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Zdenek Sekanina 《Icarus》1979,37(2):420-442
Conspicuous anisotropy in the outgassing from comets, especially from short-period ones, appears to be the factor responsible for a frequent occurrence of a fan-shaped coma, extending in the general direction of the Sun. It is proposed that the pattern of deviations from the sunward direction contains information on the orientation of the spin axis and on the time lag in the sublimation process, which in turn provides insight into the nature of the nuclear surface. A simple model of a spherical rotating nucleus is formulated and a trial-and-error technique devised to determine the axis-orientation constants and a lage angle, a meaasure of the time lag in units of the rotation period. The results of application of this method to periodic comets Encke, Tempel 2, Borrelly, and Schwassmann-Wachmann 3 are presented. It is shown that the sense of rotation determined in this fashion is consistent with the results established for three of the four comets from the transverse component of the nongravitational force affecting their orbital motions. It is found that in general the time lag is strongly time dependent and that lag angles approaching 90° are rather common near perihelion, suggesting a complex surface structure that involves an insulting crust of dust of variable thickness and strength. These results are compared with the observed lightcurves of the four comets and with the calculated distributions of integrated insolation at the nuclear surface as functions of the cometocentric latitude and time. Noticed is a tendency of the comets to turn their spin axes to the Sun near perihelion and to replace, on the outbound leg of orbit, the established fan-orientation pattern by a “late”-tail pattern indicative of old, slowly accelerated particles. It is suggested that the motion of P/Schwassmann-Wachmann 3, which is due for a favorable return in 1979, was affected by a secular deceleration in 1930. 相似文献
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Terrigenous sandstone formations of Mesopotamian geosyncline were investigated in order to reconstruct the palaeocurrents in the basin and to compare ancient and recent drainage systems. Direct and indirect methods were used; direct ones, as investigation of the dipping of cross bedding revealing directly the axis or direction of the currents, and indirect ones, as heavy mineral analyses, revealing the possible source areas.All the formations distributed in the southwestern flank of the geosyncline exhibit the palaeocurrents flowing from the southwest towards the northeast, i. e. towards the axis of the geosyncline. By contrast, all the formations distributed in the opposite flank of the geosyncline, exhibit also opposite current directions, i. e. from the northeast towards the southwest. This ancient current system resembles the recent situation; transversal basin filling from both sides towards the geosynclinal axis. In recent times oblong basin filling, executed by Twin rivers, Euphrates and Tigris, also takes place. They transport the material along the geosynclinal axis towards the remaining marine parts of the Mesopotamian geosyncline, represented by Persian Gulf.
Zusammenfassung Terrigene sandige Ablagerungen der mesopotamischen Geosynklinale wurden untersucht, um die Paläoströmungen in den Sedimentationsbecken zu rekonstruieren. Es wurden direkte (Untersuchung der Strömungsschichtung) und indirekte Methoden (Schwermineral- und Geröllanalysen) angewendet.Alle Formationen, welche am südwestlichen Rand der Geosynklinale liegen, zeigen Paläoströmungsrichtungen von Südwesten nach Nordosten. Dagegen zeigen die nordöstlichen Formationen umgekehrte Richtungen von Nordosten nach Südwesten, senkrecht zur Geosynklinalachse. Beide Paläoströmungen sind sehr ähnlich dem heutigen Bild. So können wir auch heute eine Querfüllung des Geosynklinalbeckens beobachten. Außerdem tritt heute auch eine Längsfüllung der Geosynklinale durch Euphrat und Tigris auf. Diese Flüsse transportieren das Material entlang der Geosynklinalachse zur rezenten marinen Restgeosynklinale, die heute den Persischen Golf bildet.
Résumé Les formations détritiques du géosynclinal Mésopotamique ont été étudiées en vue d'une réconstruction des paléocourants et d'une comparaison de la situation présente et ancienne. Des méthodes directes (études de la stratification croisée) et indirectes (études de minéraux lourds) ont été employées.Toutes les formations situées au bord sud-ouest du géosynclinal montrent des paléocourants dirigés du sud-ouest au nord-est. Au contraire, les formations du bord nord-est présentent la direction inverse. Les paléocourants ont toujours été dirigés perpendiculairement à l'axe du géosynclinal. Le système des paléocourants anciens ressemble au système contemporain. Le remplissage du bassin a toujours été transversal. Mais aujourd'hui le remplissage longitudinal existe aussi. Ce remplissage est effectué par les fleuves Euphrate et Tigre, qui transportent des matériaux détritiques dans le reste marin du géosynclinal Mésopotamique.
