A transmission electron microscope study of Chassigny: Evidence for strong shock metamorphism

Abstract— Chassigny is a shock-metamorphosed dunite of probable Martian origin. In order to determine its degree of shock metamorphism and to define the starting conditions prior to its ejection from Mars, the shock signature of Chassigny has been carefully examined by optical and electronoptical techniques. Dominant shock effects are the conversion of feldspars to diaplectic glass (maskelynite), the clino-/orthoenstatite inversion, strong mosaicism of olivine, and the activation of numerous planar fractures and c dislocations in olivine. High-resolution transmission electron microscopy (TEM) reveals additionally the coexistance of planar fractures with, so far, unknown discontinuous fractures in olivine. These findings point to a shock pressure of 35 GPa. Chassigny has thus experienced a high and similar degree of shock metamorphism as the shergottites. The results of this study suggest that Chassigny was at a shallow target position, close to the point of impact, when it was ejected.     

The fusion crust of the Winchcombe meteorite: A preserved record of atmospheric entry processes

Fusion crusts form during the atmospheric entry heating of meteorites and preserve a record of the conditions that occurred during deceleration in the atmosphere. The fusion crust of the Winchcombe meteorite closely resembles that of other stony meteorites, and in particular CM2 chondrites, since it is dominated by olivine phenocrysts set in a glassy mesostasis with magnetite, and is highly vesicular. Dehydration cracks are unusually abundant in Winchcombe. Failure of this weak layer is an additional ablation mechanism to produce large numbers of particles during deceleration, consistent with the observation of pulses of plasma in videos of the Winchcombe fireball. Calving events might provide an observable phenomenon related to meteorites that are particularly susceptible to dehydration. Oscillatory zoning is observed within olivine phenocrysts in the fusion crust, in contrast to other meteorites, perhaps owing to temperature fluctuations resulting from calving events. Magnetite monolayers are found in the crust, and have also not been previously reported, and form discontinuous strata. These features grade into magnetite rims formed on the external surface of the crust and suggest the trapping of surface magnetite by collapse of melt. Magnetite monolayers may be a feature of meteorites that undergo significant degassing. Silicate warts with dendritic textures were observed and are suggested to be droplets ablated from another stone in the shower. They, therefore, represent the first evidence for intershower transfer of ablation materials and are consistent with the other evidence in the Winchcombe meteorite for unusually intense gas loss and ablation, despite its low entry velocity.     

牛眠沟流域泥石流形成条件、发展趋势及其防治探讨

汶川地震使牛眠沟流域变成了一个多泥石流灾害的区域,到目前为止,已经发生7次大规模泥石流灾害。经研究,该流域诱发泥石流灾害的3个基本条件非常发育,而目前正处于发展阶段,在震后5~10年内的雨季,极易发生危害性较大、冲击力较强的大规模降雨型泥石流。如果考虑直接利用主沟内沉积的泥石流堆积物,在主沟堆积区的中上游左右两侧修建约为沟宽一半的交替式拦挡土石坝、在中下游修建与沟宽大致相等的拦挡土石坝及在相邻两坝间修建与沟向相对且具有一定坡度斜坡的土木防治工程,可实现消能与耗能双重目的。     

基于长期数据的北斗广播星历精度评估

介绍了广播星历精度评估的基本原理和方法,并利用2013-01~2015-04共28个月的广播星历数据,分析比较了北斗不同类型卫星的轨道精度、钟差精度及整体精度的短期和长期变化趋势。结果表明,GEO卫星广播星历的轨道精度约为1.8 m,钟差精度优于6 ns;IGSO和MEO广播星历的轨道精度优于1.2 m,钟差精度优于4 ns,整体上优于GEO卫星。从长期变化趋势来看,北斗广播星历的精度有逐渐提高的趋势。     

Petrology and shock metamorphism of the olivine-phyric shergottite Yamato 980459: Evidence for a two-stage cooling and a single-stage ejection history

