bluuebunny/crossref_metadata_2025 huggingface dataset:
Mindoro Strait and Sibutu Passage transports estimated from satellite data
Authors: Qu Tangdong , Song Y. Tony
Published: May 2009

This study investigates the Mindoro Strait and Sibutu Passage transports using remotely‐sensed sea surface height (SSH) and ocean‐bottom‐pressure (OBP) data. The analysis reveals the existence of a persistent baroclinic pressure gradient that drives a deepwater overflow from the South China Sea into the Sulu Sea. The application of hydraulic theory combined with the “geostrophic control” formula yields a mean transport estimate of about 2.4 Sv (1 Sv = 106 m3 s−1) through the Mindoro Strait and about 2.8 Sv through the Sibutu Passage. Most of this water enters the Sulu Sea through the bottom layer of the Mindoro Strait and exits the Sulu Sea in the upper layer of the Sibutu Passage. The analysis also provides the first satellite‐based observational evidence on the seasonal and interannual variation of the Mindoro Strait and Sibutu Passage transports for the period from January 2004 to December 2007.
Hybrid modeling of the mega‐tsunami runup in Lituya Bay after half a century
Authors: Weiss Robert , Fritz Hermann M. , Wünnemann Kai
Published: May 2009

The largest mega‐tsunami dates back half a century to 10 July 1958, when almost unnoticed by the general public, an earthquake of Mw 8.3 at the Fairweather Fault triggered a rockslide into Lituya Bay. The rockslide impact generated a giant tsunami at the head of Lituya Bay resulting in an unprecedented tsunami runup of 524 m on a spur ridge in direct prolongation of the slide axis. A forest trim line and erosion down to bedrock mark the largest runup in recorded history. While these observations have not been challenged directly, they have been largely ignored in hazard mitigation studies, because of the difficulties of even posing – much less solving – a well‐defined physical problem for investigation. We study the mega‐tsunami runup with a hybrid modeling approach applying physical and numerical models of slide processes of deformable bodies into a U‐shaped trench similar to the geometry found at Lituya Bay.
Enhanced wavelet analysis of solar magnetic activity with comparison to global temperature and the Central England Temperature record
Authors: Johnson Robert W.
Published: May 2009

The continuous wavelet transform may be enhanced by deconvolution with the wavelet response function. After correcting for the cone of influence, the power spectral density of the solar magnetic record as given by the derectified yearly sunspot number is calculated, revealing a spectrum of odd harmonics of the fundamental Hale cycle, and the integrated instant power is compared to a reconstruction of global temperature in a normalized scatterplot displaying a positive correlation after the turn of the twentieth century. Comparison of the spectrum with that obtained from the Central England Temperature record suggests that some features are shared while others are not, and the scatterplot again indicates a possible correlation.
Simple algorithms for remote determination of mineral abundances and particle sizes from reflectance spectra
Authors: Johnson Paul E. , Smith Milton O. , Adams John B.
Published: May 2009

Simple algorithms for quantitatively modeling the reflectance spectra of mineral particulates are tested. Although more sophisticated models exist, these algorithms are particularly suited for remotely sensed data, where little or no opportunity exists to independently measure reflectance versus particle size and phase function. Previously, we introduced this method in the analysis of the directional‐hemispherical reflectance spectra of binary mineral mixtures containing a single particle size distribution. In this study the technique is extended to multicomponent mixtures, various size separates, and spectra with differing illumination/viewing geometries. It is found that the theoretical calculations and measured data agree to nearly the level of experimental error. This method is also used to determine the threshold abundance at which a mineral can be detected when mixed with another mineral.
Analysis of orbital perturbations acting on objects in orbits near geosynchronous Earth orbit
Authors: Friesen Larry Jay , Jackson Albert A. , Zook Herbert A. , Kessler Donald J.
Published: May 2009

Orbital evolution has been numerically simulated for objects started in geosynchronous Earth orbit (GEO) or in orbits near GEO, during a project to study potential orbital debris problems in this region. Perturbations simulated include nonspherical terms in the Earth's geopotential field, lunar and solar gravity, and solar radiation pressure. Objects simulated include large satellites, for which solar radiation pressure is insignificant, and small particles (a few microns in diameter), for which solar radiation pressure is an important force. Results for large satellites are largely in agreement with previous GEO studies that used classical perturbation techniques; orbital evolution studies were extended to possible storage orbits slightly above or below GEO. One notes that while the orbit planes of GEO satellites initially placed in equatorial orbits precess, so that those orbits reach inclinations of 14° to 15° to the equator, a “stable plane” exists inclined approximately 7.3° to the equator. The orbit planes of GEO satellites placed in such a stable plane orbit experience very little precession, remaining always within 1.2° of their initial orientation. Solar radiation pressure generates two major effects on small particles. One is an orbital eccentricity oscillation anticipated from previous research. The other is an oscillation in orbital inclination. This orbital inclination pattern is due to a precession of the small particle's orbital angular momentum vector about an axis offset from the Earth's polar axis. The magnitude of the precession axis offset angle depends on the particle's cross‐sectional area to mass ratio. The rate for this precession differs greatly from the precession rate predicted in a previous study using perturbation techniques. This difference points up the inadequacy of those perturbation techniques for orbits with large eccentricities. For one sequence of runs with small particles, Poynting‐Robertson drag was added to the simulation in order to slowly reduce the orbital semimajor axis and probe for possible orbital period resonances near the GEO distance. A significant resonance was found at the geosynchronous distance, where small grains are trapped into a 1:1 resonance with the Earth's daily rotation.
Mars Observer mission
Authors: Albee A. L. , Arvidson R. E. , Palluconi F. D.
Published: May 2009

