贵州双河洞晚更新世大熊猫化石高度聚集现象及其意义

贵州双河洞是亚洲第一长洞,该洞先后分辨出近30多具大熊猫个体化石,是目前世界上发现完整大熊猫化石最多的洞穴。其中一支洞中竖井下方至少聚集了10余具个体化石。以此为研究对象,结合洞道发育形态特征及化石埋藏学特征,初步推测出这些大熊猫深入该支洞内经由竖井跌落并死亡的过程,表明喀斯特洞穴可以为大熊猫提供良好的栖息场所及化石保存环境。选取一例化石表面沉积的碳酸钙样品进行了铀系法测年,表明该个体至少生活在5.1万年前;对另一例个体的牙齿化石进行AMS14C测年,发现其生活在距今2.5万年左右,推断该洞道中大熊猫主要生活在晚更新世时期。     

Structure and Dynamics of the Centaur Population: Constraints on the Origin of Short-Period Comets

The origin of Jupiter-family comets is linked to the intermediate stage of evolution through the Centaur region. Thus the structure of the Centaur population provides important constraints on sources of short-period comets. We show that our model of the Oort cloud evolution gives results which are consistent with the orbital distribution of observed Centaurs. In particular, it explains the existence of the large population of Centaurs with semimajor axes greater than 60 AU. The main source for these objects is the inner Oort cloud. Both Jupiter-family and Halley-type comets are produced by Centaurs originating from the Oort cloud. The injection rate for Jupiter-family comets coming from the inner Oort cloud is, at least, not less than that for a model based on the observed sample of high-eccentricity trans-Neptunian objects.     

Surface motion of a fluid planet induced by impacts

In order to approximate the free-surface motion of an Earth-sized planet subjected to a giant impact, we have described the excitation of body and surface waves in a spherical compressible fluid planet without gravity or intrinsic material attenuation for a buried explosion source. Using the mode summation method, we obtained an analytical solution for the surface motion of the fluid planet in terms of an infinite series involving the products of spherical Bessel functions and Legendre polynomials. We established a closed form expression for the mode summation excitation coefficient for a spherical buried explosion source, and then calculated the surface motion for different spherical explosion source radii a (for cases of   a / R = 0.001  to 0.035, R is the radius of the Earth) We also studied the effect of placing the explosion source at different radii r ₀ (for cases of   r ₀/ R = 0.90  to 0.96) from the centre of the planet. The amplitude of the quasi-surface waves depends substantially on a / R , and slightly on   r ₀/ R   . For example, in our base-line case,   a / R = 0.03, r ₀/ R = 0.96  , the free-surface velocity above the source is 0.26 c , whereas antipodal to the source, the peak free surface velocity is 0.19 c . Here c is the acoustic velocity of the fluid planet. These results can then be applied to studies of atmosphere erosion via blow-off caused by asteroid impacts.     

Giant pulses from the millisecond pulsar PSR B1937+214

We have detected giant pulses from the millisecond pulsar PSR B1937+214 at the lowest frequency of 112 MHz. The observed flux density at the pulse peak is ～40 000 Jy, which exceeds the average level by a factor of 600. Pulses of such intensity occur about once per 300 000 periods. The brightness temperature of the observed giant pulses is T _B≈10³⁵ K. We estimated the pulse broadening by interstellar scattering to be τ_sc=3–10 ms. Based on this estimate and on published high-frequency measurements of this parameter, we determined the frequency dependence of the pulse broadening by scattering: τ_sc(f)=25 × (f/100)^?4.0±02.     

The electric potential on dust particles in comets and in interplanetary space

We calculate the electric surface potential reached by small dust particles in cometary atmospheres and in interplanetary space. Plasma temperature and density are varied over a wide range; a two component plasma of ions and electrons in thermodynamic equilibrium is assumed. The calculations are performed for three types of grains whose photoelectric and secondary electron emission yield are choosen to cover about the range expected for real dust. Results for vanishing secondary electron emission are given for comparison. At the beginning, a short review of the theoretical formulation and the main assumptions are presented. Wir berechnen das Oberflächenpotential kleiner Staubteilchen im Plasma einer Kometenatmosphäre und im interplanetaren Raum. Die Plasma-parameter Temperatur und Dichte werden in einem weiten Bereich variiert, es wird jedoch stets thermodynamisches Gleichgewicht zwischen Elektronen und Ionen eines Zweikomponentenplasmas angenommen. Die Rechnungen werden für drei Teilchenmaterialien ausgeführt, deren Photo-effekt und Sekundärelektronenausbeute etwa den an realen Staubteilchen vorkommenden Bereich überdecken dürften; zum Vergleich werden auch die Ergebnisse bei vernachlässigbarer Sekundärelektronenausbeute mitgeteilt. Eine kurze Zusammenfassung der theoretischen Grundlagen und der wesentlichen Voraussetzungen ist den Rechnungen vorangestellt.     

Sungrazing Comets: Properties of Nuclei and in Situ Detectability of Cometary Ions at 1 AU

A one-dimensional sublimation model for cometary nuclei is used to derive size limits for the nuclei of sungrazing comets and to estimate oxygen ion fluxes at 1 AU from their evaporation. Given that none of the ≈300 sungrazers detected by the SOlar and Heliospheric Observatory (SOHO) was observed after disappearing behind the sun, and that small nuclei with a radius of ≈3.5 m could be observed, it is assumed that all SOHO sungrazers were completely destroyed. For the case that sublimation alone is sufficient for destruction, the model yields an upper size limit as a function of nuclear density ?, albedo A, and perihelion distance q. If the density of the nuclei is that typical of porous ice (600 kg m⁻³), the maximum size is 63 m. These results confirm similar model calculations by Weissman (1983). An analytical expression is derived that approximates the model results well. We discuss possible modifications of our results by different disruption mechanisms. While disruption by thermal stress does not change the upper size limits significantly, they may be somewhat increased by tidal disruption (up to 100 m for a density of 600 kg m⁻³), dependent on the isotropy of the sublimation process and the tensile strength of the comet. Implications for the Kreutz family of sungrazers are discussed.Oxygen ions from the sublimation of sungrazing comets form a tail. Fluxes from this tail are sufficiently high to be measured at 1 AU by particle detectors on spacecraft, but the duration of a tail crossing is only about half an hour. Therefore, the probability of a spacecraft actually encountering a tail of an evaporating sungrazer is only of the order of 2% per year.     

The lost city of north‐west Queensland: a test of the model of giant Grikeland development in semi‐arid Karst

Jennings and Sweeting's (1963) model of landscape evolution in the Limestone Ranges of Western Australia has been proposed as a general scheme of semi‐arid karstification by Williams (1978) and Ford and Williams (1989). The model involves an initial plateau surface which undergoes progressive karstic dissection. The resultant landscape is characterised first by giant grikelands, then by box valleys and finally by karst towers. The depth of dissection is governed by the altitude of successive levels of pediplanation adjacent to the karst, with each karst level representing the location of a former erosion surface. The Lost City, a giant grikeland in the Barkly karst of north‐west Queensland, has evolved by the karstic dissection and stripping of a gently dipping sequence of carbonates and cherts. By contrast with the type example, the depth of dissection, the altitude of successive surfaces, the form and even the location of the grikeland are all controlled by rock structure. Not only does this cast doubt on the general applicability of Jennings and Sweeting's model, it suggests that such landscapes may also be the result of structural control and that the role played by semi‐arid climates may be less important than has previously been proposed. Given our present poverty of knowledge of semi‐arid karstification, any attempt to identify type examples of semi‐arid karst may be premature.