The basaltic Martian meteorite Yamato 980459 consists of large olivine phenocrysts and often prismatic pyroxenes set into a fine-grained groundmass of smaller more Fe-rich olivine, chromite, and an interstitial residual material displaying quenching textures of dendritic olivine, chain-like augite and sulfide droplets in a glassy matrix. Yamato 980459 is, thus, the only Martian meteorite without plagioclase/maskelynite. Olivine is compositionally zoned from a Mg-rich core to a Fe-rich rim with the outer few micrometers being especially rich in iron. With Fo₈₄ the cores are the most magnesian olivines found in Martian meteorites so far. Pyroxenes are also mostly composite crystals of large orthopyroxene cores and thin Ca-rich overgrowths. Separate pigeonite and augites are rare. On basis of the mineral compositions, the cooling rates determined from crystal morphologies, and crystal grain size distributions it is deduced that the parent magma of Yamato 980459 initially cooled under near equilibrium conditions e.g., in a magma chamber allowing chromite and the Mg-rich silicates to form as cumulus phases. Fractional crystallization at higher cooling rates and a low degree of undercooling let to the formation of the Ca-, Al-, and Fe-rich overgrowths on olivine and orthopyroxene while the magma was ascending towards the Martian surface. Finally and before plagioclase and also phosphates could precipitate, the magma was very quickly erupted quenching the remaining melt to glass, dendritic silicates and sulfide droplets. The shape preferred orientation of olivine and pyroxene suggests a quick, thin outflow of lava. According to the shock effects found in the minerals of Yamato 980459, the meteorite experienced an equilibration shock pressure of about 20-25 GPa. Its near surface position allowed the ejection from the planet’s surface already by a single impact event and at relatively low shock pressures.     

一种快速解算高维模糊度的LLL分块处理算法

由于多频多模GNSS观测数据解算的模糊度具有较高的维数和精度,当采用常规的LLL算法进行模糊度整数估计时,规约耗时显著大于搜索耗时,成为限制高维模糊度解算计算效率的主要因素。针对这一问题,通过分析规约耗时与模糊度维数和精度之间的关系,提出了一种LLL分块处理算法。该算法通过对模糊度方差协方差阵进行分块处理,降低单个规约矩阵的维数,以减少规约耗时,从而提高模糊度解算计算效率。通过两组实测高维模糊度数据对本文提出的分块处理算法进行了效果验证。结果显示,当分块选择合理时,本文提出的算法相对于LLL算法的解算效率分别可提高65.2%和60.2%。     

BDS/GPS单历元多频定向算法性能对比

针对多频多系统定向的不同需求,该文采用了BDS/GPS单历元双差非组合定向算法以及附有基线长约束的最小二乘降相关平差模糊度固定方法。4组实测数据的处理结果表明,单频GPS、单频BDS的模糊度平均固定成功率分别为55.9%、84.3%;单频双系统、多频(双频三频)单系统及多频(双频或三频)双系统定向时,其模糊度固定成功率均可达98%以上;在模糊度固定正确的情况下,BDS/GPS组合定向的精度最高,且与单GPS较为相近,但两者都略高于单BDS,频率差异所带来的精度差异并不明显。     

Ejection of Martian meteorites

Abstract— We investigated the transfer of meteorites from Mars to Earth with a combined mineralogical and numerical approach. We used quantitative shock pressure barometry and thermodynamic calculations of post‐shock temperatures to constrain the pressure/temperature conditions for the ejection of Martian meteorites. The results show that shock pressures allowing the ejection of Martian meteorites range from 5 to 55 GPa, with corresponding post‐shock temperature elevations of 10 to about 1000 °C. With respect to shock pressures and post‐shock temperatures, an ejection of potentially viable organisms in Martian surface rocks seems possible. A calculation of the cooling time in space for the most highly shocked Martian meteorite Allan Hills (ALH) 77005 was performed and yielded a best‐fit for a post‐shock temperature of 1000 °C and a meteoroid size of 0.4 to 0.6 m. The final burial depths of the sub‐volcanic to volcanic Martian rocks as indicated by textures and mineral compositions of meteorites are in good agreement with the postulated size of the potential source region for Martian meteorites during the impact of a small projectile (200 m), as defined by numerical modeling (Artemieva and Ivanov 2004). A comparison of shock pressures and ejection and terrestrial ages indicates that, on average, highly shocked fragments reach Earth‐crossing orbits faster than weakly shocked fragments. If climatic changes on Mars have a significant influence on the atmospheric pressure, they could account for the increase of recorded ejection events of Martian meteorites in the last 5 Ma.     

GPS卫星星历的精度分析

利用全球GPS永久性跟踪站WUHN(武汉)站在SA取消前后五天的广播星历文件计算得到在视卫星的位置和钟差，与事后IGS精密星历提供的卫星位置和钟差进行比较分析，说明SA取消对广播星历的影响。