The Mars Observer mission will extend the exploration and characterization of Mars by providing new and systematic measurements of the atmosphere, surface, and interior of the planet. These measurements will be made from a low‐altitude polar orbiter over a period of 1 Martian year, permitting repetitive observations of the surface and of the seasonal variations of the atmosphere. The mission will be conducted in a manner that will provide new and valuable scientific data using a distributed data system that minimizes operational complexity and cost.
Airburst origin of dark shadows on Venus
Authors: Zahnle Kevin J.
Published: May 2009

A simple analytic model for the catastrophic disruption and deceleration of impactors in a thick atmosphere is used to (1) reproduce observed Venusian cratering statistics and (2) generate radar‐dark disks by the impact of atmospheric shock waves with the surface. When used as input to Monte Carlo simulations of Venusian cratering, the model nicely reproduces the observed low diameter cutoff. Venusian craters are found to be more consistent with an asteroidal rather than a cometary source. The radar‐dark “shadows” of the title are surface features, usually circular, that have been attributed to airbursting impactors. A typical craterless airburst is the equivalent of a ∼ 106 megaton explosion. The airburst is treated as a massive, extended explosion using a thin‐shell, isobaric cavity approximation. The strong atmospheric shock waves excited by the airburst are then coupled to surface rock using the usual impedance matching conditions. Peak shock pressures experienced by surface rock typically exceed 0.2 GPa for distances 15–30 km from ground zero (the place on the surface immediately beneath the site of the airburst), and 1 GPa for 10–20 km. These high shock pressures are felt to considerable depth, often more than a kilometer. Beneath the airburst the shock could reduce surface rocks to fine rubble, while at greater distance the weaker shock would leave fields of broken blocks, perhaps in part accounting for radar‐bright halos that often surround the dark shadows.
Images and topographic relief at the North Pole of Venus
Authors: Leberl Franz W. , Maurice Kelly E. , Thomas John K. , Leff Craig E. , Wall Stephen D.
Published: May 2009

We report on the analysis of unique coverage of the north pole of Venus using a subset of 13 images from a total of about 775 synthetic aperture radar orbits during cycle 1 of the Magellan mission. We present images at 13 distinctly different azimuths that range over 360° in longitude and that show the topographic relief at vastly different imaging geometries at intervals of about 20°. A pair of intersecting or crossing orbits is used to assess the topographic relief with stereo radargrammetric techniques, and to refine this assessment using a multi‐image clinometric approach. We show that the pole is located in accentuated topography with elevation differences in the range of no more than 1 km. We conclude that pinpointing the location of the pole from the overlapping images and standard mission data is accurate to within about 100 pixels or 8 km (RMS).
Initial results from the Magellan stereo experiment
Authors: Leberl Franz W. , Thomas John K. , Maurice Kelly E.
Published: May 2009

The second global radar image coverage of Venus by Magellan included a 1 day experiment to produce a set of eight images along orbits that combined with the original nominal imagery into stereo image pairs suited for visual stereo interpretation and mapping of topographic elevations. Initial analysis shows excellent stereo radar data with good vertical exaggeration of six and more, parallax accuracies of ±0.6 pixels or ±45 m and a surface definition within ±30 m. The topographic relief can be very accentuated, with local elevation differences of 2.5 km and terrain slopes often in excess of 25°. This data set has proven the usefulness of global stereo coverage and marks the beginning of significant planetary stereo radargrammetric developments. Extensive mapping with this same stereo geometry is being carried out in cycle 3, and preparations are underway to develop a detailed stereo radargrammetric topographic model of the planet.
Control of VLF burst activity in the nightside ionosphere of Venus by the magnetic field orientation
Authors: Ho C.‐M. , Strangeway R. J. , Russell C. T.
Published: May 2009

In order to determine whether VLF bursts observed in the nightside ionosphere on Venus are better interpreted as whistler mode waves or ion acoustic waves we have performed both case studies and statistical studies of the occurrence of these signals. The correlation between burst occurrence and the angle between the magnetic field and the radial direction, as well as the spacecraft flight direction are examined. We find that narrow‐band 100 Hz bursts are more frequently associated with radial magnetic fields while wideband signals are more frequently associated with horizontal fields. Although most 100 Hz bursts appear when the field is perpendicular to the flight direction, there is no obvious dependence on the field orientation with respect to the velocity for the wideband signals. Furthermore, narrow‐band 100 Hz bursts occur more often at all altitudes when the expected propagation direction is inside the whistler mode resonance cone, assuming vertical propagation. Finally, the 100 Hz burst rate inside the resonance cone decreases slowly with increasing altitude, while the burst rate outside the resonance cone and at higher frequencies decreases with a scale height of about 20 km. The dependence on the magnetic field orientation is consistent with propagation of the narrow‐band signals in the whistler mode from an atmospheric source, such as lightning. The wideband source may also be lightning, but the mode of entry of these signals into the ionosphere remains undetermined.
Joints in Venusian lava flows
Authors: Johnson Catherine L. , Sandwell David T.
Published: May 2009

