Results of TV imaging of Phobos (Experiment VSK-Fregat)

From February to March 1989 the Phobos 2 spacecraft took 37 TV images of Phobos at a distance of 190-1100 km. These images complement Mariner-9 and Viking data by providing higher-resolution coverage of a large region West of the crater Stickney (40-160 degrees W) and by providing disk-resolved measurements of surface brightness at a greater range of wavelengths and additional phase angles. These images have supported updated mapping and characterization of large craters and grooves, and have provided additional observations of craters' and grooves' bright rims. Variations in surface visible/near-infrared color ratio of almost a factor of 2 have been recognized; these variations appear to be associated with the ejecta of specific large impact craters. Updated determinations of satellite mass and volume allow calculation of a more accurate value of bulk density, 1.90 +/- 0.1 g cm-3. This is significantly lower than the density of meteoritic analogs to Phobos' surface, suggesting a porous interior perhaps containing interstitial ice.     

Shear-induced instability and arch filament eruption: A magnetohydrodynamic (MHD) numerical simulation

We investigate, via a two-dimensional (nonplanar) MHD simulation, a situation wherein a bipolar magnetic field embedded in a stratified solar atmosphere (i.e., arch-filament-like structure) undergoes symmetrical shear motion at the footpoints. It was found that the vertical plasma flow velocities grow exponentially leading to a new type of global MHD-instability that could be characterized as a Dynamic Shearing Instability, with a growth rate of about 8{ovV} _A a, where {ovV} _A is the average Alfvén speed and a ^–1 is the characteristic length scale. The growth rate grows almost linearly until it reaches the same order of magnitude as the Alfvén speed. Then a nonlinear MHD instability occurs beyond this point. This simulation indicates the following physical consequences: the central loops are pinched by opposing Lorentz forces, and the outer closed loops stretch upward with the vertically-rising mass flow. This instability may apply to arch filament eruptions (AFE) and coronal mass ejections (CMEs).To illustrate the nonlinear dynamical shearing instability, a numerical example is given for three different values of the plasma beta that span several orders of magnitude. The numerical results were analyzed using a linearized asymptotic approach in which an analytical approximate solution for velocity growth is presented. Finally, this theoretical model is applied to describe the arch filament eruption as well as CMEs.     

The Paasivaara PGE reef in the Penikat layered intrusion,northern Finland

Summary Three major PGE-bearing mineralized zones have been found in the layered series of the early Proterozoic Penikat layered intrusion. These are designated as the Sompujärvi (SJ), Ala-Penikka (AP) and Paasivaara (PV) Reefs according to the site of their initial discovery.The uppermost of these, the PV Reef, has the highest Pt/Pd ratio. It is located in the transition zone between the fourth and the fifth megacyclic units. The main host rock is the uppermost anorthosite, disseminated sulphides and associated PGM being concentrated in the interstices of this plagioclase orthocumulate. The Reef has also been encountered in other parts of the transition zone, however, and sometimes even in the lowermost parts of the fifth megacyclic unit. The dominant sulphide paragenesis is chalcopyrite-pyrrhotite-pentlandite, whereas the PGM identified are represented by sperrylite (PtAs₂), kotulskite (PdTe), merenskyite (PdTe₂), isomertieite (Pd₁₁Sb₂As₂), stibiopalladinite (Pd₅Sb₂), cooperite (PtS) and braggite ((Pt, Pd, Ni)S).It is suggested that the PV Reef was formed in the mixing process when the fifth magma pulse intruded into the magma chamber. Mixing of the new magma with the older residual magma in the chamber accounted for the sulphide precipitation. Mixing and convection were probably turbulent at first and the sulphides were thus able to "scavenge" PGE from a large amount of silicate melt. The metal ratios in the mineralization point to a close genetic relationship with the fifth magma pulse.

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Das Paasivaara PGE Reef in der Penikat-Intrusion, Nord-Finnland

Zusammenfassung In den geschichteten Serien der frühproterozoischen Intrusion von Penikat kommen drei grössere PGE-führende Zonen vor. Diese werden als die Sompujärvi (SJ), Ala-Penikka (AP) und Paasivaara (PV) Reefs bezeichnet, entsprechend den Lokalitäten der Entdeckung.Das am höchsten gelegene PV Reef hat die höchsten Pt/Pd Verhältnisse. Es liegt in der Übergangszone zwischen der vierten und der fünften megazyklischen Einheit. Das wichtigste Wirtsgestein ist der oberste Anorthosit, wo disseminierte Sulfide und assoziierte PGM in den Zwischenräumen dieses Plagioklas-Orthokumulates vorkommen. Das Reef wurde auch in anderen Teilen der Überganszone beobachtet und manchmal sogar in den untersten Partien der fünften megazyklischen Einheit. Die dominierende Sulfidparagenese ist Kupferkies-Magnetkies-Pentlandit; PGM sind Sperrylith (PtAs₂), Kotulskit (PdTe), Merenskyit (PdTe₂), Isomertieit (Pd₁₁Sb₂As₂), Stibiopalladinit (Pd₅Sb₂), Cooperite (PtS) und Braggit ((Pt, Pd, Ni)S).Es wird angeregt, dass das PV Reef während der Mischungsvorgänge bei der Intrusion des fünften Magma Pulses in die Magmenkammer entstanden ist. Mischung des neuen Magmas mit dem alten Residual-Magma in der Kammer war für die Ausfällung der Sulfide verantwortlich. Mischung und Konvektion dürften anfangs turbulent gewesen sein, und so konnten die Sulfide die PGE aus einem beträchtlichen Anteil der Silikatschmelze entfernen. Die Metallverhältnisse dieser Vererzung lassen eine enge genetische Verbindung mit dem fünften Magmapuls erkennen.

