Some special orbits in the two-body problem with radiation pressure

The motion has been studied of a particle in a gravitational field perturbed by radiation pressure. By combining the formulation in the physical space variables with the KS variables we obtained explicit evidence for the existence of a surface of stable circular orbits with centers on an axis through the primary body. Furthermore, the effects of a sharp shadow on the two-dimensional unstable parabolic orbits were investigated. It was found that they do not survive the introduction of a shadow.     

Optical searches for venusian lightning: implications for nightside field and plasma relationships

Repeated searches for optical evidence of lightning across Venus nightside regions reported as exhibiting almost incessant activity have failed to detect any evidence of lightning. Owing to the extensive nature of these investigations, the negative results contribute strongly to the interpretation that the plasma noise initially attributed to a lightning source is instead stimulated by interaction of the solar wind and draped interplanetary magnetic field with the nightside ionosphere.     

Escape from a gravitational system of positive energy

It is proved in this article that under the condition defined in Equation (6), whent, at least two particles must escape from a Newtonian gravitational system of positive energy.     

Computation of gas pressure profiles relevant to outbursting in coal mines

High gas pressure gradients in coal seams play an important role in the occurrence of outbursts during underground coal mining. Excess stress resulting from body forces due to high pressure gradients contributes to the structural failure of coal. We present a one-dimensional model of a dry, rigid coal seam with a moving mine face and determine gas pressures by solving the nonlinear equation for gas flow in a porous medium. An implicit finite difference scheme is used. Since the structure of coal has a dual porosity nature, we consider flow through both the macropores and the micropores. We investigate the effect of changes in mining velocity, idle mining periods and coal properties on the pressure profile in the coal seam. When the yield zone behind the moving mine face is taken into account, the greatest body force occurs on the least competent section of the coal seam.     

CONSTRAINTS ON CHONDRULE ORIGIN FROM PETROLOGY OF ISOTOPICALLY CHARACTERIZED CHONDRULES IN THE ALLENDE METEORITE

Petrographic and chemical features of Allende ferromagnesian chondrules previously analyzed for oxygen and silicon isotopes by Clayton et al. (1983a) provide additional information on chondrule origin. Allende, like other carbonaceous chondrites, contains two chondrule populations, but one of these is represented by only one chondrule in this isotopically characterized set. All Allende chondrules fall along an isotopic mixing line, probably defined by an ¹⁶O-rich solid component and an isotopically heavier oxygen gaseous exchange component. Differences in the amounts of isotopic exchange for porphyritic and barred chondrules presumably resulted from varying degrees of melting. Those porphyritic chondrules containing abundant relict grains experienced the least isotopic exchange. Chondrules with high bulk FeO/(FeO + MgO) ratios apparently persisted longer as liquids and contain more of the exchange component. The distinct directions of oxygen isotopic exchange in chondrules from carbonaceous, ordinary, and enstatite chondrites indicate that each formed from different solid precursor materials. Silicon isotopic variations in Allende chondrules probably reflect evaporative loss of lighter isotopes; however, silicon loss is also controlled by chondrule sizes, which are unknown. Observed correlations point to the importance of kinetic factors in a gaseous nebula for chondrule genesis, and are not consistent with models that explain chondrules as mixtures of several solids with distinct oxygen and silicon isotopic compositions.     

The nondetection of continuum radiation from comet IRAS-Araki-Alcock (1983d) at 2- to 6-cm wavelengths and its implication on the icy-grain halo theory

Observations of Comet IRAS-Araki-Alcock have been made with the VLA (Very Large Array) at 6 and 2 cm, when the comet was at geocentric distances of 0.08 and 0.035 AU, respectively. The 3σ upper limits are 90 and 750 μJy at 6 and 2 cm, respectively. We show that the “conventional” icy-grain halo theory is not adequate to explain the data. If there is such a halo, it is either very thin, or does not contain grains with sizes larger than 10–100 μm. Comparison of our limits with a reported detection at 1.3-cm wavelengths shows that if the centimeter-wavelength radiation all arises in the halo, the halo should have an extent of the order of 300–400 km, but an effective area of 100 km². If only thermal emission from the nucleus is significant, the temperature decreases from about 200°K at the layers probed at 1.3 cm to about 50°K or less at depths probed at 2 cm (assuming unit emissivity at all wavelengths and depths). This can be due to a combination of a lower emissivity and lower physical temperature at larger depths in the comet; both effects are expected when considering theories on microwave emission from glaciers on Earth.     