Venusian plains regions, as imaged by the Magellan spacecraft, display many styles of tectonic and volcanic deformation. Radar images of several areas of the volcanic plains reveal polygonal patterns of bright lineations. Intersection geometries of the lineations defining the polygonal patterns are typical of those found in tensile networks. In addition, the polygonal patterns generally exhibit no preferred orientation, implying that they are the result of horizontally isotropic stress fields. Such stress fields usually arise on the Earth as a consequence of desiccation, freeze‐thaw cycles, or cooling and produce mud cracks, ice‐wedge polygons, and columnar joints, respectively. We propose that the polygonal patterns seen in the Magellan images of some of the volcanic plains are the result of thermal stresses. We consider two alternative scenarios which would generate sufficient tensile thermal stresses to cause failure. The first scenario is that of a cooling lava flow; the residual thermal stress which would develop (assuming no failure of the rock) is tensional and of the order of 400 MPa. This is much greater than the strength of unfractured terrestrial basalt (∼10 MPa), so we can expect joints to form during cooling of Venusian lava flows. However, the spacing of the polygonal lineations seen in Magellan images is typically 1–2 km, much larger than the largest spacings of decimeters for joints in terrestrial lavas. The second scenario involves an increased heat flux to the base of the lithosphere; the resulting thermal stresses cause the upper lithosphere to be in tension and the lower lithosphere to be in compression. Brittle tensile failure occurs near the surface due to the finite yield strength of the lithosphere. The maximum depth to which failure occurs increases with increasing elevation of the temperature gradient. For an initially 25‐km‐thick lithosphere and temperature gradient of 11°/km, this maximum depth varies from 0.5 km to 2 km as the temperature gradient is increased to 12°/km and 22°/km, respectively. Both the cooling flow scenario and the heated lithosphere scenario produce isotropic tensile surface stress patterns, but the heated lithosphere model is more compatible with the kilometer scale of the polygonal patterns seen in Magellan images.
A note on dust grain charging in space plasmas
Authors: Rosenberg M. , Mendis D. A.
Published: May 2009

Central to the study of dust‐plasma interactions in the solar system is the electrostatic charging of dust grains. While previous calculations have generally assumed that the distributions of electrons and ions in the plasma are Maxwellian, most space plasmas are observed to have non‐Maxwellian tails and can often be fit by a generalized Lorentzian (kappa) distribution. Here we use such a distribution to reevaluate the grain potential, under the condition that the dominant currents to the grain are due to electron and ion collection, as is the case in certain regions of space. The magnitude of the grain potential is found to be larger than that in a Maxwellian plasma as long as the electrons are described by a kappa distribution: this enhancement increases with ion mass and decreasing electron kappa. The modification of the grain potential in generalized Lorentzian plasmas has implications for both the physics (e.g., grain growth and disruption) and the dynamics of dust in space plasmas. These are also briefly discussed in this short note.
Pancakelike domes on Venus
Authors: McKenzie Dan , Ford Peter G. , Liu Fang , Pettengill Gordon H.
Published: May 2009

The shape of seven large domes on the plains of Venus, with volumes between 100 and 1000 km3, is compared with that of an axisymmetric gravity current spreading over a rigid horizontal surface. Both the altimetric profiles and the horizontal projection of the line of intersection of domes on the synthetic aperture radar images agree well with the theoretical similarity solution for a Newtonian fluid but not with the shape calculated for a rigid‐plastic rheology nor with that for a static model with a strong skin. As a viscous current spreads, it generates an isotropic strain rate tensor whose magnitude is independent of radius. Such a flow can account for the randomly oriented cracks that are uniformly distributed on the surface of the domes. The stress induced by the flow in the plains material below is obtained and is probably large enough to produce the short radial cracks in the surface of the plains beyond the domes. The viscosity of the domes can be estimated from their thermal time constants if spreading is possible only when the fluid is hot and lies between 1014 and 1017 Pa s. Laboratory experiments show that such viscosities correspond to temperatures of 610° to 700°C in dry rhyolitic magmas. These temperatures agree with laboratory measurements of the solidus temperature of wet rhyolite. These results show that the development of the domes can be understood using simple fluid dynamical ideas and that the magmas involved can be produced by wet melting at depths below 10 km, followed by eruption and degassing.
Magellan mission summary
Authors: Saunders R. S. , Spear A. J. , Allin P. C. , Austin R. S. , Berman A. L. , Chandlee R. C. , Clark J. , Decharon A. V. , De Jong E. M. , Griffith D. G. , Gunn J. M. , Hensley S. , Johnson W. T. K. , Kirby C. E. , Leung K. S. , Lyons D. T. , Michaels G. A. , Miller J. , Morris R. B. , Morrison A. D. , Piereson R. G. , Scott J. F. , Shaffer S. J. , Slonski J. P. , Stofan E. R. , Thompson T. W. , Wall S. D.
Published: May 2009

