Limb darkening of meteorites and asteroids

This paper presents the results of a laboratory study of the limb darkening, near opposition, of the carbonaceous chondrites Orgueil (C1), Murchison (C2), and Allende (C3), the ordinary chondrite Bruderheim (L6), and a stainless-steel powder. These materials represent possible analogs for the surface materials of C, S, and M asteroids respectively. At low phase angles, the limb-darkening behavior of all materials studied is well represented by Minnaert's law. For carbonaceous chondrites, the Minnaert limb-darkening parameter k is nearly independent of wavelength for wavelengths between 0.4 and 0.9 μm, with a typical value of k = 0.55. The reflectance parameter, B₀, varies from 0.045 to 0.065 over the same range of wavelengths. Both k and B₀ are larger for the stainless-steel powder and the ordinary chondrite, due to the increased importance of multiple scattering in the surface layer. If no limb darkening were present, k would equal $\text{1}\text{2}$ and the geometric albedo (p) of an asteroid would equal the normal reflectance $\text{(r}{}_{\mathrm{<mtext>n</mtext>}}\text{? B}{}_0\text{)}$ of its surface material. For bodies whose surface material is appreciably limb darkened, the geometric albedo measured at the telescope will be lower than the true normal reflectance of surface material; we estimate that for S and M objects r_n ? 1.05 p. In the case of nonspherical asteroids, because the distribution of incidence and emission angles varies as the asteroid rotates, the geometric albedo must change with aspect. If limb darkening is not considered when interpreting asteroid light curves, the values of b/a derived will be too extreme. This effect is probably too small to be observed for C asteroids, because of their intrinsically low reflectances, but could be appreciable for S and M objects.     

Saturn's icy satellites: Thermal and structural models

Thermal histories of the small icy Saturnian satellites Mimas, Tethys, Dione, Rhea, and Iapetus are constructed by assuming that they formed as homogeneous ice-silicate mixtures. The models include effects of radiogenic and accretional heating, conductive and subsolidus convective heat transfer, and lithospheric growth. Accretional heating is unlikely to have melted the water ice in the interiors of these bodies and solid state creep of the predominately ice material precludes melting by radiogenic heating. Mimas is so small that its thermal evolution is essentially purely conductive; at present it is a cold, nearly isothermal body. Any subsolidus convection or thermal activity in Mimas would have been confined to a brief period in its early history and would have been due to a warm formation. The four largest satellites are big enough and contain sufficient heat-producing silicates that solid state convection beneath a rigid lithosphere is inevitable independent of initial conditions. Dione and Rhea have convective interiors for most of their thermal histories, while Tethys and Iapetus have mainly conductive thermal histories with early periods of convective 0activity. The thermal histories of the five satellites for the last 4 by are independent of initial conditions; at present they have cold, conductive interiors. The model thermal histories are qualitatively consistent with the appearances of these satellites: Mimas has an ancient heavily cratered surface, Tethys and probably Iapetus have both heavily cratered and more lightly cratered areas, and Dione and Rhea have extensively modified surfaces. Because of their similar sizes and densities, Mimas and Enceladus are expected to have similar surfaces and thermal histories, but instead Enceladus has the most modified surface of all the small icy Saturnian satellites. Our results suggest a heat source for Enceladus, in addition to radiogenic and accretional heating; tidal dissipation is a possibility. Because the water ice in these bodies does not melt, resurfacing must be accomplished by the melting of a low-melting-temperature minor component such as ammonia hydrate.     

Accumulation of planetesimals in the solar nebula

Characteristic time scales relevant to the accumulation of planetesimals in a gaseous nebula are examined and the accumulation toward the planets is simulated by numerically solving a growth equation for a mass distribution function. The eccentricity and inclination of planetesimals are assumed to be determined by a balance between excitation due to mutual gravitational scattering and dissipation due to gas drag. Two kinds of mass motion in the radial direction, i.e., diffusion due to mutual scattering and inward flow due to gas drag, are both taken into account. The diffusion is shown to be effective in later stages with a result of accelerating the accumulation. As to the coalescent collision cross section, the usual formula for a binary encounter in a free space is used but the effect of tidal disruption which increases substantially the cross section is taken into account. Numerical results show that the gravitational enhancement factor (i.e., the so-called “Safronov number”), contained in the cross section formula, always takes a value of the order of unity but the accumulation proceeds relatively rapidly owing to the effects of radial diffusion and tidal disruption. That is, a proto-Earth, a proto-Jupiter, and a proto-Saturn with masses of 1×10²⁷ g are formed in 5×10⁶, 1×10⁷, and 1.6×10⁸ years, respectively. Also, a tentative numerical computation for the Neptune formation shows that a proto-Neptune with the same mass requires a long accumulation time, 4.6×10⁹ years. Finally, the other effects which are expected to reduce the above growth times further are discussed.     

