Asteroid Cooling Rates Indicated by K-Feldspar Exsolution Textures in H4 Ordinary Chondrites

Undisturbed thermal metamorphism in ordinary chondrite (OC) asteroids, produced through the radioactive decay of 26Al, is expected to result in an onion-shell-like structure. In such a structure, the inner layers of the asteroid experience more extensive thermal metamorphism, as represented by higher petrologic type, than the exterior layers. Furthermore, cooling rates are expected to be slower for OCs of high petrologic type than those of low petrologic type. However, cooling rates determined using metallographic methods and pyroxene diffusion are inconsistent with onion-shell-style cooling and have resulted in new models. These models argue for the disruption of the asteroid after peak metamorphism followed by reaccretion into a rubble pile. Improved constraints on cooling rates would provide a better understanding of the timing and scale of disruptive events. Feldspar microtextures are another tool that can be used to determine asteroid cooling rates. In OC chondrules, plagioclase is present as either a primary phase, or a secondary phase forming from the crystallization of mesostasis glass through petrologic type 4, followed by chemical and textural equilibration. Potas-sium feldspar is observed in petrologic types 3.6-6, as either patches or lamellae exsolved from albite in a perthite texture, often near pores or fractures. Exsolution occurs most commonly, and most extensively, in petrologic type 4. Because the feldspar exsolution wavelength is related to the rate at which grains cooled from the solvus temperature, determined from the minerals bulk composition, the chondrite cooling rate can be measured from regions of exsolution. We have previously reported the perthite exsolution cooling rate of Avanhandava, an H4 chondrite, to be 1 C per 1-4 months over a temperature interval of 765-670 C. A peristerite exsolution texture was also present in the Na-rich lamellae for which we estimated a cooling rate of 1 C in 103-104 years from 570-540 C. Overall, the cooling rates determined from Avanhandava are consistent with pyroxene diffusion (fast cooling at high temperatures) and metallographic rates (slow cooling at low temperatures), hence with the rubble pile model of disruption and reaccretion. Here, we characterize feldspar microtextures in four additional H4 chondrites to test the consistency of feldspar cooling rates across a range of samples. We show that all H4s are similar and support rubble pile models

Unjamming due to local perturbations in granular packings with and without gravity

We investigate the unjamming response of disordered packings of frictional hard disks with the help of computer simulations. First, we generate jammed configurations of the disks and then force them to move again by local perturbations. We study the spatial distribution of the stress and displacement response and find long range effects of the perturbation in both cases. We record the penetration depth of the displacements and the critical force that is needed to make the system yield. These quantities are tested in two types of systems: in ideal homogeneous packings in zero gravity and in packings settled under gravity. The penetration depth and the critical force are sensitive to the interparticle friction coefficient. Qualitatively, the same nonmonotonic friction dependence is found both with and without gravity, however the location of the extrema are at different friction values. We discuss the role of the connectivity of the contact network and of the pressure gradient in the unjamming response.Comment: 12 pages, 13 figure

Value at Risk models with long memory features and their economic performance

We study alternative dynamics for Value at Risk (VaR) that incorporate a slow moving component and information on recent aggregate returns in established quantile (auto) regression models. These models are compared on their economic performance, and also on metrics of first-order importance such as violation ratios. By better economic performance, we mean that changes in the VaR forecasts should have a lower variance to reduce transaction costs and should lead to lower exceedance sizes without raising the average level of the VaR. We find that, in combination with a targeted estimation strategy, our proposed models lead to improved performance in both statistical and economic terms

Self-stresses and Crack Formation by Particle Swelling in Cohesive Granular Media

We present a molecular dynamics study of force patterns, tensile strength and crack formation in a cohesive granular model where the particles are subjected to swelling or shrinkage gradients. Non-uniform particle size change generates self-equilibrated forces that lead to crack initiation as soon as strongest tensile contacts begin to fail. We find that the coarse-grained stresses are correctly predicted by an elastic model that incorporates particle size change as metric evolution. The tensile strength is found to be well below the theoretical strength as a result of inhomogeneous force transmission in granular media. The cracks propagate either inward from the edge upon shrinkage and outward from the center upon swelling

Nanoscale Mineralogy and Composition of Experimental Regolith Agglutinates Produced under Asteroidal Impact Conditions