Magellan started mapping the planet Venus on September 15, 1990, and after one cycle (one Venus day or 243 Earth days) had mapped 84% of the planet's surface. This returned an image data volume greater than all past planetary missions combined. Spacecraft problems were experienced in flight. Changes in operational procedures and reprogramming of onboard computers minimized the amount of mapping data lost. Magellan data processing is the largest planetary image‐processing challenge to date. Compilation of global maps of tectonic and volcanic features, as well as impact craters and related phenomena and surface processes related to wind, weathering, and mass wasting, has begun. The Magellan project is now in an extended mission phase, with plans for additional cycles out to 1995. The Magellan project will fill in mapping gaps, obtain a global gravity data set between mid‐September 1992 and May 1993, acquire images at different view angles, and look for changes on the surface from one cycle to another caused by surface activity such as volcanism, faulting, or wind activity.
Comparison of Goldstone and Magellan radar data in the equatorial plains of Venus
Authors: Plaut Jeffrey J. , Arvidson Raymond E.
Published: May 2009

Goldstone radar observations of the equatorial plains of Venus provide complementary information to that obtained by Magellan. Different radar scattering mechanisms dominate each system, leading to a sampling of different surface properties. Comparison of image data and derived parameters indicate that (1) relatively high dielectric constants on impact‐related parabolic features are detected in Goldstone backscatter, Magellan reflectivity, and Magellan emissivity data; the dielectric effects are overwhelmed by roughness‐related signatures in Magellan synthetic aperature radar (SAR) data, (2) lava flows in Navka Planitia show dielectric variations both among and within flows; higher dielectric constants on the perimeter of some flows may be due to a decrease in vesicularity, (3) some volcanic domes are relatively smooth at the wavelength scale and probably consist of low‐density deposits, (4) comparisons of Magellan SAR data with rough surface scattering models and SAR data of terrestrial surfaces indicate that the roughness characteristics of the equatorial plains surfaces are comparable to modified terrestrial lava flows, and (5) scattering properties of an equatorial “ridge belt” structure suggest highly weathered or soil‐dominated surfaces.
Asteroid core crystallization by inward dendritic growth
Authors: Haack Henning , Scott Edward R. D.
Published: May 2009

To better understand the geology of metallic asteroids and the crystallization of planetary cores, we have studied the crystallization of the cores of iron meteorite parent bodies. Available data for the relative magnitudes of the adiabatic gradient and the liquidas gradient across the core indicate that for the low pressures of asteroidal cores (less than a few hundred megapascals), crystallization commences at the base of the mantle and continues as dominantly inward growth. A consequence of the rejection of S from the crystallizing solid is that a light S‐rich layer forms below the crystallizing front. This inhibits concentric front growth and promotes formation of tree‐like crystals called dendrites. The mode of crystallization was therefore completely different from that of Earth. Dendrites in asteroidal cores may have grown to lengths of hundreds of meters or perhaps even as large as the core radius thereby dividing the core into separate magma chambers. Supporting evidence comes from the Cape York iron meteorites and others from the same core: the surface area of the solid/liquid interface was very large; the direction of crystallization was horizontal; and the liquid was not perfectly mixed throughout crystallization. We argue that the Cape York irons did not form as a result of a rare event; they are typical products of core crystallization. Troilite, which has different optical and mechanical properties from Fe‐Ni, formed in pockets between dendrites. The distribution of troilite may therefore provide a visual record of the crystallization history of asteroidal cores.
Geological correlations with the interior density structure of Venus
Authors: Herrick Robert R. , Phillips Roger J.
Published: May 2009

A model has been constructed by stipulating that Venusian long‐wavelength topography and geoid elevations are explained by the sum of density anomalies associated with mantle convection and lithospheric compensation mechanisms. For convenience the latter are represented by a single Airy surface because individual lithospheric components cannot be resolved at the wavelengths considered in this paper. Mantle convection is represented at a single depth by mass perturbations that drive viscous flow. We invert spherical harmonic representations of the topography and the geoid to create global representations of the mantle convection pattern and near‐surface density anomalies. The mantle convection pattern matches well with previous numerical simulations of mantle convection that show a pattern of isolated upwellings amidst an interconnected network of downwellings. The near‐surface density anomalies show no particular pattern. The inversion results have been compared with a global synthesis of Magellan data. Impact crater density does not correlate with the mantle convection pattern or the near‐surface density anomalies. Large volcanic structures are associated with upwellings. Planitiae are associated with downwellings. Coronae are anticorrelated with high‐amplitude upwellings and downwellings and apparently are passive features associated with rifting that is related to mantle convection in a complex fashion. Tesserae are associated with negative lithospheric density anomalies interpreted as areas of thickened crust. Tesserae show no definitive relationship to the present mantle convection pattern.
High‐resolution gravity model of Venus
Authors: Reasenberg R. D. , Goldberg Z. M.
Published: May 2009

