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1.
Abstract— Twenty‐three moldavites from a new locality, the Cheb Basin in Western Bohemia, were analyzed by instrumental neutron activation analysis for 45 major and trace elements. Detailed comparison of the Cheb Basin moldavites with moldavites from other substrewn fields in both major and trace element composition shows that the Cheb Basin is a separate substrewn field. The geochemical data obtained are discussed with respect to the source materials and processes leading to formation of moldavites. The data show that three groups of Cheb Basin moldavites exist. Ten samples of group 1 are characterized by the lowest content of Al, Fe, Na, and other elements representing phyllosilicate minerals, and by high Ca + Mg contents related probably to carbonates. They resemble the “poisonous green” moldavites, a subgroup of the Southern Bohemian moldavites. Seven samples of group 2 and 6 samples of group 3 are similar to typical moldavites of the Southern Bohemian substrewn field. These two groups differ from each other mainly in Al contents; with higher contents of Al and the elements associated with phyllosilicate minerals (namely Ba and Sr), group 3 also resembles the Moravian moldavites. Significant positive correlations between K, Ca, Mg, and Mn found in group 2 of the Cheb Basin moldavites and the enrichment in these elements observed generally in all moldavites, as well as other facts, e.g., high K/Na and K/Rb ratios and the reduced conditions during formation of moldavites, have been attributed to possible contribution to the moldavite source materials of the ash produced by burning of vegetation and soil organic matter present at the pre‐impact area.  相似文献   

2.
Abstract— A new 40Ar/39Ar data set is presented for tektites from the Central European strewn field (moldavites). This is the only strewn field that is entirely situated in a continental environment and still characterized by scattered ages (14–15.3 Myr). The main objectives of the study were to define more precisely the moldavite formation age and provide a good calibration for a glass standard proposed for fission‐track dating. The laser total fusion ages obtained on chips from 7 individual specimens from the Southern Bohemian and Moravian subfields are restricted to a narrow interval of time, with an average of 14.34 ± 0.08 Myr relative to the 27.95 ± 0.09 Myr of the Fish Canyon Tuff biotite. This result gives a more precise age not only for the tektite field but also for its producing impact. If the genetic link between the moldavites and the Nördlinger Ries impact crater is maintained, then this new age has to be considered a reliable estimate for the Ries crater also. This new value places the formation of Central European tektites within the Lower Serravallian period in the latest geologic timescales. Evidence of their impact products, such as glass spherules or shocked minerals, can, therefore, be sought in sedimentary marine formations in a more precisely defined age interval.  相似文献   

3.
The Czech Republic comprises two geologically diverse provinces. The western part is an ancient, long-stabilized crystalline (slowly uplifting) massif, the Bohemian Massif, whereas the eastern part is the younger and tectonically more mobile Carpathian Foredeep and the West Carpathian mountain ranges. The Late Cenozoic fluvial record, controlled in essence by cyclic climate-driven changes of the environment shows discernible differences in both provinces and, therefore, the paper deals with these separately. In the Bohemian Massif a regular development of terrace staircases is noted, making internally consistent correlation possible. Problems that remain open to debate include dating the start of fluvial terrace staircase development, and correlating the fluvial terraces with the succession of Scandinavian glaciations that have affected the northernmost Czech Republic. On the other hand, the fluvial records in the southeastern Czech Republic show significant lateral variations; thus, northern, central and southern Moravia are reviewed separately in this paper. The terrace system in northern Moravia formed in direct contact with the ice sheet during two Scandinavian glaciations, whereas only periglacial or extraglacial conditions developed further south. Nevertheless, the existence of the Main Terrace as an important stratigraphical index horizon allows the reliable correlation of the terrace systems of all the larger Moravian rivers, despite their fragmentary development.  相似文献   

