Vol 53, No 5 (2019)

The paper presents the results of the study of the dynamic structure of near-Earth orbital space in the region of the 1 : 3 resonance with the Earth’s rotation. The results of an extensive numerical-analytical experiment to study the orbital evolution of objects moving in the range of semimajor axes from 20 250 to 20 280 km, with inclinations from 0 to 90 degrees, are presented. The zones of action of five components of orbital resonance and apsidal-nodal secular resonances of low orders are revealed. Maps of the distribution of identified resonances are given. The analysis of the dynamic structure of the orbital space using the fast Lyapunov characteristic (MEGNO) is presented, and the MEGNO map of the region in the plane section (orbit inclination, semimajor axis) is given. It has been shown that the dynamic evolution of most of the orbits is chaotic, which is due to the superposition of different types of resonances.

Migration of planetesimals from the feeding zone of the terrestrial planets, which was divided into seven regions depending on the distance to the Sun, was simulated. The influence of gravity of all planets was taken into account. In some cases, the embryos of the terrestrial planets rather than the planets themselves were considered; their masses were assumed to be 0.1 or 0.3 of the current masses of the planets. The arrays of orbital elements of migrated planetesimals were used to calculate the probabilities of their collisions with the planets, the Moon, or their embryos. As distinct from the earlier modeling of the evolution of disks of the bodies coagulating in collisions, this approach makes it possible to calculate more accurately the probabilities of collisions of planetesimals with planetary embryos of different masses for some evolution stages. When studying the composition of planetary embryos formed from planetesimals, which initially were at different distances from the Sun, we considered the narrower zones, from which planetesimals came, as compared to those examined earlier, and analyzed the temporal changes in the composition of planetary embryos rather than only the final composition of planets. Based on our calculations, we drew conclusions on the process of accumulation of the terrestrial planets. The embryos of the terrestrial planets, the masses of which did not exceed a tenth of the current planetary masses, accumulated planetesimals mainly from the vicinity of their orbits. When planetesimals fell onto the embryos of the terrestrial planets from the feeding zone of Jupiter and Saturn, these embryos had not yet acquired the current masses of the planets, and the material of this zone (including water and volatiles) could be accumulated in the inner layers of the terrestrial planets and the Moon. For planetesimals which initially were at a distance of 0.7–0.9 AU from the Sun, the probabilities of their infall onto the embryos of the Earth and Venus, the mass of which is 0.3 of the present masses of the planets, differed less than twofold for these embryos. The total mass of planetesimals, which initially were in each part of the region between 0.7 and 1.5 AU from the Sun and collided with the almost-formed Earth and Venus, apparently differed by less than two times for these planets. The inner layers of each of the terrestrial planets were mainly formed from the material located in the vicinity of the orbit of a certain planet. The outer layers of the Earth and Venus could accumulate the same material for these two planets from different parts of the feeding zone of the terrestrial planets. The Earth and Venus could acquire more than half of their masses in 5 Myr. The material ejection that occurred in impacts of bodies with the planets, which was not taken into account in the model, may enlarge the accumulation time for the planets. A relatively rapid growth of the bulk of the Martian mass can be explained by the formation of Mars’ embryo (the mass of which is several times less than that of Mars) due to contraction of a rarified condensation. For the mass ratio of the Earth’s and lunar embryos equal to 81 (the same as that for the masses of the Earth and the Moon), the ratio of the probabilities for infalls of planetesimals onto the Earth’s and lunar embryos did not exceed 54 for the considered variants of calculations; and it was highest for the embryos’ masses approximately three times less than the present masses of these celestial bodies. Special features in the formation of the terrestrial planets can be explained even under a relatively gentle decrease of the semi-major axis of Jupiter’s orbit due to ejection of planetesimals by Jupiter into hyperbolic orbits. In this modeling, it is not necessary to consider the migration of Jupiter to the orbit of Mars and back, as in the Grand Tack model, and sharp changes in the orbits of the giant planets falling into a resonance, as in the Nice model.

We present the results of observations of the Galilean moons of Jupiter carried out at the Normal Astrograph of the Pulkovo Observatory in 2018. We obtained 452 positions of the Galilean moons of Jupiter in the Gaia DR1 catalog system (ICRF, J2000.0) and 671 differential coordinates of the satellites relative to each other. The obtained mean errors in the satellites’ normal positions on the right ascension and declination, which demonstrate the intrinsic convergence of the observational results, are ε_α = 0.003′′ and ε_δ = 0.003′′, respectively, for the entire observational period. The errors of one difference are σ_α = 0.070′′, and σ_δ = 0.067′′, respectively. The equatorial coordinates of the moons were compared to eight motion theories of planets and satellites. On average, the (O–C) residuals in the both coordinates relative to the motion theories are 0.014′′. The best agreement with observations is achieved by combination of all four motion theories of satellites with the planetary theory EPM2017, which yields average (O–C) residuals of approximately 0.01′′ for each of them. The new results were compared to those of the 2016−2017 observational season. As in the past, peculiarities in the behavior of the (O–C) residuals for Io and Ganymede have been noticed.

Polar areas of the Moon are prospective sites for construction of a lunar base due to the near constant illumination conditions and the potential presence of water ice in the regolith of cold traps. The mountain Mons Malapert (MM) near the South pole of the Moon is a key candidate for the location of such a base. MM is an ~30 × 50 km mountain elongated in a WNW-ESE direction with a NNE extension. Its summit stands ~5 km above the 1838 km datum, has constant visibility from Earth and long periods of sunlight (87 to 91% of the year). In this analysis we consider the topographic, geologic and trafficability characteristics of Mons Malapert, which need to be taken into account in the further consideration of MM as a lunar base location. The topography and its derivatives were studied using LROC WAC images and the LOLA-based DTM. South of MM lie the ~50 km craters Haworth and Shoemaker whose floors are in permanent shadow and show a neutron spectrometric signature of high water-ice content that may be a source of water for the base. The geology of the MM region is defined by its position on the rim of the South-Pole-Aitken basin, the largest and most ancient impact basin on the Moon. The ancient age of this area is confirmed by crater spatial density which shows ages of ~4.2 Ga. The MM slopes are mostly rather steep: from ~20 to 30°, while slopes on its summit and base are more gentle. LROC NAC images of this area show that while the summit and base of MM are covered by numerous small craters, its steep slopes show a deficit of craters and are complicated by low ridges appoximately perpendicular to the downslope direction. These characteristics of the steep slopes suggest effective downslope movement of the regolith material that, in turn, suggests that the mechanical properties of the surface layer here are relatively weak. The siting, building and operation of a lunar base implies activity not only in-base and close proximity, but also traversing to other distant sites of interest for resources and scientific investigations. So planning the Mons Malapert base requires the detailed analysis of the trafficability of the region. To consider this issue we return to experience gained by the operations of Soviet Lunokhod 1, 2 and the US Apollo Lunar Roving Vehicles. On the basis of new and evolving technology, rovers designed for the MM lunar base may significantly differ from earlier rovers, but consideration of trafficability of the earlier rovers is important for future planning. Our analysis shows that neither Lunokhods nor the Apollo LRV could successfully climb most of the slopes of Mons Malapert. The acceptable trafficability appears to be only possible along the WNW crest of the mountain. For emergency cases wheel-walking rovers may be considered. Mons Malapert seems to be a good locality for the lunar base but more studies are needed.