The anomalous gravity field of Venus shows high correlation with surface features revealed by radar. We extract gravity models from the Doppler tracking data from the Pioneer Venus Orbiter (PVO) by means of a two‐step process. In the first step, we solve the nonlinear spacecraft state estimation problem using a Kalman filter‐smoother. The Kalman filter has been evaluated through simulations. This evaluation and some unusual features of the filter are discussed. In the second step, we perform a geophysical inversion using a linear Bayesian estimator. To allow an unbiased comparison between gravity and topography, we use a simulation technique to smooth and distort the radar topographic data so as to yield maps having the same characteristics as our gravity maps. The maps presented cover 2/3 of the surface of Venus and display the strong topography‐gravity correlation previously reported. The topography‐gravity scatter plots show two distinct trends.
Impact craters and Venus resurfacing history
Authors: Phillips Roger J. , Raubertas Richard F. , Arvidson Raymond E. , Sarkar Ila C. , Herrick Robert R. , Izenberg Noam , Grimm Robert E.
Published: May 2009

Venusian impact crater size‐frequency distributions, locations, and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism. An atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km, the size‐frequency distribution is close to the atmosphere‐free case. With this result, and assuming that the surface records a crater production population (a catastrophic resurfacing model, CRM), an age of cessation of rapid resurfacing of ∼ 500 Ma is obtained. Crater locations are widely dispersed across Venus and the hypothesis that they are completely spatially random (CSR) cannot be rejected. However, craters that show embayment by plains materials or modification by throughgoing faults (i.e., tectonized) are preferentially found in areas with relatively few craters overall. The primary region where these modified craters are found is the Aphrodite volcanotectonic zone, extending from Ovda Regio on the west to the region east of Atla Regio. These results, together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy, imply that a range of surface ages are recorded by the impact crater population; e.g., the Aphrodite zone is relatively young. An end‐member model (equilibrium resurfacing model, ERM) was developed to quantify resurfacing scenarios. In the ERM, Venus has been resurfacing at an average rate of approximately 1 km2 yr−1. However, the CRM and ERM are idealized end‐member representations of possible resurfacing histories. For both models, the resurfacing rate can be expressed as the product of resurfacing patch area a (normalized by planetary surface area) and the frequency ω of resurfacing events. Numerical simulations of resurfacing showed that there are two solution branches that satisfy the CSR constraint: a < 0.0003 (4° diameter circle ) and a > 0.1 (74° diameter circle). The former range corresponds to resurfacing diameters smaller than the average intercrater distance, whereas the latter is associated with large, infrequent events, resurfacing 10% of the planet every 50 Ma to 100% every 500 Ma. The observed fraction of embayed and tectonized craters further constrains values of a and only values near 0.0003 are admissible. The resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs, in any given geological episode, in only a limited number of regions on the planet.
Caldera subsidence and magma chamber depth of the Olympus Mons volcano, Mars
Authors: Zuber M. T. , Mouginis‐Mark P. J.
Published: May 2009

Observations of the distribution of tectonic features and their relationship to local surface topography indicate that the floor of the oldest and largest crater of the summit caldera of Mars' Olympus Mons volcano may have undergone subsidence in response to depressurization of a subsurface magma chamber. In this study, we construct an axisymmetric finite element model to calculate elastic stresses in a volcanic edifice to investigate the relationship between surface tectonism, caldera subsidence, and the physical characteristics of Olympus Mons' magmatic reservoir. Constraints on the model are provided by the stress field within the crater indicated by the distribution of radial ridges and circumferential graben that we hypothesize formed due to a postcollapse and postresurfacing phase of subsidence of the caldera floor. Model results show that the surface stress state is not strongly sensitive to the aspect ratio or pressure distribution of the magma chamber, or to the contrast in stiffness between the magma chamber and surroundings, but is strongly dependent on the width and depth of the chamber. For a range of plausible model parameters, we find the maximum depth to the top of the magma chamber to have been ≤16 km, which indicates that the chamber, at the time of crater floor subsidence, was positioned within the Olympus Mons edifice, at a level much shallower than the estimated source depth of Martian magmas (∼140–200 km). The allowable range of solutions indicates that the vertical position of the magma chamber may have been controlled by the neutral buoyancy level of ascending magma. Our results suggest a gross similarity between the configurations of the magmatic plumbing systems of Olympus Mons and several well‐studied terrestrial volcanoes such as the Hawaiian shields.
Energetic electron precipitation in Jupiter's upper atmosphere
Authors: Singhal R. P. , Chakravarty S. C. , Bhardwaj A. , Prasad B.
Published: May 2009