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Zdenek Sekanina 《Icarus》1976,27(2):265-321
With the use of the most powerful stream-search technique in existence, we have detected 275 streams in the synoptic-year (December 2, 1968–December 14, 1969) sample of 19 698 radio meteors, most of which were not reported before. About 16% of all meteors in the sample belong to these streams. We have confirmed the existence of two-thirds of the 83 streams detected by us previously in the 1961–65 sample of radio meteors. Some of the new streams have most uncommon orbits. A new, rich stream with a revolution period of about 30yr has been discovered. Streams of low inclination are often detected at both nodes. A computer technique, developed for determining the two parameters of the D-distribution of meteor orbits in a stream, has been applied to the 275 streams. A number of known comet-meteor associations have been confirmed, and a few new possible associations established. The previously detected orbital similarity between the minor planet Adonis and a few radio streams (including a major, broad stream) has been reinforced, but we have so far failed to prove the evolutionary relationship more quantitatively. Several possible associations with other Earth-crossing asteroids, with the meteorite P?íbram, and with a few fireballs are suggested. The mean space density in streams is shown to be much below the sporadic density, but the central density may significantly exceed the sporadic density. The derived absolute stream-density values are in an order-of-magnitude agreement with space densities estimated from the cometary production rates of solid material of a comparable particle size. Plots on a height-velocity diagram of both the individual radio meteors and the radio streams fail to exhibit the discrete-level structure known to exist for photographic meteors. 相似文献
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Zdenek Johan 《Lithos》1976,9(2):165-171
Senegalite is orthorhombic, mm2, a:b:c:=1.296:1:1.007; , Z=4, Gcalc=2551; space group Pna2. The strongest lines in the powder pattern are: 5.41(7); 4.089(9); 3.834(10); 3.610(8); 2.990(9); 2.348(8); 2.070(7) 1.929(7); 1.505(7) Å. The chemical analysis: Al2O3 ? 46.23; Fe2O3 ? 0.28; P2O5 ? 31.85 H2O ? 21.00; sum 99.34, gives a formula Al2(PO4)(OH)3 · H2O. Colourless optically biaxial positive, nS: α=1.562, β=1.566, γ=1.587, plane of optical axies (001), Z=a, Y=c; 2V=53°, weak dispersion r > v. Measured density 2.552. The DTA curve shows endothermic reactions at 250, 370 and 440°C corresponding to the dehydration of mineral. Infrared spectrum indicates the presence of OH and H2O groups. Found in oxidation zone of Kouroudiako iron deposit, Senegal, associated with turquoise, augelite, wavellite and crandallite. 相似文献
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Anton Beran Dominik Talla Zdenek Losos Jiri Pinkas 《Physics and Chemistry of Minerals》2010,37(3):159-166
The infrared (IR) spectra of gem-quality baryte crystals from different occurrences are characterized by relatively weak but
strongly pleochroic absorption bands at 3,280, 3,220, 3,155, and 3,115 cm−1. These bands are assigned to anti-symmetric and symmetric OH stretching vibrations of two types of H2O molecules localized on vacant Ba sites. The H–H axis of the H2O I molecule is slightly tilted from the a-axis direction, its twofold axis being nearly parallel to the b-axis, thus defining the plane of the H2O molecule practically parallel to (001). The H2O II molecule has its H–H axis parallel to the b-axis direction, with its plane lying approximately parallel to (101). The values of the total water contents of the baryte
crystals, calculated on the basis of IR spectroscopic data, are ranging from about 1.7–3.8 wt.ppm. The possible presence of
H3O+ ions is also discussed. 相似文献
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Martin Mozny Mirek Trnka Zdenek Zalud Petr Hlavinka Jiri Nekovar Vera Potop Michal Virag 《Theoretical and Applied Climatology》2012,107(1-2):99-111