4.
Abstract— 40Ar/39Ar ages of four tektites (moldavites) from southern Bohemia (near ?eské Budějovice, Czech Republic) and a tektite from Lusatia (near Dresden, Germany) have been determined by 11 step‐degassing experiments. The purpose of the study was to enlarge the 40Ar/39Ar data base of moldavites and to check the age relations of the Bohemian and Lusatian samples. The mean plateau‐age of the Bohemian samples, which range from 14.42 to 14.70 Ma, is 14.50 ± 0.16 (0.42) (2σ) Ma (errors in parentheses include age error and uncertainty of standard monitor age). The plateau age of the Lusatian sample of 14.38 ± 0.26 (0.44) (2σ) Ma confirms the previously published 40Ar/39Ar age of 14.52 ± 0.08 (0.40) (2σ) Ma, and demonstrates that the fall of Lusatian and Bohemian tektites were contemporaneous. Because of their geochemistry and their ages there is no doubt that the Lusatian tektites are moldavites. Accepting that moldavites are ejecta from the Nördlinger Ries impact, the new ages also date the impact event. This age is slightly younger (about 0.2–0.3 Ma) than the age suggested by earlier K‐Ar determinations.  相似文献   

5.
The existence of either pre- or post-burst emission can provide substantial new information about the burst source and its local environment. We have data from serveral events serendipitously in or near the OSSE field of view at the time of the burst. We present pre- and post-burst flux limits from one such event, GRB 940301. The OSSE data for other periods when scheduled observations have included burst locations will enable us to search for pre- and post- burst emission on many time scales.  相似文献   

6.
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7.
《Planetary and Space Science》2007,55(11):1614-1621
Many ground-based observations of Na in Mercury's surface-bounded exosphere have been made and continued to be made in an effort to understand the sources, sinks, and distribution of Na around Mercury. These time consuming and costly efforts are made to better understand the physical processes on and around Mercury. A big step would be to discover an actual source of the Na from Mercury's crust because it is already known that meteorites and comets provide Na to the exosphere through impact. We provide ground-based CCD imagery obtained with small ground-based telescopes that show bright albedo features at locations coincident with enhanced Na emissions in Mercury's exosphere. We suggest these locations are sources for Na. We also provide a mechanism to test this hypothesis using in situ observations by instruments on the MESSENGER spacecraft during the three fly bys of Mercury that will occur in 2008 and 2009, and during the orbital mission which begins in 2011. It is necessary to prove that Na is delivered to the exosphere from one or more crustal source regions before exospheric Na can be used as a measure of the volatile content of Mercury used to infer formation and evolution from the primitive solar nebula. The same applies to other elements such as K which is known to be in Mercury's exosphere and S which is postulated to be present. We expound on the impact that the discovery of one or more source regions from Mercury's crust would have on our ability to discern between the three leading models of Mercury's formation and crustal evolution.  相似文献   

8.
The properties of the flaring sites in the active region NOAA 6555 and 6659 are presented in this paper. Although, several locations of these active regions displayed high magnetic shear, the X-class flares occurred only at one of them. Our investigations show that these locations are characterized by the `crossing' of magnetic and velocity field neutral lines. Based on a new parameter to define the stressed magnetic fields, we show that some of these flaring locations possessed highly stressed magnetic field which disappeared after the flare. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

9.
Slope streaks are a form of gravity-driven mass-movements that frequently occur on Mars today. The cause of slope streak formation remains unclear; both, dry and wet processes have been suggested. Here, we observationally constrain the time of the year during which slope streaks form. Imagery from four Mars-orbiting cameras is mined to identify locations that have been imaged repeatedly, and the overlapping images are surveyed for streak activity. A search algorithm automatically finds the locations on the surface that have been imaged most often based on a graph representation. Dark slope streaks are found to form sporadically throughout the Mars year. At one study site in the Olympus Mons Aureole, observations constrain slope streak formation to at least five distinct time intervals within a single Mars year. New slope streaks form spatially isolated or in small groups within a few kilometers of one another. The observations suggest that slope streak triggering is unrelated to season and not caused by any large regional events. Most slope streaks are caused by sporadic events of small spatial extent.  相似文献   

