Thermal history of ALH 84001 meteorite by Fe2+‐Mg ordering in orthopyroxene

Abstract— A single orthopyroxene crystal from the Martian meteorite Allan Hills (ALH) 84001 was studied by X‐ray diffraction (XRD) and electron microprobe analysis (EMPA) to retrieve information about its thermal history. Both sets of data were used to measure the Fe²⁺‐Mg order degree between the M1 and M2 sites expressed by the distribution coefficient k_D. The 529 ± 30°C closure temperature (T_c) of the Fe²⁺‐Mg ordering process of ALH 84001 orthopyroxene (Fs₂₈) was calculated using Stimpfl (2005a, 2005b) ln k_D versus 1/T equation obtained for intermediate iron sample. At this T_c, the orthopyroxene cooling rate, calculated by Ganguly's (1982) numerical method, was 0.1 °C/day. This study puts new constraints on the last high‐temperature thermal episode recorded by orthopyroxene. With reference to the geological history (Treiman 1998), we ascribe this episode to the I3 event, and we interpret the T_c of 529 °C as a lower limit for this impact heating. Our data confirm that experimentally defined physical conditions for the formation of magnetite from decomposition of carbonates took place on the Martian surface during event I3.     

Compositional properties of coexisting orthopyroxene and spinel in some Antarctic diogenites: Implications for thermal history

Abstract— We analyzed the compositional profiles of coexisting orthopyroxenes and spinels in six diogenite samples from the Antarctic meteorite collection and used the data to constrain their thermal histories. The closure temperatures of Fe²⁺‐Mg exchange between spinel and orthopyroxene in these samples vary between ～630 and 830 °C. However, those in other diogenite samples, for which the compositional data are available in the literature, extend up to ～1125 °C. This wide range of closure temperatures suggests repeated excavation of the diogenites from their original sites over a long time interval during cooling. The orthopyroxene grains were found to be homogeneous in composition while two of the relatively large spinel grains in the samples Elephant Moraine (EET) 87530 and Thiel Mountains (TIL) 82410 showed compositional zoning near the rim. Modeling of the spinel zoning in TIL 82410 suggests that it developed during cooling under a regolith or ejecta blanket, possibly at a depth of ～80–120 m, and that the spinel composition was homogeneous at ～900 °C. A nonlinear cooling model in which the cooling rate is given by ηT(K)², with η = 5.8 times 10^?3 K^?1Ma^?1, leads to simulated retrograde zoning profile in spinel which match the observed profile in TIL 82410 very well.     

Cooling by grains and H2 in collapsing clouds of different metallicities

Computations of the cooling by grains and by H₂ molecules are carried out and applied to different stages of cloud collapse during the isothermal phase. Clouds of 10⁶ M of different metallicites are considered, with the purpose of obtaining a relation between metallicity and importance of grain cooling relative to H₂ cooling. Our results show that grain cooling dominates over H₂ cooling for metallicities [M/H] > -4.0, in the absence of external UV field.     

Determination of the Fe oxidation state of the Chassigny kaersutite: A microXANES spectroscopic study

Abstract— In order to elucidate the formation of low‐H kaersutites in Martian meteorites, the Fe³⁺/ΣFe ratio of Chassigny kaersutites in magmatic inclusions was directly determined by the synchrotron microXANES analysis. XANES analysis for standard kaersutites with known Fe³⁺/ΣFe ratios shows a linear relationship between centroid energy positions of XANES pre‐edge spectra and the Fe³⁺/ΣFe ratio. Based on the linear relationship, the Fe³⁺/ΣFe ratio of Chassigny kaersutites is estimated to be about 0.05. The low Fe³⁺/ΣFe ratio clearly suggests that low‐H kaersutites in Chassigny are not likely to be formed by the oxidation‐dehydrogenation reactions. The low‐H content of the Chassigny kaersutites is mainly due to the presence of a Ti oxy‐component.     

Cooling rates of porphyritic olivine chondrules in the Semarkona (LL3.00) ordinary chondrite: A model for diffusional equilibration of olivine during fractional crystallization

Abstract— Cooling rates of chondrules provide important constraints on the formation process of chondrite components at high temperatures. Although many dynamic crystallization experiments have been performed to obtain the cooling rate of chondrules, these only provide a possible range of cooling rates, rather than providing actual measured values from natural chondrules. We have developed a new model to calculate chondrule cooling rates by using the Fe‐Mg chemical zoning profile of olivine, considering diffusional modification of zoning profiles as crystals grow by fractional crystallization from a chondrule melt. The model was successfully verified by reproducing the Fe‐Mg zoning profiles obtained in dynamic crystallization experiments on analogs for type II chondrules in Semarkona. We applied the model to calculating cooling rates for olivine grains of type II porphyritic olivine chondrules in the Semarkona (LL3.00) ordinary chondrite. Calculated cooling rates show a wide range from 0.7 °C/h to 2400 °C/h and are broadly consistent with those obtained by dynamic crystallization experiments (10–1000 °C/h). Variations in cooling rates in individual chondrules can be attributed to the fact that we modeled grains with different core Fa compositions that are more Fe‐rich either because of sectioning effects or because of delayed nucleation. Variations in cooling rates among chondrules suggest that each chondrule formed in different conditions, for example in regions with varying gas density, and assembled in the Semarkona parent body after chondrule formation.     