Since 2000, the network of stations that make up the Czech Hydrometeorological Institute (CHMI) has measured the soil moisture content at the 0- to 0.9-m layer using sensors placed within the natural soil profile under closely cropped grass cover. Using information from 8?years of continuous observation at seven stations throughout the Czech Republic, we verified the usefulness of the Soil Moisture Index (SMI). The SMI is a potentially useful index for calculating the water deficit in the Czech Republic and Central Europe. During this period, a statistically significant decrease in moisture content was detected, and negative SMI values predominated. There were frequent occurrences of flash drought, defined as a very rapid decline in soil moisture during a 3-week period. The CHMI can use SMI values below ?3 in the Integrated Warning Service System. The routine calculation of SMI values can alert agricultural producers to the development of flash drought conditions and provide them with information regarding the effectiveness of recent rainfall events. An increase in soil moisture, in contrast, could serve as a warning sign for hydrology because it creates the preconditions for flooding. The complex study of soil humidity regimes is becoming more significant in connection with current global climatic change warnings in hydrological cycles. 相似文献
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Petrogenesis of Variscan high-temperature Group A eclogites from the Moldanubian Zone of the Bohemian Massif,Czechoslovakia 总被引：1，自引：0，他引：1
Brian L. Beard L. Gordon Medaris Jr. Clark M. Johnson Hannes K. Brueckner Zdenek Mísař 《Contributions to Mineralogy and Petrology》1992,111(4):468-483
High-temperature (HT), Group A eclogites from three localities in the Moldanubian Zone of the Bohemian Massif are interpreted to have formed in the mantle and to have been transported into the crust by their enclosing garnet peridotites during Variscan orogenesis. Garnet and omphacite are compositionally zoned and contain homogeneous cores and retrograde rims. Cores of minerals yield minimum temperatures and pressures of 850 to 985°C and 16.0 to 22.5 kb, based on Fe–Mg exchange between garnet and clinopyroxene and the jadeite content of clinopyroxene. Sugh high temperatures indicate equilibration in, and derivation from, the upper mantle. Trace element compositions, including the REEs, high MgO contents, and high Mg numbers suggest that the rocks formed by high pressure accumulation of garnet and clinopyroxene and variable amounts of trapped melt. Sm-Nd ages determined on four garnet-clinopyroxene pairs from the three localities are 377±20, 342±9, 336±16, and 323±7 Ma.
Nd and initial 87Sr/86Sr are negatively correlated, varying from +6.7 to -0.1 and 0.7027 to 0.7057, respectively. Field, compositional, and isotopic data indicate that the eclogites were derived from heterogeneous mantle that included depleted and enriched compositions; this heterogeneity may have resulted from subduction processes that occurred prior to the late Variscan collision of Gondwana and Baltica. 相似文献
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Stéphane Reboulet Peter F. Rawson Josep A. Moreno-Bedmar Maria B. Aguirre-Urreta Ricardo Barragán Yuri Bogomolov Miguel Company Celestina González-Arreola Vyara Idakieva Stoyanova Alexander Lukeneder Bertrand Matrion Vasily Mitta Hasina Randrianaly Zdenek Vaši?ek Evgenij J. Baraboshkin Didier Bert Stéphane Bersac Tamara N. Bogdanova Luc G. Bulot Jean-Louis Latil Irina A. Mikhailova Pierre Ropolo Ottilia Szives 《Cretaceous Research》2011,32(6):786-793
The 4th Kilian Group meeting (Dijon, France, 30th August 2010) focused on the Aptian and Albian Stages. For the Aptian, a two-fold division of the stage was adopted for the Mediterranean area with a boundary between the Dufrenoyia furcata and Epicheloniceras martini Zones. The main changes to the zonal scheme concern the Lower Aptian with: the introduction of a Deshayesites luppovi Subzone in the upper part of the Deshayesites oglanlensis Zone; the replacement of Deshayesites weissi by Deshayesites forbesi as new index-species of the second interval zone; the introduction of a Roloboceras hambrovi Subzone in the upper part of the D. forbesi Zone; and the subdivision of the D. furcata Zone into the D. furcata and Dufrenoyia dufrenoyi Subzones. For the Albian, the upper part of the Douvilleiceras mammillatum Zone (Lower Albian) is now characterized by a Lyelliceras pseudolyelli Subzone. The main amendments concern the Upper Albian. The base of this substage is defined by the base of the Dipoloceras cristatum Zone. Above it, the Upper Albian zonal scheme comprises in stratigraphic order the Mortoniceras pricei, Mortoniceras inflatum, Mortoniceras fallax, Mortoniceras rostratum, Mortoniceras perinflatum and Arrhaphoceras briacensis Zones. 相似文献
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