10.
The mean UBV color of S-class asteroids varies markedly with distance from the Sun and may also vary with diameter, implying either that the surface properties of the asteroids have been modified by space weathering or, as suggested here, that there are least two major subclasses of S-class asteroids with different mean locations.  相似文献   

11.
高重频激光测距数据处理方法   总被引：1，自引：0，他引：1  
该文探讨了高重频激光测距工作中的数据处理问题。该数据处理采用了屏幕显示以及数学分析方法,以剔除卫星激光测距数据中的观测异常值,提取有效的数据点,并最终生成标准点数据。通过对奥地利GRAZ激光测距站高重频数据资料的处理表明,该数据处理方法是有效的。  相似文献   

12.
The local magnetic structure of the heliospheric current sheet (HCS) is observed as a boundary through which the magnetic field inverts its direction toward or away from Sun. The local variability of the HCS has been studied by means of a comparison of its local orientation estimated from data of different spacecraft. With the aim of determining possible variations in the local orientation, the selected events have been grouped according to their magnetic connection. A rough estimate of the magnetic connection (C) between two observation points has been found by considering the absolute value of the difference between the elapsed and expected times (C=|??t el??C???t ex|/??t ex). Lower values of C imply better connections, and smaller variation in the HCS orientation is expected if variations, temporal or spatial, in the HCS shape are negligible. Two periods have been analyzed: the ascending phase of Solar Cycle 23 and the minimum of the cycle in 2007??C?2008. It has been observed that, during the ascending phase, changes in the local HCS shape are mainly due to spatial variations. During minimum, the results show an increasing trend of the variation of the HCS local inclination with distance between spacecraft up to 5000 Earth radii (R E). For larger distances the results show a downward tendency. This inversion could be related to a continuous interaction of the HCS with the solar wind and with a poor magnetic connection, which could lead to changes in the local HCS shape making it unrecognizable to analyze the evolution of the structure from one observation point to another.  相似文献   

13.
Abstract— The C contents and δ13C values of eleven individual micrometeorites have been determined using a combination of stepped combustion and static mass spectrometry. A new low-blank procedure, involving pretreatment of the samples with a solvent to remove surficial contaminants, has enabled samples of 6–84 μg to be analysed successfully. The eleven samples (seven separated from Greenland cryoconite and four from Antarctic ice) were each split prior to C determination and a fragment taken for study using analytical electron microscopy. In this way, the chemical compositions were obtained thereby allowing comparison with other investigations. As with previous studies of micrometeorites collected at the Earth's surface, the major difficulty with interpreting the results involves distinguishing indigenous components from terrestrial contaminants. Overall C contents were typically <0.2 wt%, although one of the Greenland samples contained 1.5 wt% C, considered to arise mainly from algal contamination. For the other samples, around 0.05–0.15 wt% of the total C in each micrometeorite was considered to be organic in nature with at least some of this (if not all) being terrestrial in origin; the remainder was probably indigenous, being analogous to the macromolecular organic material found in primitive carbonaceous chondrites. The generally low content of this indigenous organic material, compared to conventional meteorites, is presumably a reflection of C loss from the micrometeorites either during atmospheric heating, or subsequent weathering. For that C combusting between 500 and 600 °C, ten of the samples appeared to show a simple two-component system (i.e., a mixture of blank and an isotopically light component; δ13C > ?32%). It is possible that the light component is Cδ, a fine-grained form of presolar diamond which is known to be prevalent in primitive chondritic meteorites. If so, then it is present in the micrometeorites at concentrations of ～30–600 ppm (typically 200 ppm), which is a similar level to that in meteorites. An analysis of algae separated from Greenland cryoconite shows tentative evidence for the presence of extraterrestrial silicon carbide; however, further work will be needed to substantiate this  相似文献   

