关于近地小行星轨道演化的初步探索

本文采用改进的显式辛算法和嵌套的PKF7（8）积分器同时对86颗已命名（或编号）的近地小行星的轨道演化进行了数值研究，在103－104年的时间尺度上，给出了这些小行星轨道演化的状况以及它们与几颗大行星靠近的最小距离，特别是与地球接近的最小距离可小于0．01天文单位，甚至可能比月球还更靠近地球．     

Secular resonances in the asteroid belt: Theoretical perturbation approach and the problem of their location

In this paper a theoretical perturbation approach to the problem of the dynamics in secular resonance is exposed. This approach avoids any expansion of the main term of the Hamiltonian (linear term in the masses) with respect to the eccentricity or the inclination of the asteroid, in order to achieve results valid for any value of these variables. Moreover suitable action-angle variables are introduced to take properly into account the dynamics related to the motion of the argument of perihelion of the asteroid, which is relevant at high inclination. A class of secular resonances wider than that usually considered is found. An explicit computation of the location of the main secular resonances, estimating also the contribution of the quadratic term in the masses by means of classical series expansion, is reported in the last sections. The accuracy of computations obtained by series expansion is discussed in the paper.     

Characteristics in naturally and experimentally shocked chondrites: A clue to P-T conditions of impacted asteroids

The aim of this study is to compare the experimentally shock-induced features with those in naturally shocked chondrites and to test the feasibility of experimentally calibrating naturally induced features in shocked H- and L-chondrites. Samples of the Jilin chondrite (H5) were experimentally shock-loaded at the following peak pressures: 12, 27, 39, 53, 78, 83, 93 and 133 GPa respectively. Chondritic melts were first obtained at P>78 GPa and more than 60% melting was achieved at P~133 GPa. No high-pressure phases were observed in any of the shocked samples, neither in the deformed nor in the molten regions. Textural relations and mineral assemblages of the shocked samples are comparable to those encountered in the heavily shocked H-chondrite Yanzhuang but differ considerably from those found in heavily shocked L6 chondrites. Shock melt veins in L6 chondrites contain high-pressure polymorphs of olivine and pyroxene and high pressure liquidus phases. Scaling from shock experiments on millimeter-sized samples to natural shock features on kilometer-sized asteroids poses considerable problems in quantifying the P-T conditions during natural shock events on asteroids.     

Investigation of the long-periodic orbits of Trojan-type asteroids in the outer solar system

The present paper demonstrates the results of the numerical integration of equations of motion of a infinitesimal mass pleased in the neighborhood of the triangular point of the Sun-planet system. There are presented the results for the outer solar system, i.e. for Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The long-periodic solutions were searched for the distance from the Lagrangian point changing from ±0.01 to ±0.10 in canonical units. The Trojans of those planets have the circle, tadpole, horseshoe and irregular shape of their orbits. Same of those test particles showed a close approach to planet. Other of those collided with planet and then was removed from the solar system. The tadpole, circle and same trajectories surveyed integration for 100,000 years. This revised version was published online in July 2006 with corrections to the Cover Date.     

Iridium anomalies and fractionated siderophile element patterns in impact ejecta, Brockman Iron Formation, Hamersley Basin, Western Australia: evidence for a major asteroid impact in simatic crustal regions of the early Proterozoic earth

A stratigraphically consistent <20-cm-thick unit of microkrystite spherule and microtektite-bearing impact fallout ejecta overlying volcanic tuff of the 4th Shale Macroband (DGS4) of the Dales Gorge Member (2.47–2.50 Ga), Brockman Iron Formation, Hamersley Group, Western Australia, displays anomalous platinum group element (PGE) and other trace metal patterns. The unit has high Ir (13 ppb) and Pt (35 ppb), and low Pd (2.7 ppb) and Au (1.55–1.88 ppb). The low Pd/Ir ratios and low Cr/V suggest depletion in volatile PGE and metals relative to refractory PGE and V, contrasted to the ubiquitous high Pd/Ir of most terrestrial rocks. Marked depletion in the volatile Rare Earth Element (REE) abundances in stilpnomelane spherule cores is consistent with this model. The loss of volatile PGE, analogous to relations in 3.24 Ga impact fallout units of the Barberton greenstone belt (S3 and S4), suggests fractionation related to atmospheric spherule condensation. The microkrystite spherule unit locally incorporate fragments and up to meter-scale boulders of banded chert and stromatolite carbonate, suggesting tsunami transport postdating spherule deposition. DGS4 microkrystite spherules are dominated by stilpnomelane mantled by K-feldspar shells, which consist of inward-radiating fibrous feldspar aggregates suggestive of devitrification. The K and REE enrichment of spherule margins are contrasted to flat REE patterns of the stilpnomelane cores, suggesting adsorption of lithophile elements during settling of the spherules through the hydrosphere. K-feldspar shells contain submicron-scale Ni metal, oxide, sulfide and arsenide grains and euhedral needles of feldspar-exsolved ilmenite. Associated magnetite may have high nickel (<1.25% NiO). The generally mafic composition of the spherules and high Ni/Cr and Ni/Co are consistent with a target mafic-ultramafic crust, consistent with the lack of shock-metamorphosed quartz. Mixing calculations suggest a contribution of 2.5–3% projectile component to the impact-generated volatile cloud. Conservative mass balance estimates derived from the Ir and Pt flux, assuming global extent of a 10-cm-thick spherule unit and chondritic projectile composition, suggest an asteroid diameter on the scale of 30 km. Similar estimates are obtained from spherule sizes, which in DGS4 reach a mean diameter of 2.0 mm in aerodynamically elongate spherules. The evidence implies formation of an impact basin on the scale of 400 km in simatic/oceanic regions of the early Proterozoic crust.     