A study of the object ZGC 2315+03: A candidate polar ring galaxy

We report the results of spectroscopic and photometric observations of ZGC 2315+03 with the 6-m Special Astrophysical Observatory telescope. This object is shown to belong neither to polar ring galaxies nor to close interacting pairs. The peculiar appearance of ZGC 2315+03 is explained by projection effects.     

Measurements in N2-CH4(C2H2) discharges of reaction rates and thermochemical constants for Titan atmosphere study

The kinetic reactions in N2-xCH4(C2H2) gas discharges with x less than 1% have been studied by emission spectroscopy in the afterglow of D.C. discharges and by mass spectroscopy from radiolysis ionization using alpha particles. The pressure range is from several Torr to 100 Torr. At the end of N2 D.C. discharges at room temperature, for a residence time of about 10(-2) s, the dominant active species are the N atoms with density of 10(14)-10(15) cm-3 for N2 density of about 10(17) cm-3 (3 Torr), the N2(X,V) vibrational molecules with for example [N2(X,V = 10)] approximately 10(14) cm-3 and the electronic metastable molecules N2(A 3 sigma u +) with a density of 10(12) cm-3. In such conditions, the following kinetic reactions have been studied: N2(A) + N2(A) --> N2(C,B,V') + N2(X), N2(A) + N2(X,V>5) --> N2(X) + N2(B,V') in pure N2 post-discharges and N2(A) + CH4 --> products, C + N + M2 --> CN(B,V') + M2, N2(X,V>4) + CN --> N2(X) + CN(B,A,V'), in N2-1% CH4 post-discharges. The clustering reactions of N2-(1-5%)CH4(C2H2) gas mixtures after radiolysis ionization have been studied for the H2CN+ nN2 ions and the equilibrium constants have been determined in the temperature range T = 140-300 K.     

A simple derivation of capture probabilities for the J+1:J and J+2:J orbit-orbit resonance problems

We present a simplified analytic derivation of the capture probabilities for the j+1j and j+2j orbital resonances. We apply Henrard's method which is based on an extension of the theory of adiabatic invariants and recover the results originally obtained by Yoder.Contribution number 3874 from the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, U.S.A.     

266 E+A galaxies selected from the Sloan Digital Sky Survey Data Release 2: the origin of E+A galaxies

E+A galaxies are characterized as galaxies with strong Balmer absorption lines but without any [O  ii ] or Hα emission lines. The existence of strong Balmer absorption lines indicates that E+A galaxies have experienced starburst within the past one gigayear. However, the lack of [O  ii ] and Hα emission lines indicates that E+A galaxies do not have any on-going star formation. Therefore, E+A galaxies are interpreted as post-starburst galaxies. For many years, however, it has been a mystery why E+A galaxies started starburst and why they quenched star formation abruptly. Using one of the largest samples of 266 E+A galaxies carefully selected from the Sloan Digital Sky Survey Data Release 2, we have investigated the environment of E+A galaxies from 50 kpc to 8 Mpc scale, i.e. from a typical distance to satellite galaxies to the scale of large-scale structures. We found that E+A galaxies have an excess of local galaxy density only at a scale of <100 kpc (with a 2σ significance), but not at the cluster scale (∼1.5 Mpc) nor at the scale of large-scale structure (∼8 Mpc). These results indicate that E+A galaxies are not created by the physical mechanisms associated with galaxy clusters or the large-scale structure, but are likely to be created by dynamical interaction with closely accompanying galaxies at a <100 kpc scale. The claim is also supported by the morphology of E+A galaxies. We have found that almost all E+A galaxies have a bright compact core, and that ∼30 per cent of E+A galaxies have dynamically disturbed signatures or tidal tails, which quite strongly suggest the morphological appearance of merger/interaction remnants.     

