Plasticity and the evolution of group-level regulation of cellular differentiation in the volvocine algae
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Dinah R. Davison↗
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Model of a rotating magnetic cloud
The possibility that magnetic clouds rotate while they propagate antisunward was investigated. Magnetic clouds are modeled as magnetic flux ropes which rotate rigidly about the axis of symmetry. An ideal magnetohydrodynamic model, in which the evolution of the magnetic structure is related to the time evolution of the angular frequency, is developed. A class of 'separable' magnetic fields is employed to reduce the problem to a nonlinear ordinary differential equation for the evolution function, and it is solved numerically. The corresponding effective potential gives rise to two modes of evolution--expansion and oscillation--depending on the energy and on the value of a dimensionless parameter, k. Parameter k depends on the gas pressure, the ratio of the magnetic field components, and the frequency of rotation. There is a critical value of k, k(sub c), above which the oscillatory regime disappears and the flux rope invariably expands, regardless of the energy. Below k(sub c) the energy determines whether the configuration is confined or unbounded. Rotation always helps expansion by lowering the potential barrier. A data example was studied and features which are interpreted as signatures of rotation are presented. The angular speed is comparable to the Alfven speed, and the core of the rotating cloud completes on average one full revolution every three days at 1 AU. The parameter k is calculated from observations, and it is found to be close to, but below, critical. Only three out of the nine clouds examined showed signatures of rotation. Theoretical analysis suggests that close to the Sun rotation effects may play a more important role in the evolution of magnetic clouds than 1 AU.
Farrugia, C. J.↗
Nonlinear evolution of magnetic flux ropes. 2: Finite beta plasma
In this second paper on the evolution of magnetic flux ropes we study the effects of gas pressure. We assume that the energy transport is described by a polytropic relationship and reduce the set of ideal MHD equations to a single, second-order, nonlinear, ordinary differential equation for the evolution function. For this conservative system we obtain a first integral of motion. To analyze the possible motions, we use a mechanical analogue -- a one-dimensional, nonlinear oscillator. We find that the effective potential for such an oscillator depends on two parameters: the polytropic index gamma and a dimensionless quantity kappa the latter being a function of the plasma beta, the strength of the azimuthal magnetic field relative to the axial field of the flux rope, and gamma. Through a study of this effective potential we classify all possible modes of evolution of the system. In the main body of the paper, we focus on magnetic flux ropes whose field and gas pressure increase steadily towards the symmetry axis. In this case, for gamma greater than 1 and all values of kappa, only oscillations are possible. For gamma less than 1, however, both oscillations and expansion are allowed. For gamma less than 1 and kappa below a critical value, the energy of the nonlinear oscillator determines whether the flux rope will oscillate or expand to infinity. For gamma less than 1 and kappa above critical, however, only expansion occurs. Thus by increasing kappa while keeping gamma fixed (less than 1), a phase transition occurs at kappa = kappa(sub critical) and the oscillatory mode disappears. We illustrate the above theoretical considerations by the example of a flux rope of constant field line twist evolving self-similarly. For this example, we present the full numerical MHD solution. In an appendix to the paper we catalogue all possible evolutions when (1) either the magnetic field or (2) the gas pressure decreases monotonically toward the axis. We find that in these cases critical conditions can occur for gamma greater than 1. While in most cases the flux rope collapses, there are notable exceptions when, for certain ranges of kappa and gamma, collapse may be averted.
Osherovich, V. A.↗
Shock effects in meteorites
The impacts that can occur between objects on intersecting solar system orbits can generate shock-induced deformations and transformations, creating new mineral phases or melting old ones. These shock-metamorphic effects affect not only the petrography but the chemical and isotopic properties and the ages of primordial meteoritic materials. A fuller understanding of shock metamorphism and breccia formation in meteorites will be essential not only in the study of early accretion, differentiation, and regolith-evolution processes, but in the characterization of the primordial composition of the accreted material itself.