Energy deposition by electrons having a Maxwellian energy distribution with characteristic energies 10, 30, and 100 keV, precipitating in the high‐latitude upper atmosphere of Jupiter, has been studied using a continuous slowing down approximation. Electron fluxes, volume excitation, and ionization rates have been calculated. Chemical equilibrium equations have been solved for 24 ionic species using extensive hydrocarbon chemistry and incorporating diffusive transport for the ion H+. H2 Lyman and Werner bands and H Ly α intensities are obtained considering pure absorption in hydrocarbons. Comparison with Voyager ultraviolet spectrometer data requires incident energy fluxes of about 10, 18, and 45 ergs cm−2 s−1 for characteristic energies 10, 30, and 100 keV, respectively, for polar model methane abundance. Numerical experiments have been performed to study the effect of changing atomic hydrogen and methane number density, three‐body reaction rates, incident energy flux, and H2(ν≥4) vibrational temperature on plasma densities. Electrons with characteristic energy 30 keV or somewhat higher give good overall agreement with Voyager 2 electron densities and also simulate the low‐altitude peak measured by Pioneer 11. The calculated bremsstrahlung X ray flux is smaller by 1 to 2 orders of magnitude than the observed low‐energy (<2 keV) X ray flux. New observations of high‐energy (>2 keV) bremsstrahlung X ray emissions are required to give a definite resolution of the identity and energy of the particles responsible for the aurora on Jupiter.
Mariner 10 multispectral images of the eastern limb and farside of the Moon
Authors: Robinson Mark S. , Hawke B. Ray , Lucey Paul G. , Smith Gregory A.
Published: May 2009

We present the first analysis of Mariner 10 multispectral image data for the eastern limb and farside of the Moon. A new, rigorous calibration of these data is used that provides results that are consistent with independently derived data sets. From these newly calibrated data, a color ratio image (0.48/0.58 μm) is produced to show relative TiO2 abundances in mare units on the eastern limb and farside of the Moon. Our results indicate that the farside lunar mare deposits examined have TiO2 contents that are intermediate to low (2–5% TiO2) relative to mare deposits on the nearside. We find no evidence for areally extensive deposits of either very low titanium (VLT) or high to very high titanium (VHT) basalts in these portions of the Moon. Our findings support previous interpretations that the source regions for lunar mare basalts are laterally heterogeneous. Additionally, examination of highland units reveals a relatively blue region that corresponds with a previously proposed cryptomare, suggesting enhanced TiO2 for this unit. Finally, based on these Mariner 10 data, telescopic spectral data, Galileo multispectral image data, and lunar meteorite analyses we speculate that volcanism in farside mare deposits did not include high titanium basalts (> 8 wt %).
Trapped xenon in ordinary chondrites
Authors: Lavielle Bernard , Marti Kurt
Published: May 2009

A trapped component of heavy noble gases representing a distinct solar system (presumably asteroid belt) reservoir is known to be concentrated in carbonaceous carriers of chondritic meteorites. The isotopic signature of trapped Xe in separates of the H4 chondrite Forest Vale (FV) was determined by combusting its carrier phases at 600°C in oxygen, discriminating against in situ produced nucleogenic components which are released above 600°C. The isotopic abundances of FVC (combustion)‐Xe are compared to signatures of bulk trapped Xe in chondritic meteorites. We conclude that FVC‐Xe represents the predominant trapped component in ordinary chondrites (OC) for which we adopt the term OC‐Xe. Its isotopic signature differs from Xe in ureilites, in “average carbonaceous chondrites”, in Earth's atmosphere, and in the solar wind. Additional minor Xe components were identified in type 3 chondrites and in the metal phase of chondrites. We discuss relationships among solar system Xe reservoirs and show that the OC‐Xe signature is consistent with a mixture of HL‐Xe with slightly mass fractionated solar‐type Xe.
Effects of dispersed particulates on the rheology of water ice at planetary conditions
Authors: Durham William B. , Kirby Stephen H. , Stern Laura A.
Published: May 2009

