Lunar mascons. A detailed study of the Moon's gravity field became possible after the launch of space satellites into the orbit of artificial satellites of the Moon. The satellite orbits were observed using three ground stations.

By changing the frequency of the satellite transmitter, the so-called "radial accelerations" were determined - the projections of the acceleration of gravity on the direction of the Earth - the satellite (for the central part of the visible side of the Moon, these accelerations corresponded to the vertical component).

The first construction of a picture of the gravitational field of the Moon was carried out by Soviet researchers based on the results of the flight of the Luna-10 spacecraft, later the data were refined by observations of the orbits of artificial satellites of the Lunar Orbitar series, as well as on those parts of the tracks of the Apollo spacecraft, where their orbits around the Moon were determined only by its gravity field.

The gravitational field of the Moon turned out to be more complex and non-uniform than the Earth's, the surface of equal potential of gravity is more uneven, and the sources of anomalies are located closer to the surface of the Moon. An essential feature of the lunar gravity field was large positive anomalies confined to round seas, which were called mascons (from English - "mass concentration"). When approaching the mascon, the speed of the satellite increases; after the passage, the satellite slows down slightly, while the orbital height changes by 60 - 100 m.

At first, mascons were discovered in the seas of the visible side: Rains, Clarity, Crises, Nectar, Humidity; their sizes reached 50 - 200 km (they fit into the contours of the seas), and the magnitude of the anomalies was 100-200 mgal. The anomaly of the Sea of ​​Rains corresponded to an excess of the mass of the order of (1.5–4.5) x 10 -5 of the mass of the entire Moon.

Subsequently, more massive mascons were discovered on the border of the visible and far sides in the East and Marginal Seas, as well as a huge mascon in the equatorial zone of the center of the far side of the Moon. There is no sea in this place, therefore the mask is called "Hidden". Its diameter is over 1000 km, its mass is 5 times the excess mass of the Sea of ​​Rains. A hidden mascon is capable of deflecting a satellite flying at an altitude of 100 km by 1 km. The total excess mass corresponding to positive gravity anomalies. exceeds 10 -4 lunar masses. A number of negative anomalies turned out to be associated with the lunar mountains: the Jura, the Caucasus, the Taurus, Altai.

Gravity anomalies reflect the peculiarities of the distribution of matter masses in the interior of the Moon. If, for example, we assume that mascons are created by point masses, then the depths of their occurrence should be about 200 km in the Sea of ​​Rains, 280 km in the Sea of ​​Clarity, 160 km in Crises, 180 km in Tranquility, 100 km in Abundance, and 80 km in the Cognized km, Ocean of Storms - 60 km. Thus, gravity measurements revealed a non-uniform density distribution in the upper mantle.

Electrical conductivity. None of the lunar expeditions made direct measurements of the electric field of the Moon. It was calculated from variations in the magnetic field recorded by magnetometers at the Apollo 12, -15, -16 and Lunokhod-2 stations.

The moon, devoid of a magnetosphere, during its rotation around the Earth periodically finds itself in the full moon in the undisturbed earth's magnetosphere, in the new moon - in the solar wind, and twice for 2 days - in the transitional one. shock layer.

Fluctuations of the external interplanetary magnetic field penetrate into the Moon and induce a field of eddy currents in it. The rise time of the induced field depends on the distribution of electrical conductivity in the lunar interior. Simultaneous measurements of the external alternating field above the Moon and the secondary field on the surface make it possible to calculate the lunar electrical conductivity.

The Moon is arranged "conveniently" for magnetic-telluric sounding. The interplanetary magnetic field elongated from the Sun is homogeneous, its front can be considered flat, and therefore research does not require, as on Earth, a network of laboratories. Due to the fact that the Moon has a higher electrical resistance than the Earth, two hourly observations are sufficient for sounding it, while on the Earth, annual observations are needed.

The solar wind flowing around the Moon, which has a high conductivity, as if envelops the Moon with foil, without releasing the fields induced in the bowels to the surface. Therefore, on the sunny side of the Moon, only the horizontal component of the alternating magnetic field can be used, while on the night side, where the vertical component also works, the situation is more similar to that on Earth.

The Apollo magnetometers recorded the reaction of the Moon in the solar wind on the night and day sides, as well as in the geomagnetic plume, where the plasma effects of the solar wind are minimized.

In the Lemonnier crater on the sunny side of the Moon, Lunokhod-2 recorded the formation in time of fluctuations in the solar magnetic field. In this case, the horizontal component of the magnetic field reflects the deep electrical conductivity of the Moon, and the value of the vertical component at a long time characterized the intensity of the external field of the Moon. The experimental plot of apparent resistivity was interpreted by comparison with the theoretical curves.

Soviet (L. L. Vanyan and others) and foreign (K. Sonet, P. Dayel, and others) researchers have constructed various models of the electrical conductivity of the Moon. 200 km there is a poorly conducting layer with a resistivity of more than 106 ohm m; deeper lies a layer of reduced resistance (103 ohm m) with a thickness of 150–200 km; up to 600 km, the resistance increases by an order of magnitude and then again decreases to 103 ohm m at a depth of 800 km (Fig. 9).

