Chronology of the early Solar System from chondrule-bearing calcium-aluminium-rich inclusions

Chondrules and Ca-Al-rich inclusions (CAIs) are high-temperature components of meteorites that formed during transient heating events in the early Solar System. A major unresolved issue is the relative timing of CAI and chondrule formation. From the presence of chondrule fragments in an igneous CAI, it was concluded that some chondrules formed before CAIs (ref. 5). This conclusion is contrary to the presence of relict CAIs inside chondrules, as well as to the higher abundance of 26Al in CAIs; both observations indicate that CAIs pre-date chondrules by 1-3 million years (Myr). Here we report that relict chondrule material in the Allende meteorite, composed of olivine and low-calcium pyroxene, occurs in the outer portions of two CAIs and is 16O-poor (Delta17O approximately -1 per thousand to -5 per thousand). Spinel and diopside in the CAI cores are 16O-rich (Delta17O up to -20 per thousand), whereas diopside in their outer zones, as well as melilite and anorthite, are 16O-depleted (Delta17O = -8 per thousand to 2 per thousand). Both chondrule-bearing CAIs are 26Al-poor with initial 26Al/27Al ratios of (4.7 +/- 1.4) x 10(-6) and <1.2 x 10(-6). We conclude that these CAIs had chondrule material added to them during a re-melting episode approximately 2 Myr after formation of CAIs with the canonical 26Al/27Al ratio of 5 x 10(-5).     

Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions

Primitive or undifferentiated meteorites (chondrites) date back to the origin of the Solar System, and thus preserve a record of the physical and chemical processes that occurred during the earliest evolution of the accretion disk surrounding the young Sun. The oldest Solar System materials present within these meteorites are millimetre- to centimetre-sized calcium-aluminium-rich inclusions (CAIs) and ferromagnesian silicate spherules (chondrules), which probably originated by thermal processing of pre-existing nebula solids. Chondrules are currently believed to have formed approximately 2-3 million years (Myr) after CAIs (refs 5-10)--a timescale inconsistent with the dynamical lifespan of small particles in the early Solar System. Here, we report the presence of excess (26)Mg resulting from in situ decay of the short-lived (26)Al nuclide in CAIs and chondrules from the Allende meteorite. Six CAIs define an isochron corresponding to an initial (26)Al/(27)Al ratio of (5.25 +/- 0.10) x 10(-5), and individual model ages with uncertainties as low as +/- 30,000 years, suggesting that these objects possibly formed over a period as short as 50,000 years. In contrast, the chondrules record a range of initial (26)Al/(27)Al ratios from (5.66 +/- 0.80) to (1.36 +/- 0.52) x 10(-5), indicating that Allende chondrule formation began contemporaneously with the formation of CAIs, and continued for at least 1.4 Myr. Chondrule formation processes recorded by Allende and other chondrites may have persisted for at least 2-3 Myr in the young Solar System.     

CO self-shielding as the origin of oxygen isotope anomalies in the early solar nebula

The abundances of oxygen isotopes in the most refractory mineral phases (calcium-aluminium-rich inclusions, CAIs) in meteorites have hitherto defied explanation. Most processes fractionate isotopes by nuclear mass; that is, 18O is twice as fractionated as 17O, relative to 16O. In CAIs 17O and 18O are nearly equally fractionated, implying a fundamentally different mechanism. The CAI data were originally interpreted as evidence for supernova input of pure 16O into the solar nebula, but the lack of a similar isotope trend in other elements argues against this explanation. A symmetry-dependent fractionation mechanism may have occurred in the inner solar nebula, but experimental evidence is lacking. Isotope-selective photodissociation of CO in the innermost solar nebula might explain the CAI data, but the high temperatures in this region would have rapidly erased the signature. Here we report time-dependent calculations of CO photodissociation in the cooler surface region of a turbulent nebula. If the surface were irradiated by a far-ultraviolet flux approximately 10(3) times that of the local interstellar medium (for example, owing to an O or B star within approximately 1 pc of the protosun), then substantial fractionation of the oxygen isotopes was possible on a timescale of approximately 10(5) years. We predict that similarly irradiated protoplanetary disks will have H2O enriched in 17O and 18O by several tens of per cent relative to CO.     

