerful continental crust covers the entire moon, not dividing, as on Earth, into separate “continents”, and only in some places does it thin out and overlap with basalt covers. Under the crust to a depth of 800 km lies a mantle in which, starting from a depth of about 300 km, there are signs of weak modern activity, manifested by moonquakes. Moonquakes add up to two wide eroded belts that do not coincide with the belts of dark seas. Deeper than 800 km, apparently, a significant amount of melt appears that does not transmit transverse seismic waves.

Impact craters occupy a predominant place among the structures of the Moon, filling the entire size range from microcraters to structures with diameters of 150 – 300 km. The youngest (Copernican) craters measuring less than 10 – 15 km have simple cup-shaped forms and sharp ridges of shafts, streaky and dune-like structures are found on the outer slopes of the shafts, large blocks apparently ejected from the crater, and secondary craters formed by such blocks; from the secondary craters, “luses” of material knocked out by boulders are sometimes stretched. As a rule, these craters are surrounded by light halo. In larger young craters with diameters up to many tens of kilometers, the structure is complicated.

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