Random porosity fields and their influence on the stability of granular media

It is well established that the mechanical behavior of granular media is strongly influenced by the media's microstructure. In this work, the influence of the microstructure is studied by integrating advances in the areas of geostatistics and computational plasticity, by spatially varying the porosity on samples of sand. In particular, geostatistical tools are used to characterize and simulate random porosity fields that are then fed into a nonlinear finite element model. The underlying effective mechanical response of the granular medium is governed by a newly developed elastoplastic model for sands, which readily incorporates spatial variability in the porosity field at the meso‐scale. The objective of this study is to assess the influence of heterogeneities in the porosity field on the stability of sand samples. One hundred and fifty isotropic and anisotropic samples of dense sand are failed under plane‐strain compression tests using Monte Carlo techniques. Results from parametric studies indicate that the axial strength of a specimen is affected by both the degree and orientation of anisotropy in heterogeneous porosity values with anisotropy orientation having a dominant effect, especially when the bands of high porosity are aligned with the natural orientation of shear banding in the specimen. Copyright © 2007 John Wiley & Sons, Ltd.     

Computer simulation of chemical nucleation

The problem of nucleation at chemical instabilities is investigated by means of microscopic computer simulation. The first-order transition of interest involves a new kind of nucleation arising from chemical transformations rather than physical forces. Here it is the chemical state of matter, and not matter itself, which is spatially localized to form the nucleus for transition between different chemical states. First, the concepts of chemical instability, nonequilibrium phase transition, and dissipative structure are reviewed briefly. Then recently developed methods of reactive molecular dynamics are used to study chemical nucleation in a simple model chemical reaction. Finally, the connection of these studies to nucleation and condensation processes involving physical and chemical interactions is explored.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

Evolution of the Janus-Epimetheus coorbital resonance due to torques from Saturn's ring

We analyze the interactions between Saturn's coorbital satellites, Janus and Epimetheus, and the outer edge of the A ring, which is presumably maintained by these moons at their 7:6 resonance. Using two distinct but conceptually related methods, we show that ring torques are driving these satellites into a tighter lock. Unless there is a counterbalancing force which we have neglected, their orbital configuration will evolve from the current horseshoe-type lock to one of tadpole orbits around a single Lagrange point in ～20 myr. This finding adds an additional member to the list of short time scale problems associated with the interactions between Saturn's rings and its inner moons     

Carbon and nitrogen stable isotopes in suspended matter and sediments from the Schelde Estuary

The C/N and stable C and N isotope ratios (δ¹³C, δ¹⁵N) of sedimentary and suspended particulate matter were determined in the Schelde Estuary. Suspended matter was divided into 2 to 5 size fractions by centrifugation. Four major pools of organic matter were recognized: riverine, estuarine, marine and terrestrial materials. Terrestrial organic matter (δ¹³C≈−26‰, δ¹⁵N≈3.5‰, C/N≈21) is important for the sedimentary pool, but suspended matter is dominated by the marine (δ¹³C≈−18‰, δ¹⁵N≈9‰, C/N≈8), riverine (δ¹³C≈−30‰, δ¹⁵N≈9‰, C/N≈7.5) and estuarine (δ¹³C≈−29‰, δ¹⁵N≈15‰, C/N≈8) end-members. In the upper estuary, the suspended matter size fractions vary systematically in their carbon and nitrogen biogeochemistry, with the small particles having low C/N ratios, depleted δ¹³C and enriched δ¹⁵N values relative to large particles. Moreover, sedimentary and suspended matter differ significantly in terms of C/N ratios (17 vs. 8.9), δ¹³C (−26.3 vs. −28.9‰) and δ¹⁵N (+6.9 vs. 12.0‰). In the lower estuary, suspended matter fractions are similar and sedimentary and suspended organic matter differ only in terms of δ¹³C (−23.5 vs. −20.1‰). Our data indicate that autochthonous organic matter contributes significantly to the total suspended matter and that the suspended organic matter composition cannot be explained in terms of conservative mixing of riverine and terrestrial sources on the one hand and marine sources on the other hand.     

Exploring an Ediacaran ‘nursery’: growth,ecology and evolution in a rangeomorph palaeocommunity

The recent discovery of a fossilized assemblage of juvenile Ediacaran rangeomorph macro‐organisms, fern‐like impressions from the Drook Formation of Newfoundland, suggests that the morphological diversity present within the earliest known communities of the Ediacaran biota was relatively high. The fledgling population of these Proterozoic organisms was smothered by volcanic material from a nearby eruption, preserving the standing community on the seafloor. As many as six different genera are identified, present less than three million years after the end of the Gaskiers glacial event. This hints at a hidden evolutionary history for these organisms that is yet to be discovered. Co‐occurring filamentous impressions, and other fossils interpreted to represent the remains of decayed and ‘effaced’ organisms, suggest that the bedding plane exhibits unusual ecological completeness, with the assemblage as a whole documenting an early example of secondary community succession. Discoveries such as this reveal the potential for high‐quality preservation of soft‐bodied Ediacaran organisms in siliciclastic successions, and provide valuable insights into rangeomorph development and palaeoecology.     

