Fan-shaped coma,orientation of rotation axis,and surface structure of a cometary nucleus I. Test of a model on four comets     

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   

Relative motions of fragments of the split comets: III. A test of splitting and comets with suspected multiple nuclei     

Zdenek Sekanina 《Icarus》1979,38(2):300-316

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Palaeocurrents in Mesopotamian geosyncline     

Dr. Zdenek Kukal  Adnan Saadallah 《International Journal of Earth Sciences》1970,59(2):666-686

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.

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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.

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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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Statistical model of meteor streams. IV. A study of radio streams from the synoptic year     

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   

La sénégalite,Al₂ (PO₄) (OH)₃ · H₂O,un nouveau minéral     

Zdenek Johan 《Lithos》1976,9(2):165-171

Senegalite is orthorhombic, mm2, a:b:c:=1.296:1:1.007; $\text{a}{}_0\text{=9.673,}\text{b}{}_0\text{=7.596,}\text{c}{}_0\text{=7.668}\text{A}\text{?}$, Z=4, G_calc=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, n_S: 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   

Traces of structural H2O molecules in baryte     

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   

Use of a soil moisture network for drought monitoring in the Czech Republic   总被引：1，自引：0，他引：1  

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   

Simultaneous Determination of 11 Sulfonamides by HPLC–UV and Application for Fast Screening of Their Aerobic Elimination and Biodegradation in a Simple Test     

Waleed M. M. Mahmoud  Nareman D. H. Khaleel  Ghada M. Hadad  Randa A. Abdel‐Salam  Annette Haiß  Klaus Kümmerer 《洁净——土壤、空气、水》2013,41(9):907-916

Sulfonamides (SAs) are one of the most frequently used antibiotics. SAs have been found in various environmental compartments. If SAs are not degraded in the environment, they can affect bacteria by their antibiotic properties and contribute to bacterial antibiotic resistance. Therefore, the biodegradability of 11 SAs (sulfanilamide, sulfaguanidine monohydrate, sulfadiazine, sulfathiazole, sulfapyridine, sulfamerazine, sulfamethoxypyridazine, sulfachloropyridazine, sulfamethazine, sulfamethoxazole, and sulfadimethoxine) was studied. For this purpose, the Closed Bottle Test (CBT, OECD 301D) was performed, which includes a toxicity control. In order to monitor the environmental fate of the parent compound and to check for transformation products, a simple, efficient, and reliable HPLC–UV method for the simultaneous determination of these SAs has been developed. Acetonitrile and water (with 0.1% formic acid) were used as mobile phase solvents for gradient elution. The method was validated in terms of precision, detection and quantitation limits, selectivity, and analytical solution stability. In the CBT, none of these SAs was readily biodegradable. The HPLC–UV analysis confirmed that no degradation of any SA took place. In the toxicity control, these SAs showed no toxic effect in the used concentration of environmental bacteria applied in the test.  相似文献   

Burial history determination of the Triassic succession of Central and Northern Tunisia using clay minerals     

Dorra Mahdi  Randa Ben Abdallah  Nouri Hatira  Ali Tlili  Hédi Ridha Chaftar  Fakher Jamoussi 《Arabian Journal of Geosciences》2013,6(11):4347-4355

The Oued Belif 48 and Koudiat El Halfa 5 borehole samples have been analysed in order to reveal the mineralogical composition of the Triassic successions and their burial history within the geological evolution of the Tethysian southern margin. Oued Belif 48 borehole belongs to Nefza district which is a part of the “Nappe zone” (Tellian unit, north-western Tunisia). Koudiat El Halfa 5 borehole crosses the Koudiat El Halfa diapir (north–west of the north–south axis, Central Atlas). In this paper, the burial degree of evaporitic Triassic samples was determined by the “illite crystallinity” index and by the evolution of the other phyllosilicates, essentially chlorite, talc and illite/chlorite and illite/smectite mixed layers. The studied samples of the two boreholes are characterized by the presence of abundant clay minerals. The <2-μm grain-size fraction of the samples is mostly composed of illite, chlorite and smectite and may contain a slight percentage of swelling layers (illite/smectite and illite/chlorite). The illite crystallinity value measured on ethylene glycol solvated oriented mounts of the Oued Belif 48 samples oscillates globally between 1 and 2.5 characterizing the epizonal zone with a range of 300–400 °C temperatures. The measures of Koudiat El Halfa 5 samples crystallinity index show a value ranging from 2 to 4, which indicates the anchizone and early epizone burial stage (temperatures around 200 °C). These data can be explained by Miocene magmatic activities characterizing the Triassic material of Nefza district and also by burial phenomena effects.  相似文献   

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