Young chondrules in CB chondrites from a giant impact in the early Solar System

Chondrules, which are the major constituent of chondritic meteorites, are believed to have formed during brief, localized, repetitive melting of dust (probably caused by shock waves) in the protoplanetary disk around the early Sun. The ages of primitive chondrules in chondritic meteorites indicate that their formation started shortly after that of the calcium-aluminium-rich inclusions (4,567.2 +/- 0.7 Myr ago) and lasted for about 3 Myr, which is consistent with the dissipation timescale for protoplanetary disks around young solar-mass stars. Here we report the 207Pb-206Pb ages of chondrules in the metal-rich CB (Bencubbin-like) carbonaceous chondrites Gujba (4,562.7 +/- 0.5 Myr) and Hammadah al Hamra 237 (4,562.8 +/- 0.9 Myr), which formed during a single-stage, highly energetic event. Both the relatively young ages and the single-stage formation of the CB chondrules are inconsistent with formation during a nebular shock wave. We conclude that chondrules and metal grains in the CB chondrites formed from a vapour-melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated. These findings therefore provide evidence for planet-sized objects in the earliest asteroid belt, as required by current numerical simulations of planet formation in the inner Solar System.     

Chondrule formation in particle-rich nebular regions at least hundreds of kilometres across

Chondrules are millimetre-sized spherules (mostly silicate) that dominate the texture of primitive meteorites. Their formation mechanism is debated, but their sheer abundance suggests that the mechanism was both energetic and ubiquitous in the early inner Solar System. The processes suggested--such as shock waves, solar flares or nebula lightning--operate on different length scales that have been hard to relate directly to chondrule properties. Chondrules are depleted in volatile elements, but surprisingly they show little evidence for the associated loss of lighter isotopes one would expect. Here we report a model in which molten chondrules come to equilibrium with the gas that was evaporated from other chondrules, and which explains the observations in a natural way. The regions within which the chondrules formed must have been larger than 150-6,000 km in radius, and must have had a precursor number density of at least 10 m(-3). These constraints probably exclude nebula lightning, and also make formation far from the nebula midplane problematic. The wide range of chondrule compositions may be the result of different combinations of the local concentrations of precursors and the local abundance of water ice or vapour.     

对化学多媒体CAI的功能及作用的探讨

文章结合作者使用CAI的经验和体会,从四个方面探讨了CAI的作用和功能,阐述了CAI作为辅助教学是21世纪教育改革必然趋势的观点。     

Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary

Estimates of the time of origin for placental mammals from DNA studies span nearly the duration of the Cretaceous period (145 to 65 million years ago), with a maximum of 129 million years ago and a minimum of 78 million years ago. Palaeontologists too are divided on the timing. Some support a deep Cretaceous origin by allying certain middle Cretaceous fossils (97-90 million years old) from Uzbekistan with modern placental lineages, whereas others support the origin of crown group Placentalia near the close of the Cretaceous. This controversy has yet to be addressed by a comprehensive phylogenetic analysis that includes all well-known Cretaceous fossils and a wide sample of morphology among Tertiary and recent placentals. Here we report the discovery of a new well-preserved mammal from the Late Cretaceous of Mongolia and a broad-scale phylogenetic analysis. Our results exclude Cretaceous fossils from Placentalia, place the origin of Placentalia near the Cretaceous/Tertiary (K/T) boundary in Laurasia rather than much earlier within the Cretaceous in the Southern Hemisphere, and place afrotherians and xenarthrans in a nested rather than a basal position within Placentalia.     