Growth trishear model and its application to the Gilbertown graben system, southwest Alabama

Fault-propagation folding associated with an upward propagating fault in the Gilbertown graben system is revealed by well-based 3-D subsurface mapping and dipmeter analysis. The fold is developed in the Selma chalk, which is an oil reservoir along the southern margin of the graben. Area-depth-strain analysis suggests that the Cretaceous strata were growth units, the Jurassic strata were pregrowth units, and the graben system is detached in the Louann Salt.The growth trishear model has been applied in this paper to study the evolution and kinematics of extensional fault-propagation folding. Models indicate that the propagation to slip (p/s) ratio of the underlying fault plays an important role in governing the geometry of the resulting extensional fault-propagation fold. With a greater p/s ratio, the fold is more localized in the vicinity of the propagating fault. The extensional fault-propagation fold in the Gilbertown graben is modeled by both a compactional and a non-compactional growth trishear model. Both models predict a similar geometry of the extensional fault-propagation fold. The trishear model with compaction best predicts the fold geometry.     

Sample return of interstellar matter (SARIM)

The scientific community has expressed strong interest to re-fly Stardust-like missions with improved instrumentation. We propose a new mission concept, SARIM, that collects interstellar and interplanetary dust particles and returns them to Earth. SARIM is optimised for the collection and discrimination of interstellar dust grains. Improved active dust collectors on-board allow us to perform in-situ determination of individual dust impacts and their impact location. This will provide important constraints for subsequent laboratory analysis. The SARIM spacecraft will be placed at the L2 libration point of the Sun–Earth system, outside the Earth’s debris belts and inside the solar-wind charging environment. SARIM is three-axes stabilised and collects interstellar grains between July and October when the relative encounter speeds with interstellar dust grains are lowest (4 to 20 km/s). During a 3-year dust collection period several hundred interstellar and several thousand interplanetary grains will be collected by a total sensitive area of 1 m². At the end of the collection phase seven collector modules are stored and sealed in a MIRKA-type sample return capsule. SARIM will return the capsule containing the stardust to Earth to allow for an extraction and investigation of interstellar samples by latest laboratory technologies.     

Triaxial ellipsoid dimensions and poles of asteroids from AO observations at the Keck-II telescope

Five main belt asteroids, 2 Pallas, 129 Antigone, 409 Aspasia, 532 Herculina, and 704 Interamnia were imaged with the adaptive optics system on the 10 m Keck-II telescope in the near infrared on one night, August 16, 2006. The three axis dimensions and rotational poles were determined for Pallas, Antigone, Aspasia, and Interamnia, from their changing apparent sizes and shapes as measured with parametric blind deconvolution. The rotational pole found for Interamnia is much different from all previous work, including our own at Lick Observatory the previous month. Although images of Herculina were obtained at only two rotational phases, its rotation appears to be opposite to that predicted from the lightcurve inversion model of M. Kaasalainen, J. Torppa, and J. Piironen [2002. Icarus 159, 369-395]. A search for satellites was made in all of the asteroid images, with negative results, but three trailing stars around Herculina (200 km diameter), down to 8.9 magnitudes fainter and between 1 and 115 asteroid radii (100 to 11,500 km) from the asteroid, establishes an upper limit of 3.3 km for any object with the same albedo near Herculina.     

Tidal evolution of Mimas, Enceladus, and Dione

The tidal evolution through several resonances involving Mimas, Enceladus, and/or Dione is studied numerically with an averaged resonance model. We find that, in the Enceladus-Dione 2:1 e-Enceladus type resonance, Enceladus evolves chaotically in the future for some values of k₂/Q. Past evolution of the system is marked by temporary capture into the Enceladus-Dione 4:2 ee^′-mixed resonance. We find that the free libration of the Enceladus-Dione 2:1 e-Enceladus resonance angle of 1.5° can be explained by a recent passage of the system through a secondary resonance. In simulations with passage through the secondary resonance, the system enters the current Enceladus-Dione resonance close to tidal equilibrium and thus the equilibrium value of tidal heating of 1.1(18,000/QS) GW applies. We find that the current anomalously large eccentricity of Mimas can be explained by passage through several past resonances. In all cases, escape from the resonance occurs by unstable growth of the libration angle, sometimes with the help of a secondary resonance. Explanation of the current eccentricity of Mimas by evolution through these resonances implies that the Q of Saturn is below 100,000. Though the eccentricity of Enceladus can be excited to moderate values by capture in the Mimas-Enceladus 3:2 e-Enceladus resonance, the libration amplitude damps and the system does not escape. Thus past occupancy of this resonance and consequent tidal heating of Enceladus is excluded. The construction of a coherent history places constraints on the allowed values of k₂/Q for the satellites.