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The National Academy’s current Planetary Decadal Survey (NRC, 2011) prioritizes a future Lunar Geophysical Network (LGN) mission to gather new information that will permit us to better determine how the overall composition and structure of the Moon inform us about the initial differentiation and subsequent evolution of terrestrial planets.
The data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft have revealed several surprising characteristics about the surface of Mercury, leading to its classification as a geochemical endmember among the terrestrial planets. Some of these features include elevated abundances of up to 3 wt% S, as much as 4 wt% C enrichment in low-reflectance materials (LRM) over the local mean, Na up to 5 wt% at high northern latitudes, and Fe abundances typically lower than 2 wt% [e.g., 1–4]. The S and Fe concentrations have been used to infer that Mercury’s igneous history evolved under highly reduced oxygen fugacity conditions between 2.6 and 7.3 log(sub 10) units below the iron–wüstite buffer [e.g., 5], which is more reducing than any other terrestrial planet in the solar system [e.g., 6]. This highly reduced nature has important con-sequences for the differentiation and thermal/magmatic evolution of Mercury. While the immense amount of data collected by MESSENGER revealed Mercury as a geochemical endmember, this new knowledge raised additional questions that necessitate continued exploration of the planet. Indeed, the joint ESA–JAXA dual-orbiter BepiColombo mission, launched in October 2018, is the most ambitious effort yet attempted to explore Mercury [e.g., 7]. Direct in situ elemental and mineralogical measurements on Mercury’s surface, however, are essential for addressing the new science questions that have arisen since MESSENGER.
Plant roots and associated microbes release a diverse range of functionally distinct exudates into the surrounding rhizosphere with direct impacts on soil carbon storage, nutrient availability, and contaminant dynamics. Yet mechanistic linkages between root exudation and emergent biogeochemical processes remain challenging to measure nondestructively, in real soil, over time. Here we used a novel combination of in situ microsensors with high-resolution mass spectrometry to measure, nondestructively, changing exudation and associated biogeochemical dynamics along single growing plant roots (Avena sativa). We found that metabolite and dissolved organic carbon (DOC) concentrations as well as microbial growth, redox potential (EH), and pH dynamics vary significantly among bulk soil, root tip, and more mature root zones. Surprisingly, the significant spike of rhizosphere DOC upon root tip emergence did not significantly correlate with any biogeochemical parameters. However, the presence of sugars significantly correlated with declines in EH following the arrival of the root tip, likely due to enhanced microbial oxygen demand. Similarly, the presence of organic acids significantly correlated to declines in pH upon root tip emergence. Altogether, our in situ measurements highlight how different exudates released along growing roots create functionally distinct soil microenvironments that evolve over time.
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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.
Enhancing the intrinsic activity of transition metal catalysts for the hydrogen evolution reaction (HER) remains a critical challenge in sustainable energy conversion. Herein, we report an electronegativity-guided site differentiation strategy in a single-phase CoNiCuMoW high-entropy alloy (HEA) via electrodeposition by incorporating high-electronegativity 4d/5d orbital transition metals (Mo, W) into the face-centered cubic (fcc) matrix (CoNiCu). The as-synthesized HEA demonstrates exceptional HER performance in all pH conditions, delivering an outstanding overpotential of 65 mV (alkaline), 28 mV (acidic), and 155 mV (neutral) at a current density of 100 mA cm−2, showing performance comparable to commercial Pt/C and has excellent long-term stability at high current density (1 A cm−2, 1000 h). X-Ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations reveal that the incorporation of Mo/W simultaneously alters the local coordination environment and induces element-dependent charge redistribution, accompanied by a system-level d-band center downshift, thereby optimizing the hydrogen binding strength across multimetallic sites. Meanwhile, oxophilic Mo/W sites lower the water dissociation energy barrier. These synergistic effects collectively enable efficient and durable pH-universal HER performance.
Perhaps the most basic question we can ask about cosmological correlations is how their strength changes as we smoothly vary kinematic parameters. The answer is encoded in differential equations that govern this evolution in kinematic space. In this Letter, we introduce a new perspective on these differential equations. We show that, in the simplified setting of conformally coupled scalars in power-law Friedmann-Robertson-Walker spacetimes, the equations for arbitrary tree-level processes can be obtained from a small number of simple combinatorial rules. While this “kinematic flow” is defined purely in terms of boundary data, it reflects the physics of bulk time evolution. The unexpected regularity of the equations suggests the existence of an autonomously defined mathematical structure from which cosmological correlations and the time evolution of the associated spacetime emerge.
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.
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.
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.
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.
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.
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.
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.
.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.
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).
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.
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].