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1.
张正坤 《地球学报》1988,10(1):73-78
显生宙以来,地磁场有7次频繁倒转间断,每次倒转伴随有相应的造山运动及岩浆活动。地磁场频繁变化,与之相伴的造山运动的发生,大陆与大洋的构造运动、生物的大量灭绝等,都与太阳系在银河系中的活动有关。  相似文献   

2.
The evolution of a system of a large number of particles (N～4×104) moving in planar trajectories in the gravitational field of the Galaxy is simulated with allowance for perturbations by a spiral density wave that give rise to spiral arms (the problem of a collisional N-body system in a specified external field). The particles simulate the motion of diffuse HI clouds in the Galaxy. The spiral-wave parameters are chosen so that the corotation region lies in the vicinity of the Sun. The spiral field pumps cloud out of the corotation region, creating a trough or gap in the initially monotonic radial distribution of particles near the corotation circle. At the same time, the arms proper exhibit no conspicuous features near corotation. This trough could explain the gap in the Galactic HI distribution observed near the solar circle and can be used to locate the corotation region if combined with other data.  相似文献   

3.
Large impacts are credited with the most devastating mass extinctions in Earth's history and the Cretaceous?–?Tertiary (K/T) boundary impact is the strongest and sole direct support for this view. A review of the five largest Phanerozoic mass extinctions provides no support that impacts with craters up to 180 km in diameter caused significant species extinctions. This includes the 170 km-diameter Chicxulub impact crater regarded as 0.3 million years older than the K/T mass extinction. A second, larger impact event may have been the ultimate cause of this mass extinction, as suggested by a global iridium anomaly at the K/T boundary, but no crater has been found to date. The current crater database suggests that multiple impacts, for example comet showers, were the norm, rather than the exception, during the Late Eocene, K/T transition, latest Triassic and the Devonian?–?Carboniferous transition, but did not cause significant species extinctions. Whether multiple impacts substantially contributed to greenhouse warming and associated environmental stresses is yet to be demonstrated. From the current database, it must be concluded that no known Phanerozoic impacts, including the Chicxulub impact (but excluding the K/T impact) caused mass extinctions or even significant species extinctions. The K/T mass extinction may have been caused by the coincidence of a very large impact (>?250 km) upon a highly stressed biotic environment as a result of volcanism. The consistent association of large magmatic provinces (large igneous provinces and continental flood-basalt provinces) with all but one (end-Ordovician) of the five major Phanerozoic mass extinctions suggests that volcanism played a major role. Faunal and geochemical evidence from the end-Permian, end-Devonian, end-Cretaceous and Triassic/Jurassic transition suggests that the biotic stress was due to a lethal combination of tectonically induced hydrothermal and volcanic processes, leading to eutrophication in the oceans, global warming, sea-level transgression and ocean anoxia. It must be concluded that major magmatic events and their long-term environmental consequences are major contributors, though not the sole causes of mass extinctions. Sudden mass extinctions, such as at the K/T boundary, may require the coincidence of major volcanism and a very large Impact.  相似文献   

4.
巨大撞击对地球演化的系统灾变效应   总被引：1，自引：0，他引：1  
综述了巨大地外物体撞击事件对地球产生的一系列剧烈灾变效应，包括撞击事件诱发或产生的各类地质作用变化、古气候波动、古生物灭绝和古地磁倒转等。同时指出，全面深入地研究巨大撞击的灾变效应对认识地球演化和发展具有重要意义。  相似文献   

5.
Variations in frequency of geomagnetic reversals through the Phanerozoic have been analyzed jointly with 87Sr/ 86Sr ratios in marine carbonate sediments. The time series of both parameters contain principal components with periods from 90 to 110 Ma and show a certain correlation. Namely, (i) both time series have five local minimums spaced at similar intervals (period lengths); (ii) the minimums in the Δ87Sr/86Sr curve follow in time those of the reversal frequency, with a lag from 12 Myr in the Ordovician to 38 Myr in the Cretaceous; (iii) the rate of heat transfer from processes at the core-mantle boundary (in D″ layer) which control the Earth's geological life was from 7 to 25 cm/yr in the Phanerozoic. This rate approaches the observed velocities of horizontal plate motion and the predicted mantle convection rates.  相似文献   