Stoeffler, D.↗
Dynamics of aging magnetic clouds
The dynamics of radially expanding magnetic clouds is rigorously analyzed within the framework of ideal MHD. The cloud is modelled as a cylindrically symmetric magnetic flux rope. In the force balance we include the gas pressure gradient and the Lorentz force. Interaction with the ambient solar wind due to expansion of the magnetic cloud is represented by a drag force proportional to the bulk velocity. We consider the self-similar expansion of a polytrope, and reduce the problem to an ordinary nonlinear differential equation for the evolution function. Analyzing the asymptotic behavior of the evolution function, we formulate theoretical expectations for the long-term behavior of cloud parameters. We focus on the temporal evolution of (1) the magnetic field strength; (2) the twist of the field lines; (3) the asymmetry of the total field profile; and (4) the bulk flow speed. We present data from two magnetic clouds observed at 1 AU and 2 AU, respectively, and find good agreement with theoretical expectations. For a peak magnetic field strength at 1 AU of 25 nT and a polytropic index of 0.5, we find that a magnetic cloud can be distinguished from the background interplanetary field up to a distance of about 5 AU. Taking larger magnetic fields and bigger polytropic indices this distance can double.
Osherovich, V. A.↗
Origin and thermal evolution of Mars
The thermal evolution of Mars is governed by subsolidus mantle convection beneath a thick lithosphere. Models of the interior evolution are developed by parameterizing mantle convective heat transport in terms of mantle viscosity, the superadiabatic temperature rise across the mantle and mantle heat production. Geological, geophysical, and geochemical observations of the composition and structure of the interior and of the timing of major events in Martian evolution, such as global differentiation, atmospheric outgassing and the formation of the hemispherical dichotomy and Tharsis, are used to constrain the model computations. Isotope systematics of SNC meteorites suggest core formation essentially contemporaneously with the completion of accretion. Other aspects of this investigation are discussed.
Schubert, G.↗
Hydrodynamics of rotating stars and close binary interactions: Compressible ellipsoid models
We develop a new formalism to study the dynamics of fluid polytropes in three dimensions. The stars are modeled as compressible ellipsoids, and the hydrodynamic equations are reduced to a set of ordinary differential equations for the evolution of the principal axes and other global quantities. Both viscous dissipation and the gravitational radiation reaction are incorporated. We establish the validity of our approximations and demonstrate the simplicity and power of the method by rederiving a number of known results concerning the stability and dynamical oscillations of rapidly rotating polytropes. In particular, we present a generalization to compressible fluids of Chandrasekhar's classical results for the secular and dynamical instabilities of incompressible Maclaurin spheroids. We also present several applications of our method to astrophysical problems of great current interest, such as the tidal disruption of a star by a massive black hole, the coalescence of compact binaries driven by the emission of gravitational waves, and the development of instabilities in close binary systems.
Lai, Dong↗
Hydrodynamics of coalescing binary neutron stars: Ellipsoidal treatment
We employ an approximate treatment of dissipative hydrodynamics in three dimensions to study the coalescence of binary neutron stars driven by the emission of gravitational waves. The stars are modeled as compressible ellipsoids obeying a polytropic equation of state; all internal fluid velocities are assumed to be linear functions of the coordinates. The hydrodynamics equations then reduce to a set of coupled ordinary differential equations for the evolution of the principal axes of the ellipsoids, the internal velocity parameters, and the binary orbital parameters. Gravitational radiation reaction and viscous dissipation are both incorporated. We set up exact initial binary equilibrium configurations and follow the transition from the quasi-static, secular decay of the orbit at large separation to the rapid dynamical evolution of the configurations just prior to contact. A hydrodynamical instability resulting from tidal interactions significantly accelerates the coalescence at small separation, leading to appreciable radial infall velocity and tidal lag angles near contact. This behavior is reflected in the gravitational waveforms and may be observable by gravitational wave detectors under construction. In cases where the neutron stars have spins which are not aligned with the orbital angular momentum, the spin-induced quadrupole moment can lead to precession of the orbital plane and therefore modulation of the gravitational wave amplitude even at large orbital radius. However, the amplitude of the modulation is small for typical neutron star binaries with spins much smaller than the orbital angular momentum.