Rice. 9. Deep structure of the Earth (thick lines) and the Moon (thin) according to geophysical data:

1 - longitudinal wave velocities; 2 - speed of transverse waves; 3 - electrical conductivity. Vertical scale - depths in relation to the corresponding radii of the Earth and the Moon

The electrical soundings of the Moon carried out to date reveal the following main features:

The Moon as a whole has a higher resistance than the Earth. On top of it is a powerful insulating layer; conductivity increases with depth. A radial stratification of the Moon has been discovered, and an inhomogeneity in the horizontal direction in terms of electrical resistance is outlined.

From the profiles of electrical conductivity and the dependence of conductivity on temperature, the temperature inside the Moon was estimated for different compositions of the mantle. In all cases, down to a depth of 600–700 km, the temperature lies below the melting temperature of basalts, and at greater depths it reaches or exceeds it.

Comparison of deep temperatures with the melting temperatures of rocks at various pressures allowed scientists to evaluate such an important physical parameter as the viscosity coefficient. It characterizes the ability of rocks to move under the action of stresses.

The upper 200-300-kilometer shell of the Moon has a very high viscosity coefficient of 10 26 - 10 27 poise. This is 2–3 orders of magnitude higher than at the corresponding depths of the Earth, even if we take the most rigid regions of ancient crystalline shields. From the surface to the center of the Moon, the viscosity drops; deeper than 500 km it decreases by a factor of 100 - 1000, i.e., it becomes commensurate with the viscosity of the Earth's mantle. In the Moon's asthenosphere, the viscosity sharply decreases to values ​​characteristic of the Earth's asthenosphere (10 20 - 10 21 poise).

Heat flow. Before spacecraft flights, it was believed that the content of radioactive elements 235 U, 238 U, 232 Th, 40 K in the interior of the Moon is on average the same as in chondrite meteorites or in the Earth's mantle. The heat flow coming from the bowels of the Moon through its surface was estimated by analogy with the corresponding flow of the Earth, where every second through every 1 cm 2 of the surface 1.5 - 10 -6 kal of heat "disappears" into space. The radius of the Moon is 3.6 times smaller than that of the Earth, its surface is 7.5%, and its volume is 2% of the Earth's. Under the condition of the same concentration of radioactive isotopes per unit volume, the value of the heat flux of 0.36 · 10 -6 kal/cm 2 s was predicted for the Moon.

In 1964, Soviet astronomers led by V. S. Troitsky measured the thermal radiation of the Moon in the wavelength range from 1 mm to 3 cm and obtained an unexpectedly high value of the average heat flux (0.85 - 0.95) 10 -6 kcal / cm2s, almost three times the calculated value. This could indicate a higher content of radioactive isotopes or that heat sources are concentrated near the surface.

The unexpected result was confirmed by direct measurements of the heat flux on the Moon. Direct measurements of the heat flux on the lunar surface were carried out during two expeditions of astronauts to the Moon: in July 1971 in the Hadley Rill region on the eastern edge of the Sea of ​​Rains (Apollo 15) and in December 1972 in the Taurus-Littrow region in the narrow bay in the southeast of the Sea of ​​Clarity ("Apollo 17"). The astronauts drilled wells, inserted fiberglass tubes and placed thermal probes in them to measure temperature and thermal conductivity. Each probe provided measurements at 11 depths and consisted of 8 platinum resistance thermometers and 4 thermocouples. Two probes were installed at depths of 1 and 1.4 m at the Apollo 15 station and one at 2.3 m at Apollo 17. The readings were transmitted to the Earth every 7 min. Data for 3.5 years for the first and 2 years for the second stations were processed. The signals began to be analyzed only a month after the instruments were launched, when their thermal equilibrium with the regolith was established. Despite the huge thermal contrasts on the surface (+130 °C during the day, -170 °C at night), temperature fluctuations almost died out at a depth of 0.8 m, while annual temperature fluctuations were felt at all studied depths. To measure the thermal conductivity of the lunar soil, electric heaters were turned on for 36 hours on command from the Earth. As the temperature increased, the value of thermal conductivity was determined. The thermal conductivity of regolith turned out to be very low and highly dependent on temperature. At the surface, it was only 0.3 10 -5 kcal (cm K) -1, deeper, as it compacted, it increased, reaching at a depth of 1–2 m values ​​~ 0.24 10 -4 kcal (cm K) -1, in the 250-meter upper layer, the thermal conductivity, apparently, remains very low, 2-3 orders of magnitude less than in the bowels of the Moon, 10 times less than in the excellent heat insulator - air, and 40 times less than in water. Thus, the regolith of the Moon, formed as a result of the grinding of clastic rocks by meteorite impacts, is a kind of "blanket" that plays the role of a thermostat for the Moon and reduces the loss of its heat. For example, during the formation of the Sea of ​​Rains, vast adjacent territories were covered with clastic rocks. Due to this, over the past 100 million years, the temperature at a depth of 25 km should have risen from 300 to 480 °C. The heat flux passing through the surface of the Moon was calculated from the value of thermal conductivity and temperature difference. Its values ​​for the Apennines region are 0.53 10 -6 kcal (cm 2 s) -1, in the Descartes region - 0.38 10 -6 kcal (cm 2 s) -1. The difference exceeds the measurement errors by 40%, the effect of the local relief and characterizes the horizontal variability of the content of radioactive isotopes in the lunar crust.

