The Contribution of Asteroid Dust to the Interplanetary Dust Cloud: The Impact of ULYSSES Results on the Understanding of Dust Production in the Asteroid Belt and of the Formation of the IRAS Dust Bands

Investigations of the zodiacal dust cloud give evidence for a significant contribution of asteroidal dust to the interplanetary dust cloud, a result which can now be compared to measurements of the ULYSSES dust detector during its passage of the asteroid belt. Especially we discuss the ULYSSES data with respect to the IRAS dust bands and consider geometric selection effects for the detector. From calculations of radiation pressure forces, we conclude that particles in the IRAS dust bands with massesm≥ 10⁻¹²g will stay in bound orbits after their release from asteroid fragmentation. This is already in the mass range (10⁻¹⁶–10⁻⁷g) of particles detectable with the dust detector onboard ULYSSES. The absence of these particles in the ULYSSES data cannot be explained exclusively in terms of their small detection probability. Thus we conclude that the size distribution of particles in the IRAS dust bands most probably cannot be continued to the submicrometer range. Concerning the global structure of the inner zodiacal cloud (i.e., about solar distancer< 3.5 AU) the ULYSSES data are not inconsistent with present models. Recent estimates of the total mass of the interplanetary cloud require a dust production rate of about 10¹⁴g/year of which a significant amount is assumed to result from the asteroids. Our estimate for the production of dust particles in an IRAS dust band, based on the assumption that the dust band results from a single destruction of an asteroid of 100 km size, yields a production rate of 10¹⁰g/year. Other models of the IRAS dust bands suggest production rates up to 10¹²g/year and also cannot provide a significant source of the dust cloud.     

Eo-alpine metamorphism in the uppermost unit of the Cretan nappe system — Petrology and geochronology

The uppermost unit of the Cretan nappe system consists of ophiolites on the top, and an ophiolitic mélange at the base.Among the various constituents of the mélange, there are slices of low-P/high-T metamorphics. They form a variegated series consisting of tholeiitic ortho-amphibolites, para-amphibolites, andalusite and sillimanite-cordierite-garnet bearing mica schists, calcsilicate rocks, and marbles. The metamorphic sequence is locally intruded by early tectonic magmatites of gabbroic, dioritic and granitic composition. Critical mineral assemblages lead to a maximum temperature of about 700° C reached during metamorphism, at a total pressure of 4–5 kilobars. K — Ar dating on 6 hornblendes, 7 biotites and 1 muscovite yielded cooling ages of 75–66 m.y. and confirmed earlier results according to which the metamorphism and related magmatism took place in Late Cretaceous times.In order to evaluate the age relationships between the hightemperature metamorphics within the ophiolitic mélange and the ophiolites, hornblendes from ultramafic and mafic rocks of the ophiolite complex were dated by the K — Ar method. Hornblende from one schistose hornblendite forming a constituent of the ophiolites proper yielded 156 m.y. and thus provides a middle Jurassic minimum age for the formation of this piece of oceanic lithosphere. Four hornblendes of calc-alkaline gabbrodiorite dikes within the ophiolite complex gave distinctly lower K — Ar dates of about 140 m.y.. The dikes probably intruded after the detachment of the ophiolites in an island-arc or continental-margin environment.As a consequence, the high-temperature metamorphics and related intrusives in the ophiolitic mélange of Crete are genetically unrelated to the overlying ophiolites. The paleogeographic position of the crystalline terrane, slices of which are now incorporated into the ophiolitic mélange is still open to discussion.     

Polarization of hard X-rays from solar flares

The polarization of hard solar X-radiation (> 10 keV) is calculated on the assumption that electrons get a non-isotropic velocity distribution in the initial phase of a flare. The brems-strahlung generated by nonthermal electrons spiralling around magnetic field lines with discrete pitch angles is considerably polarized if observed at approximately right angles to the magnetic field. In the energy range from 10 to 50 keV the degree of polarization is not strongly dependent on the photon energy. For pitch-angle distributions of the form sin² and cos², the polarization has opposite signs; it decreases appreciably at high photon energies. The observation of X-ray polarization will be useful in deducing the physical conditions in flares.     