Assessing global trends in Mars magma compositions using ground truth

Global magmatic trends inferred from gamma-ray, visible/near-infrared, and thermal infrared spectrometers on Mars-orbiting spacecraft have been used to constrain planetary petrogenetic processes and global thermal evolution models. Inferred magmatic trends include temporal variations in the relative proportions of low-Ca and high-Ca pyroxenes, and in the abundances of potassium (and total alkalis), silica, FeO* (total iron expressed as FeO), and thorium. These patterns are evaluated for consistency with the compositions of surface igneous rocks of different ages analyzed by Mars rovers and of martian meteorites. Trends of decreasing low-Ca pyroxene/total pyroxene ratios and of decreasing potassium (and total alkalis), with time are generally supported by surface rock analyses. However, significant differences in the GRS-measured silica in Amazonian volcanoes and in martian meteorites of equivalent age result from contamination by silica-rich dust and are problematic for a silica trend. Comparison of FeO* in Noachian and Amazonian surface data shows no decrease. An inferred temporal trend in thorium is in conflict with the complex enrichment and depletion patterns of incompatible trace elements in martian meteorites of various ages. A dearth of analyses of Hesperian-age surface rocks precludes a firm evaluation of inferred Noachian-Hesperian trends and Hesperian-Amazonian trends, but abundant Noachian rocks and a few Hesperian rocks at rover sites, and Amazonian martian meteorites, collectively representing at least 16 surface locations, afford useful comparisons with orbital remote-sensing data.     

Glasses in howardites: Impact melts or pyroclasts?

We have analyzed glasses in eight howardites with the aim of distinguishing their origins as impact melts or pyroclasts. Although theoretical calculations predict that pyroclastic eruptions could have taken place on Vesta, the occurrence of pyroclastic glasses in HED meteorites has not been documented. This study involved petrographic examination of textures, electron microprobe analysis of major and minor elements, and LA‐ICP‐MS analysis for selected trace elements. Previously documented textural and compositional differences between lunar impact‐melt and pyroclastic glasses partly guided this study. This work yielded no positive identification of pyroclastic glasses. The most likely explanation is that pyroclastic glasses never formed, either because Vesta contains insufficient volatiles to have powered explosive eruptions, or because eruptive conditions produced optically dense fire‐fountains that allowed melt drops to collect as lava ponds. The impact‐melt glasses were grouped (low‐alkali, Ca‐rich, and K‐rich) based on compositions. We suggest that these glasses are the result of impacts onto known HED lithologies. The low‐alkali glasses are impact melts of bulk HED lithologies. We hypothesize that the Ca‐rich and K‐rich glasses result from oversampling of plagioclase and of mesostasis that experienced liquid immiscibility, respectively, during micrometeorite impacts into eucrite targets.     

Paired nakhlites MIL 090030, 090032, 090136, and 03346: Insights into the Miller Range parent meteorite

Abstract– Miller Range (MIL) 03346 is the most oxidized and least equilibrated nakhlite known and displays the highest amount of intercumulus phase. The discovery of three new nakhlites, MIL 090030, MIL 090032, and MIL 090136, in the Miller Range, Antarctica, geographically close to the location of MIL 03346, suggests that they may come from the same parent meteorite. In this study, we investigate the mineralogy and texture of cumulus and intercumulus phases, in situ major and trace element compositions for the cumulus phases, as well as pyroxene crystal size distribution patterns and spatial distribution patterns of MIL 090030, 090032, and 090136. Using these combined results, we conclude that the three nakhlites studied here are paired with MIL 03346. However, modal mineral abundances of MIL 090030, 090032, 090136, and 03346 exhibit variations indicating that a single sample of a Miller Range nakhlite is not modally representative of the parent meteorite and that analyses of multiple samples for a single nakhlite may be necessary to obtain representative modal data for placement in the cumulate pile. Our calculated parental melt composition based on all the paired samples confirms a previous study suggesting that the nakhlite parent melt crystallized as a closed system.