Evolution of grains in a turbulent solar nebula     

Stuart J. Weidenschilling 《Icarus》1984,60(3):553-567

In accretion disk models of the solar nebula, turbulence is driven by convective instability. This mechanism requires high opacity, which must be provided by solid grains. Evolution of the grain size distribution in a turbulent disk is computed numerically, using realistic collisional outcomes and strengths of grain aggregates, rather than an arbitrary “sticking efficiency.” The presence of turbulence greatly increases the rate of grain collisions; the coagulation rate is initially much greater than in a nonturbulent disk. Aggregates quickly reach sizes ～0.1–1 cm, but erosion and breakup in collisions prevent growth of larger bodies for plausible aggregate impact strengths. These aggregates are too small to settle to the plane of the disk, and planetesimal formation is impossible as long as the turbulence persists. However, the opacity of the disk is reduced by aggregate formation; some combinations of opacity law and surface density produce an optically thin disk, cutting off turbulent convection. The disk may experience alternating periods of turbulence and quiescence, as grains are depleted by coagulation and replenished by infall from the presolar cloud. Planetesimals can form only during the quiescent intervals; it is argued that such episodes were rare during the lifetime of the accretion disk.  相似文献   

Seismic body waves in anisotropic media: propagation through a layer   总被引：3，自引：0，他引：3  

Colum M. Keith  Stuart Crampin 《Geophysical Journal International》1977,49(1):209-223

Summary. The square-root energy ratios and pulse shapes are presented for P, SV and SH waves transmitted through a layer of orthorhombic olivine between two isotropic half-spaces. Off incident planes of symmetry, incident P waves generate two small amplitude SH waves (one from each interface), whose amplitudes decrease slowly with increasing period. Incident SV (or SH ) waves can generate large amplitude SH (or SV ) waves which decrease rapidly with increasing period. For incident S waves, many pulses not present in isotropic models are generated, often of large relative amplitude, with many of the transmitted S pulses showing evidence of double arrivals, either in the form of S-wave splitting, or a modification of the shape of the input waveform.  相似文献   

How fast do Galilean satellites spin?     

Richard Greenberg  Stuart J. Weidenschilling 《Icarus》1984,58(2):186-196

Each of the Galilean satellites, as well as most other satellites whose initial rotations have been substantially altered by tidal dissipation, has been widely assumed to rotate synchronously with its orbital mean motion. Such rotation would require a small permanent asymmetry in the mass distribution in order to overcome the small mean tidal torque. Since Io and Europa may be substantially fluid, they may not have the strenght to support the required permanent asymmetry. Thus, each may rotate at the unknown but slightly nonsynchronous rate that corresponds to zero mean tidal torque. This behaviour may be observable by Galileo spacecraft imaging. It may help explain the longitudinal variation of volcanism on Io and the cracking of Europa's crust.  相似文献   

Wax esters and triacylglycerols in sinking particulate matter in the Peru upwelling area (15°S, 75°W)     

Stuart G Wakeham 《Marine Chemistry》1985,17(3):213-235

The flux and composition of wax esters and triacylglycerols were measured in particulate matter collected by free-drifting sediment traps in the Peru upwelling area. The purpose was to assess diurnal variations in sources and vertical transport of these lipids. Traps were deployed for periods of 8–12 h during both day and night at the base of the euphotic zone (10–14m) and at about 50m. Although the flux of total particulate organic carbon varied 4×, the flux of wax esters varied by 84× and triacylglycerols by 30×. POC, wax esters, and triacylglycerols also showed different temporal patterns. The highest wax ester flux was measured in shallow traps at night and is attributed to increased inputs from zooplankton which feed near the surface at night. Triacylglycerol flux was high in both shallow and deep nocturnal traps, consistent with inputs of both phytoplankton and zooplankton lipids. The composition of these lipids was also quite variable and attests to the complex nature of the lipid sources.  相似文献   