6.
Massive continental volcanism and/or bolide impacts are considered by many authors to have caused three major mass extinction events during the last 300 million years: the end-Permian, end-Cretaceous and end-Triassic extinctions. However, re-evaluation of the frequency of bolide impacts and plume-related flood basalt provinces indicates that both types of event occur much more frequently than mass extinctions, and so, in isolation, may not be responsible for the largest extinctions. Furthermore, the kill mechanisms associated with either flood basalts or impacts do not appear to be sufficiently powerful to cause worldwide collapse of ecosystems leading to the largest mass extinctions. Contemporaneous flood basalts and bolide impact may be prerequisites for the largest mass extinctions. We present a statistical analysis of the probability of coincidence between volcanism and impact, and show that three random coincidences of these events in the last 300 m.y. are likely. No causal relationship between impact and volcanism is necessary. The lesser mass extinctions, on the other hand, may not require juxtaposition of two such catastrophic events; such coincidences occurring on more than three occasions during the last 300 m.y. become increasingly unlikely.  相似文献   

7.
Radio and optical data are used to analyze the development of the flare in the blazar 3C 454.3 observed in 2004–2007. A detailed correspondance between the optical and radio flares is established, with a time delay that depends on the observing frequency. The variation of the delay of the radio flare relative to the optical flare is opposite to the dispersion delay expected for the propagation of radiation in the interstellar medium, testifying to an intrinsic origin for the observed outburst. Small-scale flux variations on time intervals of 5–10 days in the millimeter and optical are also correlated, with a time delay of about ten months. This may provide evidence for a single source generating the radiation at all wavelengths. Rapid flux fluctuations in the radio and optical that are correlated with the indicated time delays could be associated with inhomogeneities in the accretion disk. Detailed studies of the flux variations of Active Galactic Nuclei (AGN) can be used to analyze the structure of the accretion disk. A model for the energy release in AGN that is not associated purely with accretion onto supermassive black holes is proposed. As is the case for other active members of the AGN family, estimates of the lifetime of the binary black-hole system in 3C 454.3 suggest that this object is in a stage of its evolution that is fairly close to the coalescence of its black holes. The energy that is released as the companion of the central black hole loses orbital angular momentum is sufficient to explain the observed AGN phenomena. The source of primary energy release could be heating of the gas behind shock fronts that arise due to the friction between the companion black hole and the ambient gaseous medium. The orbit of the companion could be located at the periphery of the accretion disk of the central body at its apocenter and plunge more deeply into the accretion disk at its pericenter, inducing flares at all wavelengths. Energy-release parameters such as the temperature and density of the heated gas are estimated for 3C 454.3. The model considered assumes omnidirectional radiation of the medium in the presence of a magnetic field. The radiation corresponding to the minimum flux level (base level) could represent omnidirectional radiation due to the orbit of the moving companion. The fraction of the energy that is transferred to directed jets is small, comprising 1–2% of the total energy released due to the loss of orbital angular momentum by the companion.  相似文献   

8.
张立军  赵曌  龚一鸣 《地球科学》2015,40(2):381-396
通过系统梳理与奥陶纪-志留纪、晚泥盆世弗拉期-法门期、二叠纪-三叠纪、三叠纪-侏罗纪、白垩纪-古近纪之交 的5次生物大灭绝期遗迹化石记录相关的生物和环境事件,发现遗迹化石对5次大灭绝事件为负响应,即在大灭绝事件之后 的残存期和复苏期期间,遗迹化石的多样性、丰度、潜穴直径、生物扰动强度、遗迹组构阶层都大为减小.遗迹化石反映的造迹 生物行为习性和觅食策略在5次生物大灭绝事件后也各有不同,食沉积物性觅食策略在晚奥陶世和晚白垩世大灭绝事件之 后占据主导,滤食性觅食策略在晚三叠世大灭绝事件之后占据主导,机会主义遗迹(如Planolites)、食沉积物性和滤食性等多 种觅食策略和行为习性在晚泥盆世F-F和晚二叠世两次大灭绝事件之后占据主导.晚泥盆世F-F和晚二叠世两次大灭绝事件 之后,遗迹化石记录了底栖生物系统由简单向复杂、由二维向三维生态空间拓展的变化趋势.   相似文献   