High atmospheric NO x levels and multiple photochemical steady states

Previous zero-dimensional photochemical calculations indicate that multiple tropospheric steady states may exist, in which different NO _x (NO+NO₂) levels could be supported by the same source of NO _x . To investigate this possibility more closely, a one-dimensional photochemical model has been used to estimate the rate of removal of atmospheric NO _x compounds at different NO _x levels. At low NO _x levels NO _x is photochemically converted to HNO₃, which is removed by either wet or dry deposition. At high NO _x levels formation of HNO₃ is inhibited, and NO _x is removed by a variety of other processes, including rainout of N₂O₄ and N₂O₅, surface deposition of NO and NO₂, and direct dissolution of NO and NO₂ in rainwater. Multiple steady states are possible if surface deposition of NO _x is relatively inefficient. The NO _x source required to trigger high atmospheric NO _x levels is approximately 10 to 15 times the present global emission rate-less than half the source strength predicted by the zero-dimensional model. NO _x mixing ratios in excess of 10^-7 would cause severe damage to the ozone layer and could result in either a climatic warming or cooling, depending upon the amount of NO₂ present.     

Plate motion on Europa and nonrigid behavior of the Icy lithosphere: The Castalia Macula region

We have developed a generalized quantitative technique for determining the finite pole of rotation between two rigid plates and use it to critically examine differing reconstructions of a region surrounding a prominent dark spot on Europa, Castalia Macula. This region is located near the equator of Europa's trailing hemisphere and has been suggested as a site where crustal convergence may have occurred. Previous reconstructions of the region have indicated that a ridge set and/or a band-like complex that define a collection of tectonic plates in the region accommodated surface contraction. However, a critical examination of the differences between these reconstructions has been complicated by the lack of a finite pole of rotation for the plates involved in either reconstruction. We have applied our modeling technique, coupled with a detailed examination of the morphology and cross-cutting relationships involving this ridge set and band-like complex, to determine if a unique reconstruction exists for several tectonic plates in this region. The cross-cutting relationships involving the ridge set also allow us to test the general assumption that plates behave rigidly on Europa. Assuming rigid behavior, our results suggest that a unique reconstruction does exist, indicating the ridge set accommodated surface contraction. However, analysis performed to test the assumption of plate rigidity indicates that one or more of the plates in the region did not behave rigidly. This does not rule out surface contraction along the ridge set but does indicate that a component of nonrigid behavior must be considered.     

近地小行星轨道演化的数值研究与辛算法有效性的探讨

本文采用改进的显式辛算法（symplecticalgorithm）和嵌套的RKF7（8）积分器对43颗已命名（或编号）的近地小行星的轨道演化进行数值研究．在力学模型上,除考虑各大行星的引力振动外,还增加了后牛顿效应,而在算法上则着重探索辛算法在近地小行星轨道演化研究中的应用前景,特别是当小行星与某一大行星靠近时辛算法的有效性．本文的结果可为了解近地小行星的轨道演化状况和对它们进行监测提供可靠的信息．     

碳质球粒陨石有机物红外光谱研究

碳质球粒陨石是太阳系中最原始的物质之一.通过对碳质球粒陨石的光谱分析,可以建立其与母体小行星之间的联系,有助于探测小行星表面物质成分、研究太阳系早期的演化历史.研究了6个CM2型碳质球粒陨石和11个煤炭样品(碳质球粒陨石所含有机质的地球类比物)可见-远红外谱段反射光谱特征,并分析了它们与有机组分的关系.结果表明,对于不...     

Asteroid families: Recent results and present scenario

After several decades of frustrating results showing a generally poor agreement among different asteroid family classifications, recent studies based on high accuracy proper elements, as well as on objective statistical methods of cluster analysis have largely improved the situation. Now, a number of asteroid families have been recognized on the basis of different methods of cluster analysis, using asteroid proper elements data sets computed by means of different theories. For these reasons, they should be considered of very high reliability. Moreover, spectroscopic observations confirm in some cases these results, indicating surface compositions of the family members in agreement with a geochemically plausible parent body. However, in particular zones of the belt, like the Flora region, further efforts should be performed in order to establish the real consistence of the resulting clusterings of objects. In addition, the size distribution and the taxonomic types of some well established families seem to indicate particular features of the family sample when compared with the field objects. We recall that asteroid families, in the framework of asteroid collisional evolution, are of the highest importance for understanding the mechanisms of injection of fragments into the Earth-crossing zone through mean-motion and secular resonances and, as a consequence, for evaluating the impact rate on Earth of asteroidal objects.