Lai, Dong↗
Transient nucleation in glasses
Nucleation rates in condensed systems are frequently not at their steady state values. Such time dependent (or transient) nucleation is most clearly observed in devitrification studies of metallic and silicate glasses. The origin of transient nucleation and its role in the formation and stability of desired phases and microstructures are discussed. Numerical models of nucleation in isothermal and nonisothermal situations, based on the coupled differential equations describing cluster evolution within the classical theory, are presented. The importance of transient nucleation in glass formation and crystallization is discussed.
Kelton, K. F.↗
Ring-Down Spectroscopy for Characterizing a CW Raman Laser
.A relatively simple technique for characterizing an all-resonant intracavity continuous-wave (CW) solid-state Raman laser involves the use of ring-down spectroscopy. As used here, characterizing signifies determining such parameters as threshold pump power, Raman gain, conversion efficiency, and quality factors (Q values) of the pump and Stokes cavity modes. Heretofore, in order to characterize resonant-cavity-based Raman lasers, it has usually been necessary to manipulate the frequencies and power levels of pump lasers and, in each case, to take several sets of measurements. In cases involving ultra-high-Q resonators, it also has been desirable to lock pump lasers to resonator modes to ensure the quality of measurement data. Simpler techniques could be useful. In the present ring-down spectroscopic technique, one infers the parameters of interest from the decay of the laser out of its steady state. This technique does not require changing the power or frequency of the pump laser or locking the pump laser to the resonator mode. The technique is based on a theoretical analysis of what happens when the pump laser is abruptly switched off after the Raman generation reaches the steady state. The analysis starts with differential equations for the evolution of the amplitudes of the pump and Stokes electric fields, leading to solutions for the power levels of the pump and Stokes fields as functions of time and of the aforementioned parameters. Among other things, these solutions show how the ring-down time depends, to some extent, on the electromagnetic energy accumulated in the cavity. The solutions are readily converted to relatively simple equations for the parameters as functions of quantities that can be determined from measurements of the time-dependent power levels. For example, the steady-state intracavity conversion efficiency is given by G1/G2 1 and the threshold power is given by Pin(G2/G1)2, where Pin is the steady-state input pump power immediately prior to abrupt switch-off, G1 is the initial rate of decay of the pump field, and G2 is the final rate of decay of the pump field. Hence, it is possible to determine all the parameters from a single ring-down scan, provided that the measurements taken in that scan are sufficiently accurate and complete.
Matsko, Andrey↗
Almahata Sitta and Brecciated Ureilites: Insights into the Heterogeneity of Asteroids and Implications for Sample Return
Analysis of samples returned to terrestrial laboratories enables more precise measurements and a wider range of techniques to be utilized than can be achieved with either remote sensing or rover instruments. Furthermore, returning samples to Earth allows them to be stored and re-examined with future technology. Following the success of the Hayabusa mission, returning samples from asteroids should be a high priority for understanding of early solar system evolution, planetary formation and differentiation. Meteorite falls provide us with materials and insight into asteroidal compositions. Almahata Sitta (AS) was the first meteorite fall from a tracked asteroid (2008 TC3) [1] providing a rare opportunity to compare direct geochemical observations with remote sensing data. Although AS is predominantly ureilitic, multiple chondritic fragments have been associated with this fall [2,3]. This is not unique, with chondritic fragments being found in many howardite samples (as described in a companion abstract [4]) and in brecciated ureilites, some of which are known to represent ureilitic regolith [5-7]. The heterogeneity of ureilite samples, which are thought to all originate from a single asteroidal ureilite parent body (UPB) [5], gives us information about both internal and external asteroidal variations. This has implications both for the planning of potential sample return missions and the interpretation of material returned to Earth. This abstract focuses on multiple fragments of two meteorites: Almahata Sitta (AS); and Dar al Gani (DaG) 1047 (a highly brecciated ureilite, likely representative of ureilite asteroidal regolith).
