A study of lunar impact crater size-distributions   总被引：3，自引：0，他引：3  

Gerhard Neukum  Beate König  Jafar Arkani-Hamed 《Earth, Moon, and Planets》1975,12(2):201-229

Discrepancies in published crater frequency data prompted this study of lunar crater distributions. Effects modifying production size distributions of impact craters such as surface lava flows, blanketing by ejecta, superposition, infilling, and abrasion of craters, mass wasting, and the contribution of secondary and volcanic craters are discussed. The resulting criteria have been applied in the determination of the size distributions of unmodified impact crater populations in selected lunar regions of different ages. The measured cumulative crater frequencies are used to obtain a general calibration size distribution curve by a normalization procedure. It is found that the lunar impact crater size distribution is largely constant in the size range 0.3 km ?D ? 20 km for regions with formation ages between ≈ 3 × 10⁹ yr and ? 4 × 10⁹ yr. A polynomial of 4th degree, valid in the size range 0.8 km ?D ? 20 km, and a polynomial of 7th degree, valid in the size range 0.3 km ?D ? ? 20 km, have been approximated to the logarithm of the cumulative crater frequencyN as a function of the logarithm of crater diameterD. The resulting relationship can be expressed asN ～D ^α(D) where α is a function depending onD. This relationship allows the comparison of crater frequencies in different size ranges. Exponential relationships with constant α, commonly used in the literature, are shown to inadequately approximate the lunar impact crater size distribution. Deviations of measured size distributions from the calibration distribution are strongly suggestive of the existence of processes having modified the primary impact crater population.  相似文献   

Der Eiswinter 1986/87 im deutschen Küstengebiet zwischen Ems und Trave     

Dr. Gerhard Koslowski 《Ocean Dynamics》1987,40(3):115-123

  相似文献   

High-temperature carbon reduction of silica: A novel approach for oxygen isotope analysis of biogenic opal     

Andreas Lücke  Gerhard H. Schleser 《Geochimica et cosmochimica acta》2005,69(6):1423-1433

A new technique has been developed for the determination of the oxygen isotope composition from biogenic silica. The iHTR method (inductive high temperature carbon reduction) is based on the reduction of silica by carbon with temperatures of up to 1830°C (maximum T 2200°C) to produce carbon monoxide for isotope analysis. Basically, samples of silica are mixed with graphite and filled into a sample holder made of a glassy carbon rod. The rod is introduced into a glassy carbon cylinder liner closed at the top which itself is enclosed by a double-walled glass vessel. The glassy carbon rod is inductively heated under vacuum to the temperature needed for quantitative conversion of the particular silica material to CO. The most critical process of dehydration (in the case of opal) and reduction to CO is routinely achieved in the iHTR device. Weakly bound oxygen and oxygen-containing contaminants, like hydroxyl groups, as well as remaining minor organic constituents are volatilized stepwise under high vacuum at temperatures of 850°C and 1050°C without isotopic exchange before the reduction of the silica. After completion of dehydration, the temperature is raised to the value needed for silica reduction. For both biogenic silica and quartz a temperature of 1550°C was found to be adequate. The technical design with a standard preparation routine and various test experiments is presented proving the reliability and capability of the new iHTR method, especially with respect to fresh diatom materials and diatom opal. The amount of sample material necessary at present is ∼1.5 mg of silica and the reproducibility achieved for natural samples is better than ±0.15‰. Replicate analysis of the quartz standard NBS28 resulted in a δ¹⁸O value of 9.62‰ ± 0.11‰ (n = 17).  相似文献   

An astrobiological perspective on Meridiani Planum     

Andrew H. Knoll  Michael Carr  David J. Des Marais  Woodward W. Fischer  Scott M. McLennan  Christian Schröder  Nicholas J. Tosca 《Earth and Planetary Science Letters》2005,240(1):179-189

Sedimentary rocks exposed in the Meridiani Planum region of Mars record aqueous and eolian deposition in ancient dune and interdune playa-like environments that were arid, acidic, and oxidizing. On Earth, microbial populations have repeatedly adapted to low pH and both episodic and chronic water limitation, suggesting that, to a first approximation, the Meridiani plain may have been habitable during at least part of the interval when deposition and early diagenesis took place. On the other hand, the environmental conditions inferred for Meridiani deposition would have posed a challenge for prebiotic chemical reactions thought to have played a role in the origin of life on Earth. Orbital observations suggest that the combination of sulfate minerals and hematite found in Meridiani rocks may be unusual on the martian surface; however, there is reason to believe that acidity, aridity, and oxidizing conditions were broadly distributed on ancient Mars. When these conditions were established and how much environmental heterogeneity existed on early Mars remain to be determined. Because sulfates and iron oxides can preserve detailed geochemical records of environmental history as well as chemical, textural and microfossil signatures of biological activity, Meridiani Planum is an attractive candidate for Mars sample return.  相似文献   

Distribution of clay minerals in surface sediments of the Aegean Sea: a compilation     

Werner Ehrmann  Gerhard Schmiedl  Yvonne Hamann  Tanja Kuhnt 《International Journal of Earth Sciences》2007,96(4):769-780

Extensive published data sets and some new data on the clay mineral composition of surface sediments in the Aegean Sea, northeastern Mediterranean Sea, have been compiled in order to map the distribution patterns of clay mineral assemblages and to decipher source areas and transport paths. We distinguished six provinces, a Northwest Aegean Province, a Marmara-Dardanelles Province, a West Turkey Province, a Southeast Aegean Province, a Kithira Province and a Central Aegean Province. The clay mineral assemblages in the coastal and shelf areas carry the signature of the riverine sediment discharge from southeast Europe and Turkey, respectively. The southern Aegean Sea is probably influenced by the River Nile discharge and transport of clay minerals by surface currents. The clay minerals in the central Aegean Sea form a mixed assemblage comprising components of the other assemblages. A dispersion and dilution of clay minerals by surface currents is obvious.  相似文献   

