Native silicon and iron silicides in the Dhofar 280 lunar meteorite

The Dhofar 280 lunar highland meteorite is the first one in which native silicon was identified in association with iron silicides. This association is surrounded by silicate material enriched in Si, Na, K, and S and occurs within an impact-melt matrix. Compared to the meteorite matrix, the objects with native Si and the silicate material around them show high Al-normalized concentrations of volatile elements and/or elements with low sensitivity to oxygen but are not any significantly enriched in refractory lithophile elements. Some lithophile elements (V, U, Sm, Eu, and Yb) seem to be contained in reduced forms, and this predetermines REE proportions atypical of lunar rocks and a very low Th/U ratio. The admixture of siderophile elements (Ni, Co, Ge, and Sb) suggests that the Si-bearing objects were contaminated with meteorite material and were produced by the impact reworking of lunar rocks. The high concentrations of volatile elements suggest that the genesis of these objects could be related to the condensation of silicate vapor generated during meteorite impacts. The reduction of silicon and other elements could take place in an impact vapor cloud, with the subsequent condensation of these elements together with volatile components. On the other hand, condensates of silicate vapor could be reduced by impact reworking of impact breccias. Impact-induced vaporization and condensation seem not to play any significant role in forming the composition of the lunar crust, but the contents of the products of such processes can be locally relatively high. The greatest amounts of silicate vapor were generated during significant impact events. For example, more than 70% of the total mass of lunar material evaporated in the course of impact events should have resulted from the collision of the Moon with a cosmic body that produced the Moon??s largest South Pole-Aitken basin.     

Evolution of Planetary Chaotic Zones in Planetesimal Disks

Astronomy Letters - Extensive numerical experiments on the long-term dynamics of planetesimal disks with planets in systems of single stars have been carried out. The planetary chaotic zone...     

Anisotropic illumination of a circumbinary disk in the presence of a low-mass companion

The model of a young star with a protoplanetary disk and a low-mass companion (q ≤ 0.1) moving in a circular orbit inclined to the disk plane is considered. Hydrodynamic models of such a system have been calculated by the SPH method. The perturbations in the disk caused by the orbital motion of the companion are shown to lead to a strong dependence of the disk illumination conditions on azimuth (because of extinction variations between the star and the disk surface) and, as a result, to the appearance of a large-scale asymmetry in the disk images. Calculations show that the dependence of the disk illumination on azimuth is stronger in the central part of the disk than on the periphery. The bright and dark (shadow) regions are located asymmetrically relative to the line of nodes. The sizes of these regions and their positions on the disk depend on model parameters and orbital phase. During the orbital motion, the bright and dark regions do not follow the companion but execute small-amplitude oscillations relative to some direction. The model properties described above open up new possibilities for detecting low-mass companions in the vicinity of young stars. Stars with protoplanetary disks seen face-on or at low inclinations i are best suited for this purpose.     