Ross, A. J.↗
Petrology of Zircon-Bearing Diogenite Northwest Africa 10666
The howardite, eucrite, and diogenite (HED) meteorites are a group of achondrites thought to be derived from the asteroid 4 Vesta, though there is active debate as to whether all diogenites are part of the HED suite. Petrologic investigation of the HED meteorite group provides a means of understanding early planetary differentiation processes and early evolution of planets in our solar system. Diogenites are predominantly coarse grained ortho-pyroxenites with some samples containing appreciable amounts of clinopyroxene, olivine, chromite, and plagioclase. Accessory metal, troilite, and apatite are common. Many diogenites are brecciated, however, there are few poorly to unbrecciated samples. Diogenites are important because they may represent the lower crust of 4 Vesta. Although Mg isotope data indicates that the sources of diogenites are ancient, their crystallization ages are difficult to constrain due to their protracted thermal histories. The limited chronologic data for diogenites also limits the ability to test petrogenetic connections with eucrites and even parent body. A reliable and high closure-temperature isotope system, such as U-Pb in zircon, is needed to address the timing of diogenite igneous crystallization. Description of the textures and mineralogy of diogenites are essential to their classification and understanding their formation, in particular, whether all phases are petrogenetically related. Here, we present detailed petrographic data from a rare zircon-bearing feldspathic diogenite, Northwest Africa (NWA) 10666 and provide textural evidence for igneous crystallization of the zircon.
Tanner, T. B.↗
Chemical and Thermodynamic Constraints on the Thermal Evolution of Eucrites
Vesta is the only differentiated asteroid with a nearly intact crust, making it the candidate for studying early planetary differentiation. It is commonly thought that the howardite, eucrite, and diogenite (HED) clan of meteorites derive from Vesta, and thus the study of HEDs is important for understanding the evolution of primitive bodies in the early solar system [1]. Of particular interest are the unusual trace element abundances in Stannern group eucrites, which have been interpreted as partial melting and melt contamination events that occurred on the parent body during thermal metamorphism[2]. However, some samples that contain evidence of high temperature metamorphism, such as Elephant Moraine (EET) 90020, have anomalous REE patterns that have been interpret-ed multiple ways. For example [3] concluded that the loss of small degrees of partial melt depleted the sample in LREEs while [4] concluded that subsolidus diffusion better explained why depletion of only some highly incompatible elements is observed. The heterogeneous nature of this sample makes reconstructing its petrologic history challenging. Here, we conduct further petrographic and chemical studies on polished thin sections of EET 90020 and compare results to previous studies [3-5]. Additionally, we combine chemical analyses with thermodynamic models in order to refine the constraints on the post-crystallization thermal history of EET 90020. We additionally include studies of Graves Nunataks (GRA) 98098, which also contains evidence of high temperature metamorphism and anomalous geochemical signatures, as well as evidence of solid state diffusion at lower temperatures [6].
