Doublet craters on Venus     

Cheryl M Cook  William F Bottke Jr. 《Icarus》2003,165(1):90-100

Of the impact craters on Earth larger than 20 km in diameter, 10-15% (3 out of 28) are doublets, having been formed by the simultaneous impact of two well-separated projectiles. The most likely scenario for their formation is the impact of well-separated binary asteroids. If a population of binary asteroids is capable of striking the Earth, it should also be able to hit the other terrestrial planets as well. Venus is a promising planet to search for doublet craters because its surface is young, erosion is nearly nonexistent, and its crater population is significantly larger than the Earth's. After a detailed investigation of single craters separated by less than 150 km and “multiple” craters having diameters greater than 10 km, we found that the proportion of doublet craters on Venus is at most 2.2%, significantly smaller than Earth's, although several nearly incontrovertible doublets were recognized. We believe this apparent deficit relative to the Earth's doublet population is a consequence of atmospheric screening of small projectiles on Venus rather than a real difference in the population of impacting bodies. We also examined “splotches,” circular radar reflectance features in the Magellan data. Projectiles that are too small to form craters probably formed these features. After a careful study of these patterns, we believe that the proportion of doublet splotches on Venus (14%) is comparable to the proportion of doublet craters found on Earth (10-15%). Thus, given the uncertainties of interpretation and the statistics of small numbers, it appears that the doublet crater population on Venus is consistent with that of the Earth.  相似文献   

Reply to comment on ‘Aspects of 1D seismic modelling using the Goupillaud principle ‘by Dan Loewenthal1     

Anton Ziolkowski  Evert Slob 《Geophysical Prospecting》1994,42(1):91-92

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The biomass of macro- and interstitial fauna on clean and wrack-covered beaches in Western Australia     

Anton McLachlan 《Estuarine, Coastal and Shelf Science》1985,21(4):587-599

The benthic faunal spectrum including bacteria, protozoans, meiofauna, wrack epifauna and macrofauna, was quantitatively surveyed on two modally reflective, moderate energy, Western Australian beaches. The more exposed beach had coarser sand, no intertidal macrofauna and a poor interstitial fauna. The less exposed beach had a large deposit of wrack totalling 161 kg m^?1 dry mass concentrated on the lower shore. The amphipod Allorchestes compressa was abundant in the fresh wrack comprising most of the macrofauna. There were also fairly abundant small epifauna on the wrack. Dry biomass of macrofauna, epifauna, meiofauna, protozoans and bacteria was 0, 0, 15, 4 and 180 g m^?1 on the more exposed beach and 160, 3, 112, 9 and 901 g m^?1 on the less exposed beach with wrack. On the latter beach there was an inverse correlation between meiofaunal densities and the densities of protozoans and bacteria, suggesting grazing by the former on the latter. On both beaches meiofauna was concentrated in the mid- to upper beach, protozoans near the surface and bacteria in the mid- to lower beach. It is estimated that bacteria are responsible for most of the secondary production on both beaches.  相似文献   

Optimization of grid-drilling using computer simulation     

J. William Miller Jr. 《Mathematical Geology》1991,23(2):201-218

A computer simulation method has been developed to find efficient drilling grids for mineral deposits. A well-known ore deposit is used as a model to develop an efficient pattern for undiscovered ore bodies in the same area or in other prospects where similar geometry is suspected. The model for this study is the Austinville, Virginia deposit, a Mississippi Valley-type deposit composed of 17 ore bodies totaling 34 million short tons (30 million metric tons). The method employs a computer program that simulates drilling the model deposit with different patterns, including various levels of follow-up drilling. Follow-up holes are drilled in fences at one half the original spacing around holes in the grid that show ore-grade mineralization. Each pattern is drilled 100 times from random starting locations to provide a range of outcomes of drilling, including the best, worst, and most likely. For this study, patterns of 100 drill holes were composed of 10 fences spaced 1000–5000 feet (305–1524 m) apart, each with 10 holes spaced 200–1000 feet (61–305 m) apart. In all, 25 grids were used with zero to three levels of follow-up drilling. The 600/2000 grid, with drill holes spaced 600 feet (183 m) apart in fences spaced 2000 feet (610 m) apart, was compared with the 200/5000 grid because they represented contrasting outcomes. The 600/2000 grid penetrated many ore bodies consistently but with few multiple hits to individual ore bodies; whereas the 200/5000 grid inconsistently penetrated few ore bodies with many multiple hits. The 600/2000 grid was more efficient than the 200/5000 grid at hitting large ore bodies of 1,000,000 short tons or greater (900,000 metric tons or greater) and was made more effective by adding one cycle of follow-up drilling. The 600/2000 grid had a 97% chance of hitting one or more large ore bodies with at least one drill hole per ore body, and the 200/5000 grid had a 64% chance. Once hit, there was an 82% chance that the largest ore body would be penetrated by three or more holes when using the 600/2000 grid and an 88% chance using the 200/5000 grid.  相似文献   

Error estimates with L1 solutions     

Richard L. Branham Jr. 《Celestial Mechanics and Dynamical Astronomy》1986,39(3):239-247