工程制图网络CAI初探

随着因特网的推广 ,人们的学习方式发生了极大的改变 足不出户就可“走”遍全球 ,坐在家中就可“上学” 显然传统的CAI课件不能满足这些要求 ,它必走向网络化 ,实现优秀教育资源共享 ,实现“个性化教育” 然而现有的CAI课件是多年来广大教育工作者和计算机开发人员积累的宝贵财富 ,如果弃之不用 ,将造成巨大的浪费 文中采用AutoCAD 2 0 0 0创建超级链接、发布网络图形文件 ,用goldwave 3 .2X和GIFAnimator软件将传统的CAI声频文件 .wav和视频文件 .avi分别转化成 .au和 .gif格式 ,用Java进行二次开发 ,用JavaScript增强人机交互性 ,使之升级为基于Internet方式运行的网络CAI软件     

A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites

Determining the chronology for the assembly of planetary bodies in the early Solar System is essential for a complete understanding of star- and planet-formation processes. Various radionuclide chronometers (applied to meteorites) have been used to determine that basaltic lava flows on the surface of the asteroid Vesta formed within 3 million years (3 Myr) of the origin of the Solar System. Such rapid formation is broadly consistent with astronomical observations of young stellar objects, which suggest that formation of planetary systems occurs within a few million years after star formation. Some hafnium-tungsten isotope data, however, require that Vesta formed later (approximately 16 Myr after the formation of the Solar System) and that the formation of the terrestrial planets took a much longer time (62(-14)(+4504) Myr). Here we report measurements of tungsten isotope compositions and hafnium-tungsten ratios of several meteorites. Our measurements indicate that, contrary to previous results, the bulk of metal-silicate separation in the Solar System was completed within <30 Myr. These results are completely consistent with other evidence for rapid planetary formation, and are also in agreement with dynamic accretion models that predict a relatively short time (approximately 10 Myr) for the main growth stage of terrestrial planet formation.     

The habitat and nature of early life

Earth is over 4,500 million years old. Massive bombardment of the planet took place for the first 500-700 million years, and the largest impacts would have been capable of sterilizing the planet. Probably until 4,000 million years ago or later, occasional impacts might have heated the ocean over 100 degrees C. Life on Earth dates from before about 3,800 million years ago, and is likely to have gone through one or more hot-ocean 'bottlenecks'. Only hyperthermophiles (organisms optimally living in water at 80-110 degrees C) would have survived. It is possible that early life diversified near hydrothermal vents, but hypotheses that life first occupied other pre-bottleneck habitats are tenable (including transfer from Mars on ejecta from impacts there). Early hyperthermophile life, probably near hydrothermal systems, may have been non-photosynthetic, and many housekeeping proteins and biochemical processes may have an original hydrothermal heritage. The development of anoxygenic and then oxygenic photosynthesis would have allowed life to escape the hydrothermal setting. By about 3,500 million years ago, most of the principal biochemical pathways that sustain the modern biosphere had evolved, and were global in scope.     

雉科四种鸟类线粒体DNAR的分子进化

采用聚合酶链式反应(PCR)和直接测序的方法分析了4种雉科(Phasianidae)鸟类的线粒体DNA(mtDNA)控制区基因(D-loop)415bp基因序列。4种鸟类遗传距离在5.90％--23．67％．变异位点数为116个，大石鸡和石鸡的遗传距离为5．90％，二者分歧时间大约是290万年．斑翅山鹑与环颈雉遗传距离为21．06％，分歧时间为1000万年，山鹑属与雉属的遗传距离为21．06％，与石鸡属的遗传距离为22．41％；雉属与石鸡属的遗传距离为22．85％．三者分歧时间在1000—1150万年间，与化石材料相符．在系统树中斑翅山鹑与环颈雉属同一枝，说明1)在雉科系统发生中，雉属与山鹑属是近缘属或2)环颈雉与斑翅山鹑的分化较晚，关系密切，也与Randi得出的斑翅山鹑称为小雉的结论相吻合。     

专业课多媒体CAI课件制作方法研究

多媒体CAI教学已经广泛应用于高校的教学当中,成为教育改革的一种趋势。但由于专业的差别,专业课CAI课件成型作品较少,不利开展CAI教学,笔者多年来从事多媒体课件的开发工作,积累了一些经验和体会,对专业课多媒体CAI课件的制作方法及流程提出新的认识。     

Strict evolutionary conservation followed rapid gene loss on human and rhesus Y chromosomes