Gorce, Jennifer S.↗
The Return of Astromaterials to Earth Over the Next Decade
We are entering a new and golden age of sample return missions. In the coming decade we will harvest samples from Comet P/Wild II and interstellar dust courtesy of the STARDUST Mission (Brownlee et al., 1997), an asteroid (probably 4660 Nereus or 1989ML) by the ISAS MUSES-C Mission (ISAS, 1997), and solar wind by the Genesis Mission. A sample return from Mars is also envisioned as early as 2008, and possibly one from the two moons of Mars. It is, however, sobering to realize that MUSES-C aims to return 3-10 g of sample, STARDUST will provide micrograms of comet and interstellar dust, and Genesis will harvest only few micrograms of atoms. The diminutive size of the returning samples may be a source of concern for petrologists used only to looking at hefty lunar rocks and meteorites. How much sample is really needed to achieve prime science objectives, while maintaining a cost effective mission? The range of geological processes that we will want to address with these samples is staggering, encompassing not merely the entire history of the Solar system, but the history of the elements themselves. The interstellar processes include element formation, production and interactions with radiation, formation of organics, grain condensation and evolution, and interactions with magnetic fields. In the pre-accretionary (nebular) environment we wish to understand grain condensation, evaporation and recondensation, shock, radiation processing, solar energetic particle implantation, gas composition, the magnetic environment, and the evolution of organics. Finally, for solid bodies we wish to examine accretion history, shock, brecciation, impact gardening, metamorphism, aqueous alteration, weathering, exposure history, volcanism, fumarolic activity, differentiation, the magnetic environment, atmosphere evolution, and the evolution of organics. Since 1981, NASA has supported asteroid and comet science by collecting dust grains from these bodies in the stratosphere, and making them available for analysis in laboratories worldwide (Warren and Zolensky, 1994). Over the succeeding 17 years, many new techniques have been developed for these painstaking analyses, by at least 24 different laboratories across the globe. Despite the fact that the particle supply has always exceeded the demand, the painstaking efforts required for most of the nano-scale analyses have resulted in only 1520 grains having been analyzed, with a total mass of only 0.52 micrograms. Thus we really require less sample for analysis than one might imagine.
Zolensky, Michael E.↗
The Abundances of F, Cl, and H2O in 4Vesta from Eucrites
The abundance and distribution of magmatic volatiles (i.e., H, C, N, F, S, and Cl) within the silicate portion of a differentiated planetary body has important consequences on its thermochemical evolution. However, the abundances of magmatic volatiles within differentiated bodies are difficult to quantify, and they are often depleted by varying degrees relative to CI chondrites. The mechanisms of depletion are not well constrained and could relate to intrinsic volatile depletion of the building blocks that formed the bodies, high temperature processes that result from accretion, post-accretion loss through parent body geological processes and large-scale impacts, and/or redistribution within a parent body through processes like core formation [1–4]. In the present study, we aim to constrain the abundances of F, Cl, and H2O in eucrites to better understand the magnitude of volatile depletion on 4Vesta. To accomplish this objective, we report electron microprobe analyses of apatite from seven unbrecciated, non-cumulate eucrites (i.e., CMS 04049,GRA 98098, LEW 88010, MAC 02522, MAC 041169,QUE 94484, and QUE 97053) and two monomict, non-cumulate eucrites (i.e., Berthoud and Stannern). In combination with previously published data on eucrite-hosted apatite, we determine Cl/F and H2O/F ratios in bulk rock eucrites through the application of apatite-based melt hygrometry and chlorometry [e.g., 5–7].Additionally, we estimate the bulk rock abundances of F in six non-cumulate eucrites (i.e., GRA 98098, MAC041169, PCA 91078, QUE 97053, Stannern, and Berthoud), which we combine with previously published bulk rock F data on non-cumulate eucrites[8] to constrain the abundances of F, Cl, and H2O in 4Vesta using appropriately paired volatile/refractory element ratios for F, followed by Cl/F and H2O/F ratios for Cl and H2O, respectively.