9.
In dynamical models for open clusters, virial equilibrium is not achieved over the violent relaxation time scale τvr. The stars form an equilibrium distribution in (?, ?ζ, l) space, where ? and l are the energy and angular momentum per unit stellar mass in the combined field of the Galaxy and cluster and ?ζ is the energy of motion perpendicular to the Galactic plane per unit mass of cluster stars in the gravitational field of the Galaxy. This distribution of stars changes little when t>τvr. The stellar phase-space distribution corresponding to this type of equilibrium and the regular cluster potential vary periodically (or quasi-periodically) with time. This phase-space equilibrium is probably possible due to an approximate balance in the stellar transitions between phase-space cells over times equal to the oscillation period for the regular cluster field.  相似文献   

10.
The Chandrayaan-1 mission proposes to put a 550 kg lunarcraft into Geostationary Transfer Orbit (GTO) using the Polar Satellite Launch Vehicle (PSLV) which will subsequently be transferred into a 100 km circular lunar polar orbit for imaging purposes. In this paper, we describe certain aspects of mission strategies which will allow optimum power generation and imaging of the lunar surface. The lunar orbit considered is circular and polar and therefore nearly perpendicular to the ecliptic plane. Unlike an Earth orbiting remote sensing satellite, the orbit plane of lunar orbiter is inertially fixed as a consequence of the very small oblateness of the Moon. The Earth rotates around the Sun once a year, resulting in an apparent motion of Sun around this orbit in a year. Two extreme situations can be identified concerning the solar illumination of the lunar orbit, noon/midnight orbit, where the Sun vector is parallel to the spacecraft orbit plane and dawn/dusk orbit, where the Sun vector is perpendicular to the spacecraft orbit plane. This scenario directly affects the solar panel configuration. In case the solar panels are not canted, during the noon/midnight orbit, 100% power is generated, whereas during the dawn/dusk orbit, zero power is generated. Hence for optimum power generation, canting of the panels is essential. Detailed analysis was carried out to fix optimum canting and also determine a strategy to maintain optimum power generation throughout the year. The analysis led to the strategy of 180‡ yaw rotation at noon/midnight orbits and flipping the solar panel by 180‡ at dawn/dusk orbits. This also resulted in the negative pitch face of the lunarcraft to be an anti-sun panel, which is very useful for thermal design, and further to meet cooling requirements of the spectrometers. In principle the Moon’s surface can be imaged in 28 days, because the orbit chosen and the payload swath provide adequate overlap. However, in reality it is not possible to complete the imaging in 28 days due to various mission constraints like maximum duration of imaging allowed keeping in view the SSR sizing and payloads data input rate, time required for downlinking the payload data, data compression requirements and visibility of the lunarcraft for the Bangalore DSN. In each cycle, all the latitudes are swept. Due to the constraints mentioned, only 60‡ latitude arc coverage is possible in each orbit. As Bangalore DSN is the only station, half of the orbits in a day are not available. The longitudinal gaps because of non-visibility are covered in the next cycle by Bangalore DSN. Hence, in the firstprime imaging season, only 25% of the prime imaging zones are covered, and an additional threeprime imaging seasons are required for a full coverage of the Moon in two years. Strategy is also planned to cover X-ray payload coverage considering swath and orbit shift.  相似文献   

11.
We obtained the photometric distances and radial velocities for the molecular gas for 270 star-forming regions and estimated the distance to the Galactic center from ten tangent points to be R0 = 8.01 ± 0.44 kpc. Estimates of R0 derived over the last decade are summarized and discussed; the average value is R0 = 7.80 ± 0.33 kpc. We analyze deviations from axial symmetry of the gas motion around the Galactic center in the solar neighborhood. Assuming a flat rotation curve, we obtain Θ0 ～ 200 km/s for the circular velocity of the Sun from regions beyond the Perseus arm. We used these Galactic constants to construct the Galactic rotation curve. This rotation curve is flat along virtually its total extent from the central bar to the periphery. The velocity jump in the corotation region of the central bar in the first quadrant is 20 km/s. We present analytical formulas for the rotation curves of the Northern and Southern hemispheres of the Galaxy for R0 = 8.0 kpc and Θ0 = 200 km/s.  相似文献   