14.
Under the two initial 1‐D one parameter velocity distribution forms (one is normal, the other is exponential), the z direction scale height evolution of normal neutron stars in the Galaxy is studied by numerical simulation. We do statistics for the cases at different time segments, also do statistics for the cumulative cases made of each time segment. The results show in the cumulative cases the evolution curves of the scale heights are smoother than in the each time segment, i.e., the cumulation improve the signal‐to‐noise ratio. Certainly the evolution cases are different at different Galactic disk locations, which also have very large difference from the average cases in the whole disk. In the initial stages of z evolution of normal neutron stars, after the beginning transient states, the cumulative scale heights increase linearly with time, and the cumulative scale height increasing rates have linear relationship with the initial velocity distribution parameters, which have larger fluctuation in the vicinity of the Sun than in the whole disk. We utilize the linear relationship of the cumulative scale height increasing rates vs. the initial velocity distribution parameters in the vicinity of the Sun to make comparison with the observation near the Sun. The results show if there is no magnetic decay, then the deserved initial velocity parameters are obvious lower than the present well known results from some authors; whereas if introducing magnetic decay, for the 1‐D normal case we can make consistence among concerning results using magnetic decay time values which are supported by some authors, while for the 1‐D exponential case the results show the lackness of young pulsar samples in the larger z in the vicinity of the Sun (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
We investigate some of Tifft's recent statistical analyses of periodicities in extragalactic redshift samples. The values of the periodicities are refinements of those predicted by Lehto. The redshifts have been corrected for the apparent motion of the solar system relative to the cosmic background radiation and have been filtered by applying criteria such as 21 cm profile width and redshift. In all cases except one, our Monte-Carlo simulations show general agreement with Tifft's results. However, we find that one of his analyses is weakened by applying an inappropriate Bernoulli-trials statistic. We apply a new, more straightforward statistic that shows high statistical significance for some of the periodicities. We conclude that although some of Tifft's procedures seem to be open to some criticism, the periodicities are present at a level that is statistically significant.  相似文献   

16.
We present a detailed analysis of the magnetic topology of AR 2776 together with Hα UV, X-rays, and radio observations of the November 5, 1980 flares in order to understand the role of the active region large-scale topology on the flare process. As at present the coronal magnetic field is modeled by an ensemble of sub-photospheric sources whose positions and intensities are deduced from a least-square fit between the computed and observed longitudinal magnetic fields. Charges and dipole representations are shown to lead to similar modeling of the magnetic topology provided that the number of sources is great enough. However, for AR 2776, departure from a potential field has to be taken into account, therefore a linear force-free field extrapolation is used. The locations of the four bright off-band Hα kernels in quadrupolar active regions have been studied previously. In this new study the active region is bipolar and shows a two-ribbon structure. We show that these two ribbons are a consequence of the bipolar photospheric field (the four kernels of quadrupolar regions merge into two bipolar regions). The two ribbons are found to be located at the intersection of the separatrices with the chromosphere when the shear, deduced from the fibril direction, is taken into account. This study supports the hypothesis that magnetic energy is stored in field-aligned currents and released by magnetic reconnection at the location of the separator, before being transported along field lines to the chromospheric level. It is also possible that part of the magnetic energy could be stored and released on the separatrices. Our study shows that meeting just one of two conditions- the presence of intense coronal currents or of a separator in a magnetic field configuration - is not sufficient for flaring. In order to release the stored energy, the coronal currents need either to be formed along the separatrices or to be transported towards the separator or separatrices. The location of the observed photospheric current concentrations on the computed separatrices supports this view. Member of the Carrera del Investigador Científico, CONICET.  相似文献   