F M McCubbin↗
Electron Backscatter Diffraction (EBSD) Analysis and U-Pb Geochronology of the Oldest Lunar Zircon: Constraining Early Lunar Differentiation and Dating Impact-Related Deformation
The evolution of the early moon was dominated by two processes (i) crystallization of the Lunar Magma Ocean (LMO) and differentiation of potassium-rare earth element-phosphorous-rich residual magma reservoir referred to as KREEP, and (ii) an intense meteorite bombardment referred to as lunar cataclysm . The exact timing of these processes is disputed, and resolution relies on collection and interpretation of precise age data. This study examines the microstructure and geochronology of zircon from lunar impact breccias collected during the Apollo 17 mission. A large zircon clast within lunar breccia 72215,195 shows sector zoning in optical microscopy, cathodoluminescence (CL) imaging and Raman mapping, and indicates that it was a relict fragment of a much larger magmatic grain. Sensitive high resolution ion microprobe (SHRIMP) U-Pb analysis of the zircon shows that U and Th concentration correlate with sector zoning, with darkest CL domains corresponding with high-U and Th (approx.150 and approx.100 ppm respectively), and the brightest-CL sectors containing approx.30-50 ppm U and approx.10-20 ppm Th. This indicates that variations in optical CL and Raman properties correspond to differential accumulation of alpha-radiation damage in each sector. Electron backscatter diffraction (EBSD) mapping shows that the quality of electron backscatter patterns (band contrast) varies with sector zoning, with the poorest quality patterns obtained from high-U and Th, dark-CL zones. EBSD mapping also reveals a deformation microstructure that is cryptic in optical, CL and Raman imaging. Two orthogonal sets of straight discrete and gradational low-angle boundaries accommodate approx.12 misorientation across the grain. The deformation bands are parallel to the crystallographic {a}-planes of the zircon, have misorientation axes parallel to the c-axis, and are geometrically consistent with formation by dislocation creep associated with <100>{010} slip. The deformation bands are unlike curved morphology of crystal-plastic microstructures in tectonically deformed terrestrial zircon, and geometrically similar to dislocation microstructures reported in experimentally shocked zircon. We interpret these crystal-plastic deformation microstructures to have resulted from a significant impact, either directly from impact shock, or during ductile flow directly following the impact. The deformation bands appear to continue undeflected through the non-indexed, radiation-damaged areas of the grain, which suggests that the orientation variation predates any significant mechanical weakening from radiation damage in the grain, and therefore occurred early in its history.
Timms, Nick↗
Experimental studies of crystal-melt differentiation in planetary basalt compositions
An important process that controls the evolution of magmas on and within planetary bodies is crystal-melt differentiation. Experimental studies of silicate melt solidification were performed on several planetary and terrestrial melt compositions, and experiments on one of these compositions in the microgravity environment of the space station would provide an opportunity to understand the factors that control crystal growth and crystal-melt exchange processes at crystal-melt interfaces during solidification. Experimental requirements are presented.
Grove, T. L.↗
Trace Element Abundances of Olivine-Hosted Melt Inclusions in Shergottites Northwest Africa 7397 and Robert Massif 04262
Olivine-hosted melt inclusions (MIs) may retain trapped parent magma compositions as well as record progressive differentiation while magmas crystallize and ascend towards the surface [1,2 and references therein]. Major element compositions of the MIs, especially Fe and Mg, can be affected by post-entrapment re-equilibration with their host olivine [1,2]. Therefore, Fe/Mg ratio correction is required to obtain MI bulk compositions following equilibrium with their host olivine. Partition coefficients of most of the trace elements in olivine are very low (i.e. DOL/melt<0.001). Thus, ratios of trace elements of olivine-hosted MIs are unlikely to be affected by post-entrapment re-equilibration and no correction is necessary [2]. Hence, tracking trace element behavior in MIs may constrain the composition of the parent magma and its evolution yielding insights on magma differentiation of shergottites. However, analyzing MIs for chemical compositions is a challenging task due to their low abundances and small sizes. Using a highly sensitive and precise micro-beam technique is essential to examine olivine-hosted MIs in order to measure trace element abundances. For this purpose, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is an excellent tool due to its wide range of laser spot sizes (1-150 μm), ability to obtain raster analysis (several mm2) and lower detection limits (0.1 ppb) [3].
Ferdous, J.↗