12.
集群绝灭与生物复苏研究进展   总被引：3，自引：1，他引：3  
近年来，科学家们开始把研究重点从生物绝灭转移到大绝灭后生态系的复苏上。本文简要介绍了目前国际上对集群绝来－残存－复苏过程的研究宗旨和研究方法，截止１９９４年底在显生宙各大绝灭与复苏事件研究上所得的主要进展及取得的一些有意义的成果和认识。  相似文献   

13.
Galactic orbits have been constructed over long time intervals for ten globular clusters located near the Galactic center. A model with an axially symmetric gravitational potential for the Galaxy was initially applied, after which a non-axially symmetric potential corresponding to the central bar was added. Variations in the trajectories of all these globular clusters in the XY plane due to the influence of the bar were detected. These were greatest for the cluster Terzan 4 in the meridional (RZ) plane. The globular clusters Terzan 1, Terzan 2, Terzan 4, Terzan 9, NGC 6522, and NGC 6558 always remained within the Galactic bulge, no farther than 4 kpc from the Galactic center.  相似文献   

14.
In the Phanerozoic, there are three major geological boundaries: Precambrian/Cambrian.Permian/Triassic and Cretaceous/Tertiary. Studies of these boundaries in China and over the world stronglysuggest that they have the following similar features: mass extinctions of many taxa, positive anomalies ofplatinum-group metals, and abrupt changes of stable isotopes (δ~(13)C). It is quite probable that these were theconsequences of some rare catastrophic events of extraterrestrial origin. Hence, the three above-mentionedmass extinction events may be regarded as key indicators for the division of the geological history of thePhanerozoic.  相似文献   

15.
We consider the astrophysical evolution of the Galaxy over large time scales, from early stages (an age of ～108 yrs) to the end of traditional stellar evolution (～1011 yrs). Despite the fact that the basic parameters of our stellar system (such as its size, mass, and general structure) have varied little over this time, variations in the characteristics of stars (their total luminosity, color, mass function, and chemical composition) are rather substantial. The interaction of the Galaxy with other stellar systems becomes an important factor in its evolution 100–1000 Gyr after its origin; however, we take the Galaxy to be isolated. In the model considered, the basic stages of Galactic evolution are as follows. The Galaxy forms as the result of the contraction (collapse) of a protogalactic cloud. The beginning of the Milky Way’s life—the relaxation period, which lasts about 1–2 Gyr—is characterized by active star formation and final structurization. The luminosity and colors of the Galaxy are correlated to the star formation rate (SFR). The young Galaxy intensely radiates high-energy photons, which are mostly absorbed by dust and re-emitted at IR wavelengths. In the subsequent period of steady-state evolution, the gas content in the Galactic disk gradually decreases; accordingly, the SFR decreases, reaching 3–5M ⊙/yr at the present epoch and decreasing to 0.03M ⊙/yr by an age of 100 Gyr. Essentially all other basic parameters of the Galaxy vary little. Later, the decrease in the SFR accelerates, since the evolution of stars with masses exceeding 0.4M ⊙ (i.e., those able to lose matter and renew the supply of interstellar gas) comes to an end. The Galaxy enters a period of “dying”, and becomes fainter and redder. The variation of its chemical composition is manifested most appreciably in a dramatic enrichment of the interstellar gas in iron. The final “stellar epoch” in the life of the Galaxy is completed ～1013 yrs after its formation, when the evolution of the least massive stars comes to an end. By this time, the supplies of interstellar and intergalactic gas are exhausted, the remaining stars become dark, compact remnants, there is no further formation of new stars, and the Galactic disk no longer radiates. Eventually, infrequent outbursts originating from collisions of stellar remnants in the densest central regions of the Galaxy will remain the only source of emission.  相似文献   