The retention of spallation products in interstellar grains

Stimulated by recent studies indicating the possible survival of presolar grains in certain meteorites, the importance of recoil loss for the retention of spallation products in submicrometer-sized interstellar dust during irradiation by high-energy cosmic ray protons has been calculated. The model presented incorporates range straggling effects, a realistic distribution of interstellar grain sizes, and utilizes an accurate theoretical formalism for the fragmentation recoil momenta. Apart from an only vague understanding of possible grain composition, the greatest uncertainty concerns the intrinsic material density of interstellar dust which determines the recoil range for any given momentum. It is found that even allowing for a fivefold variation of density values, the retention against recoil is substantial: for example, some 20 to 50% of all ³⁸Ar nuclei resulting from high-energy interaction remain trapped, depending on the target element considered. Retentivities for the various spallation reactions contributing to ³⁸Ar have been calculated and are used to deduce an interstellar spallogenic production rate. The results are then considered in the light of recent discoveries of ⁴⁰Ar³⁹Ar apparent ages in excess of 4.53×10⁹ years for some inclusions from the meteorite Allende. Limitations of both the theoretical and experimental efforts presently preclude conclusive statements regarding the question of interstellar grain survival. However, a procedure is outlined whereby this issue might be clarified in future investigations.     

Correspondence analysis applied to a comparison of some rhyolitic zones in the Noranda area (Quebec, Canada)

Correspondence analysis was applied to 177 chemical analyses (major elements) of rocks from some rhyolitic zones of the Noranda area (Quebec). The objective was to grade the individual sample sections with respect to their attractiveness considering the chemical alterations related to mineralization. The results obtained show that it is possible to differentiate sections from different rhyolitic horizons.     

Angular momentum drain: A mechanism for despinning asteroids

It is proposed that a new mechanism—angular momentum drain—helps account for the relatively slow rotation rates of intermediate-sized asteroids. Impact ejecta on a spinning body preferentially escape in the direction of rotation. This material systematically drains away spin angular momentum, leading to the counterintuitive result that collisions can reduce the spin of midsized objects. For an asteroid of mass M spinning at frequency ω, a mass loss δM correspond to an average decrease in rotation rate $\text{δω ≈}\text{ωδM}\text{M}$. A. W. Harris' (1979), Icarus40, 145–153) theory for the collisional evolution of asteroidal spins is significantly altered by inlusion of this effect. While the modified theory is still somewhat artificial, comparison of its predictions with the data of S. F. Dermott, A. W. Harris, and C. D. Murray (1984, Icarus57, 14–34) suggests that angular momentum drain is essential for understanding the statistics of asteroidal rotations.     

XDL-1800型全液压岩心钻机的研发

为满足深孔绳索取心钻探需求,研究开发了XDL-1800型全液压岩心钻机。阐明了钻机的研制背景,介绍了钻机的特点、性能参数及结构和功能。     

LXZJ4000型履带式坑道钻机的研制

介绍了LXZJ4000型履带式坑道钻机的研制背景和总体结构布局设计,并分别叙述了钻机主要组成部分的功能、结构特点、工作原理及该钻机的试验和应用情况。     

艾比湖干涸湖底6种景观类型不同深度富盐沉积物粒径的分形特征

干旱区干涸湖底在强烈的风蚀作用下成为盐碱尘暴的重要来源之一。为研究干涸湖底不同景观类型的抗风蚀效应,选择新疆准噶尔盆地西部艾比湖干涸湖底6种不同景观类型下的富盐沉积物为研究对象,基于沉积物样品的体积粒径分布数据,运用单重、多重分形理论和典范对应分析(CCA)排序方法,重点探讨艾比湖干涸湖底不同景观类型下富盐沉积物粒径分布分维值的分异特征。结果显示:胡杨林带 (L1)、乔本结合带 (L2)、草本结合带 (L3)、芦苇荒漠带 (L4)、梭梭荒漠带 (L5)、无植被覆盖 (L6)6种景观类型0～30 cm和30～60 cm的富盐沉积物分维值分别存在显著性差异。各景观0～30 cm分维值介于1.94～2.40,30～60 cm分维值介于1.92～2.53,总体均处于低位水平。0～30 cm平均分维值依次为L5>L1>L6>L4>L2>L3,30～60 cm平均分维值为L5=L1>L4>L6>L2>L3。富盐沉积物粒径分布分维值与粒径小于73.63 μm的体积含量呈显著正相关。研究表明多重分形维数和CCA排序方法可以有效的揭示干涸湖底自然植被条件下不同深度富盐沉积物粒径分布的均匀性特征。指出干涸湖底风蚀控制研究方向应集中在根据湖区的富盐沉积物-植物特征,对富盐沉积物性状进行改良,提高原生物种幼苗存活率,通过生态修复防治盐尘暴。     

历代安徽名人的地理分布及其成因初探

从人才数量、类型和籍贯3个方面对安徽历代名人的资料进行统计与分析,在此基础上,探讨了历代安徽名人地理分布的重心与区域分异的原因。研究发现：由于时代背景、地理环境、政治、经济、文化等方面的差异,安徽名人时空分异特点显著。时间上,不同时期的名人数量差别较大,名人的影响领域随时间推移呈扩大之势;空间上,军政名人多出自皖北和皖中,而科学、文化教育和经济方面的名人以皖南居多,呈现出明显的“南文北武”分布格局。此外,安徽名人地理分布还呈现明显的南移趋势。