A possible lunar outcrop: a study of lunokhod-2 data

The remotely controlled vehicle Lunokhod-2 travelled extensively around the edges of a linear depression unofficially called Fossa Recta. The edges of the Fossa are marked by elongated boulder fields. Three lines of reasoning suggest that the boulder fields are not the usual erratic boulders found on a normal mare surface, but are bedrock protuberances: (1) The morphology of many boulders is reminiscent of primary lava features, (2) toward the edge of the Fossa the regolith thins out; (3) a model of lunar gardening indicates that no regolith is to be expected in the upper portion of a non-impact cliff.Part of the research was conducted while L.B.R. was a participant of an exchange program between the Academy of Sciences, U.S.S.R. and the Academy of Sciences, U.S.A.     

A TEM study of exsolution in Ca-rich pyroxenes from the Paris and Renazzo chondrites: Determination of type I chondrule cooling rates

We conducted a transmission electron microscope study of the exsolution microstructures of Ca-rich pyroxenes in type I chondrules from the Paris CM and Renazzo CR carbonaceous chondrites in order to provide better constraints on the cooling history of type I chondrules. Our study shows a high variability of composition in the augite grains at a submicrometer scale, reflecting nonequilibrium crystallization. The microstructure is closely related to the local composition and is thus variable inside augite grains. For compositions inside the pyroxene miscibility gap, with a wollastonite (Wo) content typically below 40 mole%, the augite grains contain abundant exsolution lamellae on (001). For grain areas with composition close to Wo₄₀, a modulated texture on (100) and (001) is the dominant microstructure, while areas with compositions higher than Wo₄₀ do not show any exsolution microstructure development. To estimate the cooling rate, we used the spacing of the exsolution lamellae on (001), for which the growth is diffusion controlled and thus sensitive to the cooling rate. Despite the relatively homogeneous microstructures of augite grains with Wo < 35 mole%, our study of four chondrules suggests a range of cooling rates from ~10 to ~1000 °C h⁻¹, within the temperature interval 1200–1350 °C. These cooling rates are comparable to those of type II chondrules, i.e., 1–1000 °C h⁻¹. We conclude that the formation of type I and II chondrules in the proto-solar nebula was the result of a common mechanism.     

A new mineralogical approach for CO3 chondrite characterization by X-ray diffraction: Identification of primordial phases and thermal history

Using the in-plane rotation of polished thin section, the X-ray diffraction patterns exhibiting a high degree of randomness similar to powder pattern were obtained for 10 CO3 chondrites, which distinguished 130 reflections of olivine in the chondrules from that in the matrix, and showed systematic differences among subtypes based on the full width at half maximum intensity of two olivine 130 peaks. A lower petrologic subtype is characterized by sharp and strong peaks for forsteritic olivines in type I chondrules and by a weak and broad peak for ferroan matrices, and the higher petrologic subtypes are characterized by sharp and strong peaks for recrystallized matrices and a weakened or absent peak of magnesian olivines. The systematic change in the split peak of olivine 130 was linked with the mean diffusion length of Mg-Fe in olivine phenocrysts in type I chondrules. Fe-Ni diffusion in metals was considered to estimate the peak temperature of CO3.0, near the surface on the parent body. The peak metamorphic temperatures were estimated to be ~600–910 K using the onion-shell model when the cooling time was 10⁶–10⁸ yr on the parent body. A weak peak for ferroan olivine of CO3.0 suggests the amorphous silicate in matrices. The modal abundance of the amorphous Fe-silicate for subtype 3.0 (15% for Allan Hills [ALH] 77307 and 9% for Yamato [Y]-81020) was also evaluated from the deviation in trend of the relative peak ratios of the Fe-rich (≥Fa₂₅) and Mg-rich (<Fa₂₅) olivines for subtypes. The existence of martensites was suggested for ALH 77307. Amorphous silicate in matrices is a more resistant primordial component that produced the CO3 chondrites than martensite.     

A ROSAT study of the cores of clusters of galaxies — I. Cooling flows in an X-ray flux-limited sample

This is the first part of a study of the detailed X-ray properties of the cores of nearby clusters. We have used the flux-limited sample of 55 clusters listed by Edge et al., and archival and proprietary data from the ROSAT observatory. In this paper an X-ray spatial analysis based on the surface-brightness-deprojection technique is applied to the clusters in the sample with the aim of studying their cooling flow properties. We determine the fraction of cooling flows in this sample to be 70–90 per cent, and estimate the contribution of the flow region to the cluster X-ray luminosity. We show that the luminosity within a strong cooling flow can account for up to 70 per cent of a cluster X-ray bolometric luminosity. Our analysis indicates that about 40 per cent of the clusters in the sample have flows depositing more than 100 M⊙ yr⁻¹ throughout the cooling region, and that these possibly have been undisturbed for many Gyr, confirming that cooling flows are the natural state of cluster cores. New cooling flows in the sample are presented, and previously ambiguous ones are clarified. We have constructed a catalogue of some intracluster medium properties for the clusters in this sample. The profiles of the mass deposited from cooling flows are analysed, and evidence is presented for the existence of breaks in some of the profiles. Comparison is made to recent optical and radio data. We cross-correlate our sample with the Green Bank, NVSS and FIRST surveys, and with the volume-limited sample of brightest cluster galaxies presented by Lauer &38; Postman. Although weak trends exist, no strong correlation between optical magnitude or radio power of the brightest cluster galaxy and the strength of the flow is found.     