17.
Differences in texture and discovery location prompted us to analyze 16 irons from Morasko; one from Seeläsgen, known to have a similar composition; and a new mass found at Jankowo Dolne. These were analyzed in duplicate by instrumental neutron‐activation analysis (INAA). The results show that all 18 samples have very similar compositions, distinct from all other IAB irons except Burgavli; we conclude that they are all from a single shower. Eight of the samples were from regions with large amounts of cohenite (but were largely free of inclusions) and six were from samples with very little cohenite; we could find no resolvable difference in composition between these sets, a fact that suggests that the C contents of the metal phases were similar in the two areas. Although Morasko has been classified into the IAB main group (IAB‐MG), its Ir plots well outside the main group field on an Ir‐Au diagram. We considered the possibility that the low Ir reflected contamination by a melt from a IAB region that ponded and experienced fractional crystallization; however, because Morasko has Pt, W, and Ga values that are the same as the highest values in IAB‐MG, we rejected this model. We therefore conclude that Morasko formed from a different melt than the IAB‐MG irons; the Morasko melt was produced by impact heating, but one or more of the main Ir carriers did not melt, leaving much of the Ir in the unmelted residue. Copper is the only element that shows resolvable differences among Morasko samples. Most (13 of 18) samples have 149 ± 4 μg g?1 Cu, but three have 213 ± 10 μg g?1; we interpret this to mean that the low‐Cu samples have equilibrated with a Cu‐rich phase, whereas there was none of the latter phase within a few diffusion lengths of the samples with high Cu contents.  相似文献   

18.
Sets of ridges of uncertain origin are seen in twenty-nine high-resolution Galileo images, which sample seven locales on Io. These ridges are on the order of a few kilometers in length with a spacing of about a kilometer. Within each locale, the ridges have a consistent orientation, but the orientations vary from place to place. We investigate whether these ridges could be a result of tidal flexing of Io by comparing their orientations with the peak tidal stress orientations at the same locations. We find that ridges grouped near the equator are aligned either north-south or east-west, as are the predicted principal stress orientations there. It is not clear why particular groups run north-south and others east-west. The one set of ridges observed far from the equator (52° S) has an oblique azimuth, as do the tidal stresses at those latitudes. Therefore, all observed ridges have similar orientations to the tidal stress in their region. This correlation is consistent with the hypothesis that tidal flexing of Io plays an important role in ridge formation.  相似文献   

19.
Radio absorption records obtained in or near the zone of totality at two solar eclipses (May 30, 1965 and July 20, 1963) have been examined in detail. It is concluded that all major radio absorption changes during an eclipse are ionization controlled and occur in the D-E layer. Corrections for the ultraviolet sensitivity of the region below 150 kilometers have been applied, so the results indicate effects attributable to the X-ray flux alone. The residual curves clearly show a threshold effect similar to that described by Rastogi et al. (1956) and later by Schmidt and Sharp (1965). Arguments are presented for interpreting this effect in terms of limb configurations involving small, hot X-ray sources similar to that described previously (Meisel, 1968). Once again it is necessary to postulate that grazing incidence reflections from the lunar limb occur. Source positions have been derived from intersections of lunar arcs as seen from different geographic locations.So far, a total of five sources have been identified with reasonable certainty for three eclipses. All the source positions fall within the zones of maximum coronal X-ray intensity predicted by Elwert (1961). Except for one source located very close to the solar limb, all have positions which are well correlated with optical plages. Wien temperatures have been derived from the lunar reflection trends. These show a pronounced correlation with the relative strengths of the 5303 (Fe xiv) and 6374 (Fe x) corona near the source locations. Only the strongest and highest temperature sources correlate well with coronal white light structure.  相似文献   

20.
Twenty-seven samples of matrix and clast materials from Boulder 1 at Station 2, Apollo 17 have been analyzed for major and trace elements as part of the study of this boulder by Consortium Indomitabile. Both unusual and common types of material have been characterized. Gray and black competent breccia (GCBx and BCBx) and anorthositic breccia (AnBx) have compositions which are common at the Apollo 17 site and were common at the site of boulder formation. Light friable breccias (LFBx) have compositions which are not found at the Apollo 17 site other than in the boulder. Pigeonite basalt is a new type of lunar rock and has characteristics that would be expected of a highland volcanic rock. It is associated with LFBx material, and like LFBx material it is exotic to the Apollo 17 site. Coarse norite is an old primitive rock which is no longer (if ever) found as millimeter fragments at the Apollo 17 site. It was, however, present as millimeter fragments associated with GCBx and BCBx materials at the site and time of boulder formation. Therefore the boulder-forming process combined materials from at least two different localities or vertical strata; at least one of these (LFBx) has not been previously sampled and analyzed.  相似文献   

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