A method is given, based on the pseudoinverse of the equations of condition, to obtain error estimates for the solution in the normL ₁ of an over-determined linear system. The computational labor to obtain the errors, while not trivial, is less than that for various competing methods, particularly if there are many more equations of condition than unknowns. The error estimates for anL ₁ solution are substantially larger than those for a least squares solution of the some data. It is suggested that a complete discussion of a linear system include at least bothL ₁ and least squares solutions with their respective errors and the condition number of the linear system.  相似文献   

Deuterated hydrogen molecule and search for early structure-formation signatures in the Universe     

Ramona Núñez-López  Anton Lipovka  Vladimir Avila-Reese 《Monthly notices of the Royal Astronomical Society》2006,369(4):2005-2012

Possible detection of signatures of structure formation at the end of the 'dark age' epoch  ( z ∼ 40–20)  is examined. We discuss the spectral–spatial fluctuations in the cosmic microwave background radiation (CMBR) temperature produced by elastic resonant scattering of CMBR photons on deuterated hydrogen (HD) molecules located in protostructures moving with peculiar velocity. Detailed chemical kinematic evolution of HD molecules in the expanding homogeneous medium is calculated. Then, the HD abundances are linked to protostructures at their maximum expansion, whose properties are estimated by using the top-hat spherical approach and the Λ cold dark matter (ΛCDM) cosmology. We find that the optical depths in the HD three lowest pure rotational lines for high-peak protohaloes at their maximum expansion are much higher than those in LiH molecule. The corresponding spectral–spatial fluctuation amplitudes, however, are probably too weak to be detected by current and forthcoming millimetre telescope facilities. We extend our estimates of spectral–spatial fluctuations to gas clouds inside collapsed CDM haloes by using results from a crude model of HD production in these clouds. The fluctuations for the highest peak CDM haloes at redshifts ∼20–30 could be detected in the future. Observations will be important to test model predictions of early structure formation in the Universe.  相似文献   

Longevity of fluorine-bearing tremolite on Venus     

Natasha M. Johnson  Bruce Fegley Jr. 《Icarus》2003,165(2):340-348

There is a general belief that hydrous minerals cannot exist on Venus under current surface conditions. This view was challenged when Johnson and Fegley (2000, Icarus 146, 301-306) showed that tremolite (Ca₂Mg₅Si₈O₂₂(OH)₂), a hydrous mineral, is stable against thermal decomposition at current Venus surface temperatures, e.g., 50% decomposition in 4 Ga at 740 K. To further explore hydrous mineral thermal stability on Venus, we experimentally determined the thermal decomposition kinetics of fluorine-bearing tremolite. Fluor-tremolite is thermodynamically more stable than OH-tremolite and should decompose more slowly. However how much slower was unknown. We measured the decomposition rate of fluorine-bearing tremolite and show that its decomposition is several times to greater than ten times slower than that of OH-tremolite. We also show that F-bearing tremolite is depleted in fluorine after decomposition and that fluorine is lost as a volatile species such as HF gas. If tremolite ever formed on Venus, it would probably also contain fluorine. The exceptional stability of F-bearing tremolite strengthens our conclusions that if hydrous minerals ever formed on Venus, they could still be there today.  相似文献   

Dynamics of Two Planets in the 3/2 Mean-motion Resonance: Application to the Planetary System of the Pulsar PSR B1257+12     

N. Callegari Jr  S. Ferraz-Mello  T. A. Michtchenko 《Celestial Mechanics and Dynamical Astronomy》2006,94(4):381-397

This paper considers the dynamics of two planets, as the planets B and C of the pulsar PSR B1257+12, near a 3/2 mean-motion resonance. A two-degrees-of-freedom model, in the framework of the general three-body planar problem, is used and the solutions are analyzed through surfaces of section and Fourier techniques in the full phase space of the system.On the possibility of existence of a fourth planet in distant orbit, see Wolszczan et al., 2000  相似文献   

Neutron techniques     

Glenn M. Frye Jr.  Philip P. Dunphy  Edward L. Chupp  Paul Evenson 《Solar physics》1988,118(1-2):321-346

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An estimate of the contribution of high nergy cosmic-ray protons to the absorbed dose inventory of a cometary nucleus     

M. P. Ryan Jr.  I. G. Draganić 《Astrophysics and Space Science》1986,125(1):49-67

We discuss the contribution of cosmic-ray protons at all energies above 1 MeV to the absorbed doses of the surface layers of a comet. Since there exists no calculation which takes into account proton energy losses by means of losses to electrons and nuclear collisions (in a cascade process), and losses due to the low energy end-products of the cascade, we have made a rough estimate of all of these contributions. An analytical formula is proposed that allows a rapid estimate of ranges and the dose absorbed at any depth. We give dose-depth curves for two extreme values of the energy at which nuclear collisions begin to dominate the slowing-down process, and for an intermediate value we display the dose-depth curve down to 20 m from the surface. The relevance of these findings to dosimetry in comets and some alterations of cometary material are considered. The need for improving the analytical expression proposed is stressed.  相似文献