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With 8 Figures     

Les gisements d'émeraude de la Cordillère Orientale de la Colombie: Nouvelles données métallogéniques

Emerald deposits of Colombia are confined to lower Cretaceous shales of the Eastern Cordillera. The tectonic pattern of the deposits is related to deep reverse and large regional fault systems. Hydrofracturing is the main factor controlling emerald mineralization. It permitted to the hydrothermal solutions to permeate through fractures but also along stratification planes forming in this case stratabound mineralizations. Emerald occurs in calcite veins, veinlets, pockets and brecciated zones associated mainly with pyrite, quartz, parisite, codazzite and fluorite. Emerald mineralization belongs to an epigenetic hydrothermal process. The alternance of arenite-shale formations in the Cretaceous probably played an important role in the accumulation of solutions and in the propagation of the hydrothermal channels. The origin of emerald involves chemical elements mobilized by the fluids in the Cr-V-Fe-Al-Si-bearing black shales. The source of beryllium remains a problem and is discussed in the paper.     

The structure of the broad line region in Fairall 9

We find that the profile changes observed for the UV emission lines L,Civ 1549, and MgIII 2798, in the Seyfert 1 galaxy Fairall 9 occur mainly near the line centre for L andCiv and in the wings for MgIII. These results indicate that the broad line region (BLR) has a complex structure, possibly with non-spherical components.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

An optical transform and Monte Carlo study of the diffuse X-ray scattering in mullite,Al2(Al2+2xSi2−2x)O10−x

Diffuse X-ray scattering distributions have been recorded for reciprocal layers normal to c ^* of a synthetic mullite with composition Al₂(Al_2+2xSi_2–2x) O_10–xx, where represents an oxygen vacancy and x is close to 0.4. The effect on the diffuse scattering pattern of different schemes for ordering of the oxygen vacancies and the accompanying cation shifts within a single ab-layer of mullite has been investigated using optical diffraction analogue experiments in conjunction with Monte Carlo simulation.A simple scheme in which the only driving force for the ordering is that the O_c oxygens are precluded from being bonded to four T/T^* cations, results in a 2D diffuse intensity distribution which displays many of the features that are observed in the x-ray scattering patterns. An alternative scheme in which bonding to four T/T^* cations was favoured, resulted in a domain structure consisting of islands of the -alumina structure within a matrix of sillimanite. This gave diffraction patterns bearing no resemblance to the observed x-ray patterns and can, we believe, be discounted completely.     

Intertidal distribution and feeding habits of the mud crab,Eurytium limosum

Average summer densities of the xanthid crab,Eurytium limosum, in an intertidal salt marsh on Sapelo Island, Georgia were in the range of 7.5 to 80.0 individuals m^?2. Crab densities were lowest in wet, lowlying marsh and highest in well-drained creekbank and mussel mound habitats. An analysis of crab stomach contents indicated that feeding occurred mostly around high tide, especially at night. Although the diet included some plant material,Eurytium limosum is primarily predatory and consumed crabs, polychaetes, ostracods, bivalves, and snails. In the laboratory, under simulated low-tide field conditions, both small (11–15 mm carapace width) and large (20–37 mm CW)Eurytium could capture and consume young killifish (Fundulus heteroclitus). Large crabs consumed the entire size range (7–19 mm total length) of larval/juvenile fish offered, but small crabs did not prey upon fish >11.5 mm TL. The potential importance ofE. limosum as a predator on young killifish may not be realized in the field because alternative prey are available and the crabs feed primarily at high tide, when young killifish are dispersed in the water column and are less vulnerable to benthic predators.     

Optical variability of NGC 7469 in 1967–1987

Within the framework of a five-year campaign of coordinated multifrequency observations of the 3 Tauri star RU Lupi, we present results obtained in X-ray, UV, optical, and IR spectral regions. In five cases, simultaneous data from UV to IR were collected. Data analysis shows a large variability at different wavelengths both in the continuum and lines, stronger in the UV and weaker in the IR. Correlations among variations in the UV and optical continua have been found. Moreover flare-like events (FLEs), probably due to strong activity in the surface layers of the star and burst phenomena on large scale, were detected. No forbidden coronal lines were detected in UV and optical regions with IUE and ESO 1.4m CAT telescope, respectively. This means either the used instruments have not enough sensitivity to detect the predicted forbidden lines, or the low X-ray detected intensity — with ASTRON satellite (2–25 keV) — is due to the absorption of the X-rays in the circumstellar envelope. In this paper we will discuss these results within the framework of the current models.Based on observations with IUE satellite (VILSPA) and ESO telescopes.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain     

Transmission and reflection of Rayleigh waves at a thin low velocity vertical layer: A laboratory and theoretical study

A three-dimensional ultrasonic modelling was used to study the Rayleigh waves reflected from a thin low velocity vertical layer and those transmitted through the same zone. The reflection and transmission coefficients of the Rayleigh waves have been calculated by an approximate method based on the Green function technique. Nonrigid contact boundary conditions were used for the low velocity layer modelling. A rather good correspondence between the theoretical and experimental absolute values of the reflection and the transmission coefficients was obtained. Some discrepancies between experimental and theoretical results, in particular with regard to phase shifts, may be explained by a certain inadequacy of the theoretical model to the low velocity layer investigated by the ultrasonic modelling.