We have investigated the effects of initial grain size and hard particulate impurities on the transient and steady state flow of water ice I at laboratory conditions selected to provide more quantitative constraints on the thermomechanical evolution of the giant icy moons of the outer solar system. Our samples were molded with particulate volume fractions, ϕ, of 0.001 to 0.56 and particle sizes of 1 to 150 μm. Deformation experiments were conducted at constant shortening rates of 4.4 × 10−7 to 4.9 × 10−4 s−1 at pressures of 50 and 100 MPa and temperatures 77 to 223 K. For the pure ice samples, initial grain sizes were 0.2–0.6 mm, 0.75–1.75 mm, and 1.25–2.5 mm. Stress‐strain curves of pure ice I under these conditions display a strength maximum σu at plastic strains ε ≤ 0.01 after initial yield, followed by strain softening and achievement of steady state levels of stress, σss, at ε = 0.1 to 0.2. Finer starting grain size in pure ice generally raises the level of σu. Petrography indicates that the initial transient flow behavior is associated with the nucleation and growth of recrystallized ice grains and the approach to σss evidently corresponds to the development of a steady state grain texture. Effects of particulate concentrations ϕ < 0.1 are slight. At these concentrations, a small but significant reduction in σu with respect to that for pure water ice occurs. Mixed‐phase ice with ϕ ≥ 0.1 is significantly stronger than pure ice; the strength of samples with ϕ = 0.56 approaches that of dry confined sand. The magnitude of the strengthening effect is far greater than expected from homogeneous strain‐rate enhancement in the ice fraction or from pinning of dislocations (Orowan hardening). This result suggests viscous drag occurs in the ice as it flows around the hard particulates. Mixed‐phase ice is also tougher than pure ice, extending the range of bulk plastic deformation versus faulting to lower temperatures and higher strain rates. The high‐pressure phase ice II formed in ϕ = 0.56 mixed‐phase ice during deformation at high stresses. Bulk planetary compositions of ice + rock (ϕ = 0.4–0.5) are roughly 2 orders of magnitude more viscous than pure ice, promoting the likelihood of thermal instability inside giant icy moons and possibly explaining the retention of crater topography on icy planetary surfaces.
Axisymmetric spherical shell models of mantle convection with variable properties and free and rigid lids
Authors: Leitch A. M. , Yuen D. A. , Lausten C. L.
Published: May 2009

Axisymmetric spherical shell numerical simulations of mantle convection were carried out to investigate the influence of two end‐member surface stress conditions: stress‐free and rigid. These correspond approximately to a subducting or a rigid lithosphere and can be seen as end‐member models of the surface of Venus. Our model assumed an effective Rayleigh number of 3×106, similar to that for Earth, and included uniform internal heating and depth‐dependent thermal expansivity α and thermal conductivity k. The simulations utilized a Newtonian viscosity which was constant or varied with depth and/or temperature. We show how the temperature, speed and vorticity fields change qualitatively and quantitatively with surface temperature, surface stress condition, internal heating and viscosity distribution. Among the significant results, we find that a rigid lid and viscosity which increases with depth both promote steady large‐scale circulation with smaller‐scale circulation in the upper mantle.
Regulation of actin function by protein kinase A‐mediated phosphorylation of Limk1
Authors: Nadella Kiran S , Saji Motoyasu , Jacob Naduparambil K , Pavel Emilia , Ringel Matthew D , Kirschner Lawrence S
Published: May 2009

Proper regulation of the cAMP‐dependent protein kinase (protein kinase A, PKA) is necessary for cellular homeostasis, and dysregulation of this kinase is crucial in human disease. Mouse embryonic fibroblasts (MEFs) lacking the PKA regulatory subunit Prkar1a show altered cell morphology and enhanced migration. At the molecular level, these cells showed increased phosphorylation of cofilin, a crucial modulator of actin dynamics, and these changes could be mimicked by stimulating the activity of PKA. Previous studies of cofilin have shown that it is phosphorylated primarily by the LIM domain kinases Limk1 and Limk2, which are under the control of the Rho GTPases and their downstream effectors. In Prkar1a−/− MEFs, neither Rho nor Rac was activated; rather, we showed that PKA could directly phosphorylate Limk1 and thus enhance the phosphorylation of cofilin. These data indicate that PKA is crucial in cell morphology and migration through its ability to modulate directly the activity of LIM kinase.
Simulation and analysis of electron channeling contrast images of threading screw dislocations in 4H-SiC
Authors: Twigg M. E. , Picard Y. N.
Published: May 2009

Using methods developed for modeling diffraction contrast of extended defects in thin foils, electron intensity profiles are simulated and found to qualitatively match channeling contrast of threading screw dislocations (TSDs) experimentally recorded by electron channeling contrast imaging (ECCI) using scanning electron microscopy. Plan-view images of TSDs axially penetrating (0001) 4H-SiC surfaces were computed using the Sturkey scattering matrix approach incorporating surface relaxation effects. Simulated diffraction contrast of the TSD allows identification of these threading defects as well as facilitates the determination of the dislocation Burgers vector. The directionality of TSD contrast features, simulated for various Bragg reflections and deviation parameters, is consistent with both ECCI and diffraction contrast imaging by transmission electron microscopy. Topographically enhanced imaging of atomic step spirals, generated by the TSDs, provides a direct determination of the TSD Burgers vector, which is shown to agree with the simulated ECCI contrast. The convergence of the incident electron beam is also shown to influence channeling contrast.
Bragg diffraction of guided optical waves by spin dipole waves in a ferrimagnetic heterostructure
Authors: Kolokoltsev O. V. , Grishin A. M. , Pérez Oliver Cortés , Ordóñez-Romero C. L.
Published: May 2009

The peculiarities of the Bragg diffraction of guided optical waves (GOWs) by spin-dipole waves (SDWs) in a ferrite heterostructure, based on Bi3Fe5O12 (BIG) thin film deposited on a standard YIG/GGG sample, is presented. It is shown that the efficiency of waveguide magneto-optic interaction between GOWs and SDWs in BIG/YIG/GGG can be 4–14 times large compared to the standard YIG/GGG waveguide, even in the case when the interacting waves are localized in YIG waveguide layer.
High temperature neutron diffraction studies of 0.9BiFeO3–0.1PbTiO3
Authors: Comyn Tim P. , Stevenson Tim , Al-Jawad Maisoon , Turner Stuart L. , Smith Ronald I. , Bell Andrew J. , Cywinski Robert
Published: May 2009