Die Strukturgeschichte der Zentralsahara

The author has investigated paleogeographic and structural problems in the middle part of the Sahara desert since 1959. Detailed studies of thickness changes, of disconformities and unconformities and of structural events resulted in the definition of the different tectonical eras and their individual paleogeographic elements. The middle part of North Africa is characterized by three major periods of structural development:

1. Folding and consolidation in Precambrian time.
2. Formation of NW to NNW striking horsts during Cambrian time, which became the core of uplifts (separated by troughs) in Silurian and Devonian time. This structural relief of the early Paleozoic era possibly is the result of regional stretch in NE-SW direction.
3. Formation of uplifts and troughs striking NE during late Paleozoic and Mesozoic time. Blockfaulting occurred along the edge of some uplifts during Jurassic time or at the Jurassic-Cretaceous transition. These movements were the result of regional compression from SE toward NW. The formation of this large scale undulation of the earth's crust coincides with the separation of the Sahara platform from the so-called Tethys (geosynclinal area in NW Africa, the Mediterranean and parts of Asia). This separation most probably began in Northwest Africa during late Carboniferous or early Permian time, it reached Northeastern Libya in Jurassic time. The Sirte grabens were formed as the result of east-west shearing movements, during Upper Cretaceous. Finally, in late Tertiary to Pleistocene time, volcanic activity formed large basalt plateaus. Volcanism occurs mainly along well defined old structural elements.

The results of this analysis were used to interpret structural aspects of larger parts of Africa. The structural relief of the early Paleozoic Era seems to extend far southeast into areas of the old African shield indicating that there is no principal structural difference between the shield and the Sahara platform. The orientation of the late Paleozoic to Mesozoic large scale undulation indicates that the reason for the SE-NW compression is the rotation tendency of Africa which began in late Carboniferous time and culminated during the Tertiary, when Africa was separated from Asia along the Red Sea graben. At approximately the same time, the Atlas area of Northwest Africa was folded.     

Direct measurements of volume transports through Fram Strait

Heat and freshwater transports through Fram Strait are understood to have a significant influence on the hydrographic conditions in the Arctic Ocean and on water mass modifications in the Nordic seas. To determine these transports and their variability reliable estimates of the volume transport through the strait are required. Current meter moorings were deployed in Fram Strait from September 1997 to September 1999 in the framework of the EU MAST III Variability of Exchanges in the Northern Seas programme. The monthly mean velocity fields reveal marked velocity variations over seasonal and annual time scales, and the spatial structure of the northward flowing West Spitsbergen Current and the southward East Greenland Current with a maximum in spring and a minimum in summer. The volume transport obtained by averaging the monthly means over two years amounts to 9.5 ± 1.4 Sv to the north and 11.1 ± 1.7 Sv to the south (1 Sv = 10⁶ m³s⁻¹). The West Spitsbergen Current has a strong barotropic and a weaker baroclinic component; in the East Greenland Current barotropic and baroclinic components are of similar magnitude. The net transport through the strait is 4.2 ± 2.3 Sv to the south. The obtained northward and southward transports are significantly larger than earlier estimates in the literature; however, within its range of uncertainty the balance obtained from a two year average is consistent with earlier estimates.     

SUJETS A TRAITER POUR L'ASSEMBLÉE DE 1960

ABSTRACT

The present paper reports on some practical experiences gained in computing unit hydrographs (UH) according to the “classical” linear theory. The sizes of the drainage basins varied from 50 to 800 km² of hilly country.     

<Emphasis Type="Italic">Scombroclupea occidentalis</Emphasis> sp. nov. (Clupeiformes,Teleostei) from the Cenomanian (Cretaceous) Plattenkalk deposits of NE Mexico

A new species of the clupeid genus Scombroclupea is described from the Upper Cretaceous (Cenomanian) Plattenkalk locality “Loma la Mula” in Coahuila, northeastern Mexico. The material provides new osteological data for the genus, which clarify its systematic position. The Mexican Scombroclupea points to an occurrence of the genus in the western Tethys and indicates a vicariant event between S. macrophthalma from Lebanon and Croatia and the new species described herein. The analysis of the functional morphology of Scombroclupea supports the reconstruction of an oscillatory locomotion mode in this genus.     

Shock waves—Phenomenology,experimental, and numerical simulation

Abstract— The purpose of this paper is to review the results of long‐term cooperation between Dieter Stöffler and the authors in the field of shock wave deformation of minerals and rocks. First, the principal phenomena of shock wave generation and propagation, predominantly in solid media, are presented, and then analytical and numerical mathematical treatment of shock wave processes on the basis of mass, momentum, and energy conservation laws will be described and discussed. Experimental methods of shock wave investigations by means of impact and explosive techniques are summarized, including hypervelocity acceleration facilities and high‐pressure explosive devices. Shock pressure barometry by means of mineralogical evidence of distinct material phase transitions and characteristic shock structures is also discussed.