The 2dF Galaxy Redshift Survey: spectra and redshifts     

Matthew Colless  Gavin Dalton  Steve Maddox  Will Sutherland  Peder Norberg  Shaun Cole  Joss Bland-Hawthorn  Terry Bridges  Russell Cannon  Chris Collins  Warrick Couch  Nicholas Cross  Kathryn Deeley  Roberto De Propris  Simon P. Driver  George Efstathiou  Richard S. Ellis  Carlos S. Frenk  Karl Glazebrook  Carole Jackson  Ofer Lahav  Ian Lewis  Stuart Lumsden  Darren Madgwick  John A. Peacock  Bruce A. Peterson  Ian Price  Mark Seaborne  Keith Taylor 《Monthly notices of the Royal Astronomical Society》2001,328(4):1039-1063

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Small-scale spatial distribution ofmeiofauna on a mud- and sandflat     

Stuart E.G. Findlay 《Estuarine, Coastal and Shelf Science》1981,12(4):471-484

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Preface     

Stuart Crampin  Roger G. Hipkin  Eugenie M. Chesnokov 《Geophysical Journal International》1984,79(1):1-1

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Shear-wave polarizations near the North Anatolian Fault – I. Evidence for anisotropy-induced shear-wave splitting     

David C. Booth  Stuart Crampin  Russ Evans  Graham Roberts 《Geophysical Journal International》1985,83(1):61-73

Summary. The Turkish Dilatancy Projects (TDP1 in 1979 and TDP2 in 1980) recorded small earthquakes near the North Anatolian Fault with closely-spaced networks of three-component seismometers in order to investigate the possibility of diagnosing dilatancy from its effects of shear-wave propagation. This paper examines the polarizations of shear wavetrains recorded in the shear-wave window immediately above the earthquake foci. Abrupt changes in the orientation and/or ellipticity of the shear-wave polarizations are almost always observed during the first few cycles following the initial shear-wave arrival on each seismogram. The horizontal projections of the polarizations of the first shear-wave arrivals at any given station show nearly parallel alignments with approximately the same orientations at each of the recording sites (with one exception). It is difficult to explain this uniform alignment over a wide area in terms of scattering at the irregular surface topography or by earthquake focal mechanisms. We demonstrate that the shear-wave splitting is likely to be the result of anisotropy in the region above the earthquake foci, which could produce polarizations displaying the observed alignments. The temporal change of the azimuth of alignment, observed at one locality between 1979 and 1980, may be due to the release of a local stress anomaly by a very near earthquake.  相似文献   

Collisional history of asteroids: Evidence from Vesta and the Hirayama families     

Donald R. Davis  Clark R. Chapman  Stuart J. Weidenschilling  Richard Greenberg 《Icarus》1985,62(1):30-53

Collisional evolution studies of asteroids indicate that the initial asteroid population at the time mean collisional velocities were pumped up to ～5 km/sec was only modestly larger than it is today; i.e., the asteroid belt was already depleted relative to the mean surface density elsewhere in the planetary region. Numerical simulations of the collisional evolution of hypothetical initial asteroid populations have been run, subject to three constraints: they must (a) evolve to the present observed asteroid size distribution, (b) preserve Vesta's basaltic crust, and (c) produce at least the observed number of major Hirayama families. A “runaway growth” initial asteroid population distribution is found to best satisfy these constraints. A new model is presented for calculating the fragmental size distribution for the disruption of large, gravitationally bound bodies in which the material strength is increased by hydrostatic self-compression. This model predicts that large asteroid behave as intrinsically strong bodies, even if they have had a history of being collisionally fractured. This model, when applied to the breakup of the Themis and Eos family parent bodies, gives size distributions in reasonably good agreement with those observed.  相似文献   

Surface shear stress, strain, and shear displacement for screw dislocations in a vertical slab with shear modulus contrast   总被引：1，自引：0，他引：1  

Stuart McHugh  Malcolm Johnston 《Geophysical Journal International》1977,49(3):715-722

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