Lunar regolith breccia Dhofar 287B: A record of lunar volcanism

Abstract— Dhofar 287 (Dho 287), a recently found lunar meteorite, consists in large part (95%) of low‐Ti mare basalt (Dho 287A) and a minor, attached portion (?5%) of regolith breccia (Dho 287B). The present study is directed mainly at the breccia portion of this meteorite. This breccia consists of a variety of lithic clasts and mineral fragments set in a fine‐grained matrix and minor impact melt. The majority of clasts and minerals appear to have been mainly derived from the low‐Ti basalt suite, similar to that of Dho 287A. Very low‐Ti (VLT) basalts are a minor lithology of the breccia. These are significantly lower in Mg# and slightly higher in Ti compared to Luna 24 and Apollo 17 VLT basalts. Picritic glasses constitute another minor component of the breccia and are compositionally similar to Apollo 15 green glasses. Dho 287B also contains abundant fragments of Mg‐rich pyroxene and anorthite‐rich plagioclase grains that are absent in the lithic clasts. Such fragments appear to have been derived from a coarse‐grained, Mg#‐rich, Na‐poor lithology. A KREEP component is apparent in chemistry, but no highlands lithologies were identified. The Dho 287 basaltic lithologies cannot be explained by near‐surface fractionation of a single parental magma. Instead, magma compositions are represented by a picritic glass; a low‐Ti, Na‐poor glass; and a low‐Ti, Na‐enriched source (similar to the Dho 287A parental melt). Compositional differences among parent melts could reflect inhomogeneity of the lunar mantle. Alternatively, the low‐Ti, Na‐poor, and Dho 287A parent melts could be of hybrid compositions, resulting from assimilation of KREEP by picritic magma. Thus, the Dho 287B breccia contains lithologies from multiple magmatic eruptions, which differed in composition, formational conditions, and cooling histories. Based on this study, the Dho 287 is inferred to have been ejected from a region located distal to highlands terrains, possibly from the western limb of the lunar nearside, dominated by mare basalts and KREEP‐rich lithologies.     

Determination of the reduced sulfur species in the anoxic zone of the Black Sea: A comparison of the spectrophotometry and iodometry techniques

The report presents the results of the studies of the reduced sulfur species in the water of the anoxic zone of the Black Sea. The content of hydrogen sulfide was determined by means of spectrophotometry using dilution with oxygen-free distilled water. The detection limit of the H₂S amounted to 0.3 μM with the method’s precision below 3%. The accuracy of the spectrophotometric determination was verified by iodometry after the fixation of the hydrogen sulfide in zinc acetate under the distillation with argon from the acidified seawater sample.     

“Horseshoe” Structures in the Debris Disks of Planet-Hosting Binary Stars

The formation of a planetary system from the protoplanetary disk leads to destruction of the latter; however, a debris disk can remain in the form of asteroids and cometary material. The motion of planets can cause the formation of coorbital structures from the debris disk matter. Previous calculations have shown that such a ring-like structure is more stable if there is a binary star in the center of the system, as opposed to a single star. To analyze the properties of the coorbital structure, we have calculated a grid of models of binary star systems with a circumbinary planet moving in a planetesimal disk. The calculations are performed considering circular orbits of the stars and the planet; the mass and position of the planet, as well as the mass ratio of the stars, are varied. The analysis of the models shows that the width of the coorbital ring and its stability significantly depend on the initial parameters of the problem. Additionally, the empirical dependences of the width of the coorbital structure on the parameters of the system have been obtained, and the parameters of the models with the most stable coorbital structures have been determined. The results of the present study can be used for the search of planets around binary stars with debris disks.     

Numerical thin-sheet modelling of the telluric field distortions by the hybrid technique. I. Theory and an example for the Baltic Shield

The distortions of the electromagnetic-plane wave field, due to near-surface conductivity inhomogeneities, are considered. The real three-dimensional distribution of the shallow conductivity is approximated by Price's thin-sheet model. As low-frequency magnetotelluric variations are considered it is possible to neglect the self-induction in the thin sheet.The calculation of the current function has been carried out by the alternative Schwarz method. The method combines, in the iterative sense, the finite element solution in the inhomogeneous region and the analytical solution outside of it. The primary electric field is assumed to be circularly polarized. An example for the Baltic Shield shows that the telluric field is mostly very distorted.     

Accretion activity of young binary systems with low-mass secondary components

The accretion activity of young binaries with low-mass (q = M ₂/M ₁ ≤ 0.1) secondary components is studied. The source of accreted matter is a common disk surrounding the binary system and coplanar with its orbit. Gas dynamic models of these systems are used to calculate the rates of accretion to the components and their dependence on the phase of the orbital period is studied. It is shown that, despite its low mass, the secondary accretes matter at a relatively higher rate than the primary. This result can be regarded as an extension of the work of Artymowicz and Lubow for young binaries with components that have unequal masses. Possible astrophysical applications of the theory are discussed.