卷 52, 编号 6 (2018)

The variations of the bulk composition of the silicate Moon (crust + mantle = Bulk Silicate Moon, BSM) depending on the thermal state are explored based on the joint inversion of gravitational, seismic, and petrologic data within the Na₂O–TiO₂–CaO–FeO–MgO–Al₂O₃–SiO₂ system. The mantle bulk temperature T_mean determining the mineral composition and physical properties of the Moon is adopted as the integral characteristic of thermal state. By parameter T_mean, all thermal models of the Moon can be conventionally broken down into the “cold” with T_mean ~ 690–860°C and the “hot” with T_mean ~ 925–1075°C. The estimations of refractory oxide abundance in lunar rocks depending on the thermal state are included in two different groups. Cold models of BSM are comparable by the bulk content of Al₂O₃ ~ 3.0–4.6 wt % to those for the silicate Earth (Bulk Silicate Earth, BSE), while hot models of BSM are significantly enriched with Al₂O₃ ~ 5.1–7.3 wt % (Al₂O₃ ~ 1.2–1.7 × BSE) as compared with BSE. On the contrary, independent of the temperature distribution, both types of BSM models are characterized by nearly constant values of bulk concentrations of FeO ~ 12–13 wt % and magnesian number MG# 80–81.5 (MG# = [MgO/(MgO + FeO) × 100]), which differ markedly from those for BSE (FeO ~ 8% and MG# 89). It means that for all possible temperature distributions, the silicate fraction of the Moon is FeO-enriched and MgO-depleted in relation to BSE. These arguments discard the possibility of the Moon’s formation out of the material of the Earth’s primitive mantle. In spite of the almost complete coincidence of the isotopic systems, this apparently undeniable fact has no adequate explanation in the existing canonical models of the Moon’s origin and should result in additional constraints on the dynamic processes in models of the formation of the Earth–Moon system. However, the problem of the similarity of and/or difference between compositions of the Moon and the Earth regarding the abundance of refractory elements, which is very important for the geochemistry of the Moon and the Earth’s mantle, remains unresolved and requires further study.

Results of astrometric and BVRI photometric observations of the active asteroid (596) Scheila are presented. The observations were carried out at the Zeiss-1000 telescope of the Sanglokh International Astronomical Observatory of the Institute of Astrophysics of the Academy of Sciences of the Republic of Tajikistan on June 16−17 and from July 30 to August 1, 2017. The coordinates of the object and its orbit were determined; and the apparent brightness in four filters, the absolute brightness in the V and R filters, and the color indices were obtained. The light curves suggest that no substantial changes in the asteroid’s brightness occurred during the observations. The absolute brightness of the asteroid in the V and R filters was (9.1 ± 0.05)^m and (8.8 ± 0.03)^m, respectively. The mean value of the asteroid diameter was (119 ± 2) km. The mean values of the color indices (B−V = (0.72 ± 0.05)^m, V−R = (0.29 ± 0.03)^m, and R−I = (0.31 ± 0.03)^m) agree well with the values for asteroids of the P- and D-types and its averages. The rotation period of the asteroid estimated from photometric observations was 16.1 ± 0.2 h. The analysis of the data has shown that the asteroid continues to exhibit the same values of absolute brightness and other characteristics as those before the collision with a small body in December 2010, though the latter resulted in the outburst event and cometary activity of the asteroid. Most likely, the collision of asteroid (596) Scheila with a small body did not lead to catastrophic changes in the surface of the asteroid or to its compete break-up.

We consider gravitational instability of the dust layer in the midplane of a protoplanetary disk with turbulence and shear stresses between the gas in the disk and that in the dust layer. We solve a linearized system of hydrodynamic equations for perturbations of dust (monodisperse) and gas phases in the incompressible gas approximation. We take into account the gas drag of solid particles (dust aggregates), turbulent diffusion and the velocity dispersion of particles, and the perturbation of the azimuthal velocity of gas in the layer upon the transfer of angular momentum from solid particles to it and from this gas to the surrounding gas in the disk. We obtain and solve the dispersion equation for the layer with the ratio of surface densities of the dust phase and gas being well above unity. The following parameters of gravitational instability in the dust layer are calculated: the critical surface density of solid matter and the Stokes number of particles corresponding to the onset of instability, the wavelength range in which instability occurs, and the rate of its growth as a function of the perturbation wavelength in the circumsolar disk at radial distances of 1 and 10 AU. We show that at 10 AU, the maximum instability growth rate increases due to the transfer of angular momentum of gas in the layer to gas outside it, a new maximum emerges at a longer wavelength, a long-wavelength instability “tail” forms, and the critical surface density initiating instability decreases relative to that determined without the transfer of angular momentum to gas outside the layer. None of these effects are observed at 1 AU, since instability in this region probably develops faster than the transfer of angular momentum to the surrounding gаs of a protoplanetary disk occurs.

The numerical analysis of the propagation of shock waves initiated by either a space body striking the Earth’s surface, or underground explosions, allows us to compare the energies required to attain the same amplitudes of shock waves at impacts and explosions. Proceeding from this and based on the data of seismic efficiency of underground explosions, the authors have estimated the fraction of the kinetic energy of a space body transformed into the energy of seismic disturbances when the body strikes the Earth. This fraction is about 10^–3, which is an order of magnitude more than the most common estimates. Space bodies decelerating and collapsing in the atmosphere also generate seismic waves in the ground due to the impact of the air-shock wave on the Earth’s surface. In this case, the seismic efficiency is considerably lower, according to the calculations, it is about 10^–5.