On the Moon, the energetics of smaller impactors and the physical/chemical characteristics of the granular regolith target combine to form a key product of lunar space weathering: chemically reduced shock melts containing optically-active nanophase Fe metal grains (npFe0) [1]. In addition to forming the optically dark glassy matrix phase in lunar agglutinitic soil particles [1], these shock melts are becoming increasingly recognized for their contribution to optically active patina coatings on a wide range of exposed rock and grain surfaces in the lunar regolith [2]. In applying the lessons of lunar space weathering to asteroids, the potential similarities and differences in regolith-hosted shock melts on the Moon compared to those on asteroids has become a topic of increasing interest [3,4]. In a series of impact experiments performed at velocities applicable to the asteroid belt [5], Horz et al. [6] and See and Horz [7] have previously shown that repeated impacts into a gabbroic regolith analog target can produce melt-welded grain aggregates morphologically very similar to lunar agglutinates [6,7]. Although these agglutinate-like particles were extensively analyzed by electron microprobe and scanning electron microscopy (SEM) as part of the original study [7], a microstructural and compositional comparison of these aggregates to lunar soil agglutinates at sub-micron scales has yet to be made. To close this gap, we characterized a representative set of these aggregates using a JEOL 7600 field-emission scanning electron microscope (FE-SEM), and JEOL 2500SE field-emission scanning transmission electron microscope (FE-STEM) both optimized for energy dispersive X-ray spectroscopy (EDX) compositional spectrum imaging at respective analytical spatial resolutions of 0.5 to 1 micron, and 2 to 4 nm

Apolipoprotein M mediates sphingosine-1-phosphate efflux from erythrocytes

Abstract Sphingosine-1-phosphate (S1P) is a bioactive lipid implicated in e.g. angiogenesis, lymphocyte trafficking, and endothelial barrier function. Erythrocytes are a main source of plasma S1P together with platelets and endothelial cells. Apolipoprotein M (apoM) in HDL carries 70% of plasma S1P, whereas 30% is carried by albumin. The current aim was to investigate the role of apoM in export of S1P from human erythrocytes. Erythrocytes exported S1P more efficiently to HDL than to albumin, particularly when apoM was present in HDL. In contrast, export of sphingosine to HDL was unaffected by the presence of apoM. The specific ability of apoM to promote export of S1P was independent of apoM being bound in HDL particles. Treatment with MK-571, an inhibitor of the ABCC1 transporter, effectively reduced export of S1P from human erythrocytes to apoM, whereas the export was unaffected by inhibitors of ABCB1 or ATPase. Thus, ABCC1 could be involved in export of S1P from erythrocytes to apoM

Water Storage and Transport Processes During Impact Experiments on Nominally Anhydrous Minerals

Water, in the form of structurally bound hydrogen in the crystal lattice of nominally anhydrous minerals (NAMs), strongly influences many important physical processes on terrestrial planets and planetary objects. Water enhances the rates of plastic deformation and controls the degree of partial melting in silicate rocks, which influences the generation of melt and therefore the nature of planetary volcanism. Water has also been experimentally demonstrated to influence the nature of lattice preferred orientation in deformed aggregates, and thus may be important in the interpretation of seismic anisotropy data collected from planetary bodies, such as from the current InSight mission on Mars. Therefore, much attention has been focused on characterizing the distribution and concentration of water in the planets and rocky bodies of our solar system

Shear strength properties of wet granular materials

We investigate shear strength properties of wet granular materials in the pendular state (i.e. the state where the liquid phase is discontinuous) as a function of water content. Sand and glass beads were wetted and tested in a direct shear cell and under various confining pressures. In parallel, we carried out three-dimensional molecular dynamics simulations by using an explicit equation expressing capillary force as a function of interparticle distance, water bridge volume and surface tension. We show that, due to the peculiar features of capillary interactions, the major influence of water content over the shear strength stems from the distribution of liquid bonds. This property results in shear strength saturation as a function of water content. We arrive at the same conclusion by a microscopic analysis of the shear strength. We propose a model that accounts for the capillary force, the granular texture and particle size polydispersity. We find fairly good agreement of the theoretical estimate of the shear strength with both experimental data and simulations. From numerical data, we analyze the connectivity and anisotropy of different classes of liquid bonds according to the sign and level of the normal force as well as the bond direction. We find that weak compressive bonds are almost isotropically distributed whereas strong compressive and tensile bonds have a pronounced anisotropy. The probability distribution function of normal forces is exponentially decreasing for strong compressive bonds, a decreasing power-law function over nearly one decade for weak compressive bonds and an increasing linear function in the range of tensile bonds. These features suggest that different bond classes do not play the same role with respect to the shear strength.Comment: 12 page

Про ефективність діяльності товарних бірж України в роки НЕПу

Об’єктом дослідження статті є економічна та соціальна ефективність діяльності товарних бірж України в період непу. При цьому основна увага зосереджується на таких аспектах проблеми: посередництво в купівлі й продажу товарів; реєстрація позабіржових угод; надання послуг учасникам біржового торгу; підтримка комерційної освіти, благодійність тощо.Объектом исследования статьи является экономическая и социальная эффективность деятельности товарных бирж Украины в период нэпа. При этом основное внимание сосредоточивается на таких аспектах проблемы: посредничество в купле и продаже товаров; регистрация внебиржевых сделок; предоставление услуг участникам биржевого торга; поддержка коммерческого образования, благотворительность и т. п