16.
自从地球诞生以来经历了许多重大事件：早期生命的出现、大气氧化事件（Atmospheric oxygenation）、雪球地球及多次生物绝灭与复苏事件。稳定同位素记录在古环境和生命演化研究中具有重要意义,在记录环境变化和生命演化的重大事件中发挥着重大作用：碳同位素的分馏记录了最早生命的开始,硫同位素的非质量相关分馏（Independent mass fractionation of sulphur isotopes）记录了大气中氧含量的重大变化,而在显生宙的几次重大生物灭绝事件中,均有碳同位素的负向漂移。  相似文献   

17.
Published and new data on the Earth’s past magnetic field have been interpreted in terms of its links with the frequency of magnetic polarity reversals and with tectonic events such as plume-related eruptions and rifting. The paleointensity and reversal frequency variations show an antiphase correlation between 0 and 160 Ma, and the same tendency likely holds for the past 400 Myr. The geomagnetic field intensity averaged over geological ages (stages) appears to evolve in a linearly increasing trend while its variations increase proportionally in amplitude and change in structure. Both paleointensity and reversal frequency patterns correlate with rifting and eruption events. In periods of high rifting activity, the geomagnetic field increases (15 to 30%) and the reversals become about 40% less frequent. Large eruption events between 0 and 150 Ma have been preceded by notable changes in magnetic intensity which decreases and then increases, the lead being most often within a few million years.  相似文献   

18.
The formation of hypervelocity stars due to the dynamical capture of one component of a closebinary system by the gravitational field of a supermassive black hole (SMBH) is modeled. The mass of the black hole was varied between 106 and 109 M ⊙. In the model, the problem was considered first as a three-body problem (stage I) and then as an N-body problem (stage II). In the first stage, the effect of the inclination of the internal close-binary orbit (the motion of the components about the center of mass of the binary system) relative to the plane of the external orbit (the motion of the close binary around the SMBH) on the velocity with which one of the binary components is ejected was assessed. The initial binary orbits were generated randomly, with 10 000 orbits considered for each external orbit with a fixed pericenter distance r p . Analysis of the results obtained in the first stage of the modeling enables determination of the binary-orbit orientations that are the most favorable for high-velocity ejection, and estimation of the largest possible ejection velocities V max. The boundaries of the region of stellar disruption derived from the balance of tidal forces and self-gravitation are discussed using V max-r p plots, which generalize the results of the first stage of the modeling. Since a point-mass representation does not enable predictions about the survival of stars during close passages by a SMBH, there is the need for a second stage of the modeling, in which the tidal influence of the SMBH is considered. An approach treating a star like a structured finite object containing N bodies (N = 4000) enables the derivation of more accurate limits for the zone of efficient acceleration of hypervelocity stars and the formulation of conditions for the tidal disruption of stars.  相似文献   

19.
We have analyzed the distribution of young objects (open clusters, classical Cepheids, and HII regions) projected onto the plane of the Galaxy using wavelet smoothing. This smoothing technique enables investigation of the large-scale structure of the distribution of young objects. All the studied objects display a similar spiral structure, whose appearance essentially corresponds to a regular alternation of young and older stellar complexes along sections of the spiral arms. A four-arm spiral structure with its arms originating at the Galactic center is obtained if the major arms are taken to be the Carina-Sagittarius arm and an outer arm behind the Perseus arm. If all the observed arms are taken to be major arms, we obtain a 12-arm structure, in contradiction with the size of the region in which the structure is observed in the Galaxy. This discrepancy can be removed if the arms originate from a ring formed by a sufficiently long bar-like structure, rather then from the Galactic center.  相似文献   

20.
３５Ｍａ－ＡｎＩｍｐｏｒｔａｎｔＮａｔｕｒａｌＰｅｒｉｏｄｉｃｉｔｙｉｎＧｅｏｌｏｇｉｃａｌＨｉｓｔｏｒｙ：ＣｏｎｃｅｐｔｏｆＮａｔｕｒａｌ Ｃｒｉｓｉｓ ａｎｄ Ｉｔｓ Ｐｏｓｓｉｂｌｅ ＣａｕｓｅＳｈｉＸｉａｏｙｉｎｇ（ＤｅｐａｒｔｍｅｎｔｏｆＧｅｏ?..  相似文献   

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