Neutron diffraction data were collected from sintered polycrystalline 0.9BiFeO3–0.1PbTiO3 at temperatures between 293 and 793 K and the crystal and magnetic structures refined by the Rietveld method. An antiferromagnetic Néel temperature of 592 K was determined from a Brillouin fit to the refined magnetic moments with a ground state of 4.34μB, showing no loss of moment per Fe site compared to BiFeO3. Some magnetic order was observed to persist above the Néel temperature. A maximum in the spontaneous rhombohedral strain was observed close to the Néel temperature, in a significant departure from typical Landau behavior for a ferroelectric and suggesting a coupling between the magnetic and nuclear structures.
Effects of electron-boundary scattering on changes in thermoreflectance in thin metal films undergoing intraband excitations
Authors: Hopkins Patrick E.
Published: May 2009

As characteristic sizes and lengths scales continue to decrease in nanostructures, carrier scattering processes at the geometric boundaries and interfaces in nanosystems become more prevalent. These scattering events can lead to additional resistances. This paper investigates electron-boundary scattering processes by examining changes in thermoreflectance signals in thin films after short pulsed laser heating. To take electron-boundary scattering into account, an additional scattering term is introduced into the Drude model for the complex dielectric function. Using an intraband thickness-dependent reflectance model, transient thermoreflectance data of Au films subject to intraband excitations are analyzed with the electron-boundary scattering Drude model introduced in this work. The electron-boundary scattering rate is determined from Au thermoreflectance data, showing that after short pulsed laser heating, electron-boundary scattering rates can be almost three orders of magnitude greater than the electron-electron and electron-phonon scattering rates. The scattering rates determined from the thermoreflectance data agree well with the theoretical predictions for electron-boundary scattering calculated from an electron-boundary scattering model for disordered conductors in the event of an electron-phonon nonequilibrium.
Controllable modification of self-assembled monolayer surface by using N2 neutral beam process
Authors: Ishikawa Yasushi , Okumura Keiji , Ishida Takao , Samukawa Seiji
Published: May 2009

A novel technique of low-energy N2 neutral beam (NB) (<10 eV) irradiation was investigated as a means of controlling the surface nitridation of self-assembled monolayers (SAMs). This low-energy process without UV photons was able to nitride the surfaces of terphenyl SAMs. Furthermore, by varying the on/off period for irradiation, a pulse-time modulated NB process could be used to control the atomic ratio of nitrogen in terphenyl molecules. Time-of-flight secondary ion mass spectra revealed that one H in the terphenyl group was substituted by a NH2 group because of irradiation by the N2 beam. The data suggested that the low-energy NB technique caused less damage to modified surfaces than the conventional plasma process did. Therefore, we believe that it is a potential damage-free process for modifying the surfaces of soft organic materials and films.
Beam loading by electrons in nonlinear plasma wakes
Authors: Tzoufras M. , Lu W. , Tsung F. S. , Huang C. , Mori W. B. , Katsouleas T. , Vieira J. , Fonseca R. A. , Silva L. O.
Published: May 2009

An analytical theory for the interaction of an electron bunch with a nonlinear plasma wave is developed to make it possible to design efficient laser- and/or beam-driven accelerators that generate high quality monoenergetic electron beams. This theory shows how to choose the charge, the shape, and the placing of the bunch so that the conversion efficiency from the fields of the bubble to the accelerating electrons reaches nearly 100% and the beam quality is optimized. For intense drivers the nonlinear wake is described by the shape of the bubble and beam loading arises when the radial space-charge force of the beam acts back on the electron sheath surrounding the ion channel. The modification of the wake due to the presence of flat-top electron bunches is studied and it is shown that the energy spread of an externally injected flat-top electron bunch can be kept low. The bunch profile that leads to zero energy spread is also derived.
Electronic transport of unimolecular devices with a group coadsorbed on one electrode surface: A density functional study
Authors: Zhang Z. H. , Qiu M. , Deng X. Q. , Ding K. H. , Zhang H.
Published: May 2009

The first-principles calculations based on the density functional theory are applied to investigate the effect of a chemical group coadsorbed on one electrode surface on the electronic transport of a molecular device. We find that the types of the coadsorbed groups and their sites on one electrode surface affect the electronic transport significantly, and the resulting shift of the molecular levels upon coadsorption depends jointly on various effects, such as the electrostatic interaction, shift of the Fermi level of the electrode, the chemical interaction, and so on. Among these factors, the chemical interaction-induced the charge transfer across metal-molecule interface is identified as a determining factor resulting in the variation of transport properties. Our findings suggest that the coadsorption may offer the novel possibility to modify the transport behaviors of a molecular device in a controlled way and can improve/add some particular functionalities or should be avoided in order to keep a stable transport for a molecular device.