Energy dependence of Ti/Fe ratio in the Galactic cosmic rays measured by the ATIC-2 experiment

Titanium is a rare, secondary nucleus among Galactic cosmic rays. Using the Silicon matrix in the ATIC experiment, Titanium has been separated. The energy dependence of the Ti to Fe flux ratio in the energy region from 5 GeV per nucleon to about 500 GeV per nucleon is presented. The article was translated by the authors.     

The formation of prominences by thermal instability: A numerical study

A two-dimensional model of prominence formation in a region containing a magnetic neutral sheet is constructed for a variety of initial conditions, assuming the coronal plasma to be described by the usual hydromagnetic approximation, with infinite electric conductivity. In each case the magnetic field is initially vertical, varying antisymmetrically with respect to the neutral sheet, to a maximum value at a distance of 70 000 km from the neutral sheet. In the first case, the plasma is initially in hydrostatic equilibrium, whereas in successive cases, the pressure is assumed to be of such a value that the plasma is in lateral equilibrium of total pressure (gas plus magnetic). In a variation of this case, the value of the solar gravitational field was artificially reduced, and the effects considered. Large lateral motions are produced in each case, thus apparently inhibiting the condensation of prominences, with the exception of the unrealistic case of artificially reduced gravity. The results suggest that consideration either of a third component of the magnetic field (horizontal and parallel to the neutral sheet), or a finite conductivity, allowing magnetic recombination across the neutral sheet, or both, would more realistically represent the problem and might thus show the development of prominences.     

A systematic study on energy dependence of quasi-periodic oscillation frequency in GRS 1915+105

Systematically studying all the RXTE/PCA observations for GRS 1915+105 before November 2010, we have discovered three additional patterns in the relation between Quasi-Periodic Oscillation (QPO) frequency and photon energy, extending earlier outcomes reported by Qu et al. (Astrophys. J. 710:836, 2010). We have confirmed that as QPO frequency increases, the relation evolves from the negative correlation to positive one. The newly discovered patterns provide new constraints on the QPO models.     

A statistical study of MHD discontinuities in the inner solar system: Helios 1 and 2

A statistical study is made of the magnetic field directional discontinuities observed in early 1976 onboard Helios 1 and 2. Strong day-to-day variations of occurrence rates are found on either tangential (TD) or rotational (RD) discontinuities. No large variation (if any) is found versus either heliocentric distance or heliographic latitude. This contradicts previous findings obtained by the same technique on Pioneer 8 data in 1968–69; however, reasons are given to expect different results, under different solar conditions. The most interesting results come from the study of the morphology of discontinuities: first of all, the orientation of TD's and RD's normals (identified by a minimum variance technique) are strongly organized by the average magnetic field, following their progressive directional change when approaching the Sun. The inclination (θ _n ) and azimuthal (? _n ) distributions are gaussian and strongly peaked along the field lines for RD's; as regards TD's the normals are perpendicular to the average field and follow its progressive variation; the θ _n distribution is isotropic in solid angle, which is interpreted as evidence of crossing of flux tubes on the order of one/hour. Implications of this interpretation in contrast with a turbulent approach are also discussed.     

Trapping and adsorption of CO2 in amorphous ice: A FTIR study

The interaction of carbon dioxide and amorphous water ice at 95 K is studied using transmission infrared spectroscopy. Samples are prepared in two ways: co-deposition of the gases admitted simultaneously or sequential deposition, in which amorphous water ice (ASW) is grown first and CO₂ vapor is added subsequently. In either case, a fraction of the CO₂ molecules is found to interact with water in a way that gives rise to shifts and splittings in the infrared bands with respect to those of a pure CO₂ solid. In co-deposition experiments, a larger amount of carbon dioxide is trapped within the amorphous water than in sequential deposition samples, where a substantial proportion of molecules appears to be trapped in macropores of the ASW. The specific surface area of sequential samples is evaluated and compared to previous literature results. When the sequential samples are heated to 140 K, beyond the onset temperature at which water ice undergoes a phase transition, the CO₂ molecules at the pores relocate inside the bulk in a structure similar to that found in co-deposited samples, as deduced by changes in the shape of the CO₂ infrared bands.