The human X and Y chromosomes evolved from an ordinary pair of autosomes during the past 200-300 million years. The human MSY (male-specific region of Y chromosome) retains only three percent of the ancestral autosomes' genes owing to genetic decay. This evolutionary decay was driven by a series of five 'stratification' events. Each event suppressed X-Y crossing over within a chromosome segment or 'stratum', incorporated that segment into the MSY and subjected its genes to the erosive forces that attend the absence of crossing over. The last of these events occurred 30 million years ago, 5 million years before the human and Old World monkey lineages diverged. Although speculation abounds regarding ongoing decay and looming extinction of the human Y chromosome, remarkably little is known about how many MSY genes were lost in the human lineage in the 25 million years that have followed its separation from the Old World monkey lineage. To investigate this question, we sequenced the MSY of the rhesus macaque, an Old World monkey, and compared it to the human MSY. We discovered that during the last 25 million years MSY gene loss in the human lineage was limited to the youngest stratum (stratum 5), which comprises three percent of the human MSY. In the older strata, which collectively comprise the bulk of the human MSY, gene loss evidently ceased more than 25 million years ago. Likewise, the rhesus MSY has not lost any older genes (from strata 1-4) during the past 25 million years, despite its major structural differences to the human MSY. The rhesus MSY is simpler, with few amplified gene families or palindromes that might enable intrachromosomal recombination and repair. We present an empirical reconstruction of human MSY evolution in which each stratum transitioned from rapid, exponential loss of ancestral genes to strict conservation through purifying selection.     

多媒体CAI课件制作存在的问题与分析

结合笔者近年来的课件创作和教学实践,以及对郑州市中小学教师进行"Intel未来教育"培训所掌握的情况,对当前多媒体课件制作中存在的问题及原因进行分析,阐述了CAI课件制作过程中应遵循的原则和注意事项,并着重强调如何实现计算机辅助教学到信息技术和课程的整合.     

基于联系数的CAI软件开发评价及其应用

CAI软件评价是一项比较复杂的多目标决策,许多指标具有模糊性与不确定性．本文在集对分析（SPA）方法的联系数基础上,建立了一种基于联系数的CAI软件开发评价模型,并给出了具体的实例应用．实例表明,评价不仅合理,而且是一种行之有效的CAI软件开发评价方法．     

The oldest articulated chondrichthyan from the Early Devonian period

Chondrichthyans (including living sharks, skates, rays and chimaeras) have a fossil record of scales and dermal denticles perhaps dating back to the Late Ordovician period, about 455 million years ago. Their fossil tooth record extends to the earliest Devonian period, almost 418 million years ago, whereas the oldest known articulated shark remains date from the Early Devonian period, about 394 million years ago. Here we report the discovery of an articulated shark that is almost 409 million years old from the Early Devonian (early Emsian) period of New Brunswick, Canada. The specimen, identified as Doliodus problematicus (Woodward), sheds light on the earliest chondrichthyans and their interrelationships with basal jawed vertebrates. This species has been truly problematic. Previously known only from isolated teeth, it has been identified as an acanthodian and a chondrichthyan. This specimen is the oldest shark showing the tooth families in situ, and preserves one of the oldest chondrichthyan braincases. More notably, it shows the presence of paired pectoral fin-spines, previously unknown in cartilaginous fishes.     

《体育与健康》课程CAI系统制作流程与应用

CAI广泛应用于体育教育教学之中能够解决体育教学中多年面临的难题 ,并开拓新的教学方法 .本文就体育教学中的 CAI制作流程与应用前景进行初步研究 ,以创造出合乎教师、学生所需要的较理想的教学环境 ,从而获得更好的教育效益     

基于灰色理论的CAI课件多层次综合评价模型

为促进CAI课件的应用,进一步提高课件的标准化、规范化程度,使CAI课件管理部门能科学地评价CAI课件、用户能正确地选用CAI课件、开发研究人员能有标准、有目的地开发CAI课件,这就需要建立先进的量化评价方法和合理、完善的CAI课件评价指标体系。     

ICAI 系统结构

计算机辅助教学（ＣＡＩ）是指用计算机帮助或代替教师完成部分教学任务的一种应用形式,将人工智能技术与ＣＡＩ相结合可以克服传统ＣＡＩ的许多弱点,改善教学效果,提高学习效率。文章提出一个智能计算机辅助教学（ＩＣＡＩ）系统结构,并对其主要组成部分的功能和相互之间的关系,及其中的知识库结构进行了讨论。