Inhomogeneities in the distribution of masses are reflected in gravitational anomalies. Gravitational anomalies, that is, deviations of the value of gravity from the "natural", normal value. Since the Moon differs very little from a ball, a constant value can be considered a normal potential. The parameters of this ball: the average radius is 1738 km, average density 3.3440.004 g/cm, dimensionless moment of inertia .

The gravitational potential of the Moon is usually written in the form of three terms

where is the attraction potential, is the centrifugal potential, is the tidal potential. The latter makes a significant contribution to the gravitational potential of the Moon. In the lecture devoted to the deformation of the level surface of the planet under the action of a tidal perturbation, we showed that the level surface "stretches" towards the attracting body. The moon can be approximated by a triaxial ellipsoid with semi-axes , , m, oriented so that its major axis is directed towards the Earth.

A detailed study of the figure of the moon became possible only after the launch of artificial satellites of the moon (ASL). However, the study of the Moon was carried out long before the launch of the ISL. Employees of the SAI M.U. Sagitov and N.P. Grushinsky, using astrometric observations, obtained that the force of gravity on the lunar triaxial ellipsoid varies according to the law

where , . This formula shows that the force of gravity towards the pole does not increase, as is the case on Earth, but decreases! This is contrary to common sense. Moreover, the geometric contraction is positive:

According to Clairaut's theorem, if the Moon is an equilibrium body, then . Maybe the value is abnormally small? Most likely - the Moon is not an equilibrium body. She stopped her rotation after she had received her hydrostatic contraction, then solidified. All these questions lie in line with the cosmogony of the Earth-Moon system.

In the satellite era, the gravitational potential of the Moon was determined repeatedly. We indicate only the result of Ferrari

As you can see, again, the force of gravity towards the pole does not increase, but decreases.

The map of Ferrari's selenoid clearly shows an increase in the height of the level surface above the ball towards the Earth by 400 meters and over 300 meters from the far side of the Moon. That is, the elongation of the selenoid towards the Earth is obvious. True, calculations show that the tidal potential of the Earth is an order of magnitude smaller! Let's fantasize a little. We know that the Moon is moving away from us due to the tidal action of the Earth. Once upon a time, the Moon was much closer to us, and the tidal effect is much greater than today. If the Moon were 2.7 times closer, then the observed elongation of the selenoid towards the Earth could be explained by the tidal influence. But then the conclusion follows that even then the rotation of the Moon and its revolution around the Earth were synchronous!

Observations of the ASL made it possible to determine the gravitational field of the Moon, and, based on it, regional (covering large areas) anomalies. The determination of local anomalies requires the performance of physical experiments. As we already mentioned, American astronauts made gravitational measurements with special lunar gravimeters, but these measurements were very few. One of the universal measurement methods is the observation of a freely falling body. The main difficulty for the implementation of the method is to ensure the accuracy of determining the acceleration of free fall of the body.

In 1968, a year before the landing of a man on the moon, American scientists P. Muller and W. Sjögren investigated the ray accelerations of ASL Lunar Orbiter 5. They found on the seas where they must be negative gravitational anomalies, in fact there are large positive anomalies that cannot be explained by anything other than the concentration of heavy masses. They called such structures mascons (mass concentrations). At the satellite flight altitude (100 km) gravitational anomalies reached 200 mGal and more. In particular, over the Sea of ​​Rains, the gravitational anomaly is 250 mGal, over the Sea of ​​Clarity -- 220 mGal, over the sea of ​​Crises - 130 mGal. Various "scenarios" for the formation of these anomalies have been proposed. Muller and Sjögren themselves believed that the positive anomaly was created by an iron-nickel meteorite that fell on the Moon and remained in the lunar crust. Later this hypothesis prevailed. An asteroid-sized body falls on the Moon and forms a "sea trench". This depression creates a small negative anomaly. That hour, lava outpourings rise up and fill the cracks to complete isostatic compensation. The bark hardens, acquires high strength and withstands additional loads without deformation. The pool is filled with material, an excess mass is created, which gives a positive gravitational anomaly. True, modern data indicate that lava outpourings did not occur immediately, but after 0.5 billion years. The initially formed negative anomaly disappears, the crust becomes isostatically compensated. The resulting lava outpourings are strong enough to withstand the crust, and for 3 billion years the isostatically uncompensated crust has positive anomalies due to the intrusion of denser masses from the interior of the Moon.