The influence of different rheological parameters on the surface deformation and stress field of the Aegean–Anatolian region     

Kasper D. Fischer 《International Journal of Earth Sciences》2006,95(2):239-249

The Aegean–Anatolian region is characterised by an inhomogeneous deformation pattern with high strain rates and a high seismicity both at the boundaries and in the plate interior. This pattern is controlled by the geometry and rheology of the structural units involved and their tectonic setting. A numerical analysis with a finite-element model of the region is used to quantify the influence of different rheological parameters. Viscoelastic material behaviour is implemented for the mantle lithosphere, whereas the crust is modelled with an elastic–plastic rheology. The variation of the inelastic material properties (viscosity and plastic strength) quantifies the influence of these material parameters on the deformation, stress, and strain patterns. Comparison of the modelled results with geodetic and geophysical observations reveals that the viscosity of the mantle lithosphere is the key to explaining the inhomogeneous deformation pattern. The best-fit model yields a viscosity of 10²⁰ Pa s beneath Anatolia, whereas adjacent regions have viscosities between 10²¹ and 10²³ Pa s. The model also explains the intra-plate seismicity and the stress field as well as its partitioning into regions with strike-slip and normal faulting. The final model is in good agreement with seismological, geodetic, and geological observations. Local deviations can be tracked down to small-scale structures, which are not included in the model.  相似文献   

Placer formation and placer minerals   总被引：1，自引：0，他引：1  

Gerhard Els  Pat Eriksson   《Ore Geology Reviews》2006,28(4):373

  相似文献   

Cenozoic intra-plate magmatism in the Darfur volcanic province: mantle source, phonolite-trachyte genesis and relation to other volcanic provinces in NE Africa     

Friedrich Lucassen  Dieter Pudlo  Gerhard Franz  Rolf L. Romer  Peter Dulski 《International Journal of Earth Sciences》2013,102(1):183-205

Chemical and Sr, Nd and Pb isotopic compositions of Late Cenozoic to Quaternary small-volume phonolite, trachyte and related mafic rocks from the Darfur volcanic province/NW-Sudan have been investigated. Isotope signatures indicate variable but minor crustal contributions. Some phonolitic and trachytic rocks show the same isotopic composition as their primitive mantle-derived parents, and no crustal contributions are visible in the trace element patterns of these samples. The magmatic evolution of the evolved rocks is dominated by crystal fractionation. The Si-undersaturated strongly alkaline phonolite and the Si-saturated mildly alkaline trachyte can be modelled by fractionation of basanite and basalt, respectively. The suite of basanite–basalt–phonolite–trachyte with characteristic isotope signatures from the Darfur volcanic province fits the compositional features of other Cenozoic intra-plate magmatism scattered in North and Central Africa (e.g., Tibesti, Maghreb, Cameroon line), which evolved on a lithosphere that was reworked or formed during the Neoproterozoic.  相似文献   

Mechanical design of the solar telescope GREGOR     

R. Volkmer  P. Eisentrger  P. Emde  A. Fischer  O. von der Lühe  H. Nicklas  D. Soltau  W. Schmidt  U. Weis 《Astronomische Nachrichten》2012,333(9):816-822

The mechanical structure of the GREGOR telescope was installed at the Observatorio del Teide, Tenerife, in 2004. New concepts for mounting and cooling of the 1.5‐meter primary mirror were introduced. GREGOR is an open telescope, therefore the dome is completely open during observations to allow for air flushing through the open, but stiff telescope structure. Backside cooling system of the primary mirror keeps the mirror surface close to ambient temperature to prevent mirror seeing. The large collecting area of the primary mirror results in high energy density at the field stop at the prime focus of the primary which needs to be removed. The optical elements are supported by precision alignment systems and should provide a stable solar image at the optical lab. The coudé train can be evacuated and serves as a natural barrier between the outer environmental conditions and the air‐conditioned optical laboratory with its sensitive scientific instrumentation. The telescope was successfully commissioned and will start its nominal operation during 2013 (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

A retrospective of the GREGOR solar telescope in scientific literature     

C. Denker  O. von der Lühe  A. Feller  K. Arlt  H. Balthasar  S.‐M. Bauer  N. Bello Gonzlez  Th. Berkefeld  P. Caligari  M. Collados  A. Fischer  T. Granzer  T. Hahn  C. Halbgewachs  F. Heidecke  A. Hofmann  T. Kentischer  M. Klva&#x;a  F. Kneer  A. Lagg  H. Nicklas  E. Popow  K.G. Puschmann  J. Rendtel  D. Schmidt  W. Schmidt  M. Sobotka  S.K. Solanki  D. Soltau  J. Staude  K.G. Strassmeier  R. Volkmer  T. Waldmann  E. Wiehr  A.D. Wittmann  M. Woche 《Astronomische Nachrichten》2012,333(9):810-815

In this review, we look back upon the literature, which had the GREGOR solar telescope project as its subject including science cases, telescope subsystems, and post‐focus instruments. The articles date back to the year 2000, when the initial concepts for a new solar telescope on Tenerife were first presented at scientific meetings. This comprehensive bibliography contains literature until the year 2012, i.e., the final stages of commissioning and science verification. Taking stock of the various publications in peer‐reviewed journals and conference proceedings also provides the “historical” context for the reference articles in this special issue of Astronomische Nachrichten/Astronomical Notes (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献