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The Ritland impact structure (western Norway), 2.7 km in diameter and about 350 m deep, is a depression partly fi lled by post‐impact Cambrian shales (upper photo; courtesy of Knut Vindfallet). The lower left photo shows well‐exposed, fractured Precambrian basement, while the lower right photo illustrates the melt rock with dark inclusions of devitrifi ed melt clasts in a light gray matrix (weathered surface) (coin is about 2.5 cm across). Images courtesy of E. Kalleson and H. Dypvik.     

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Vesta image (left) acquired by the 750 nm filter of Dawn's Framing Camera (FC), photometrically corrected to remove effects of illumination conditions. The 18‐km diameter crater at upper left is located on the floor of the Rheasilvia basin. FC color‐ratio mosaic (right) of red, green, and blue filters illustrates compositional variations. Images courtesy of NASA/JPL‐Caltech/UCLA/MPS/DLR/IDA.     

The Mechanisms of Particle Kinetics and Dynamics Leading to Seismic Emission and Sunquakes

In this paper the mechanisms responsible for observational features associated with sunquakes induced by different classes of solar flares are compared. The role of high-energy particle beams via Coulomb and Ohmic heating of the ambient plasma and nonthermal excitation and ionization is explored for different beam parameters at various atmospheric depths. On the one hand, only hard electron beams with high-energy fluxes are found producing extensive nonthermal hydrogen ionization, four orders of magnitude higher than in the quiet atmosphere. This excess ionization leads to the white-light flares associated with the seismic emission appearing simultaneously with hard X-ray emission and, consequently, to a strong increase of Ni-line emission observed as the seismic emission measured with the holographic technique. On the other hand, the ambient plasma hydrodynamic response to heating by such beam electrons forms hydrodynamic shocks just below the transition region, in the upper chromosphere, and they travel with supersonic velocity for up to five minutes before reaching the photosphere. These hydrodynamic responses caused by the beam electrons are maximized in the lower chromosphere for moderate electron beams because of their smaller Ohmic losses in the upper atmosphere compared to those for higher-energy electron beams whose bulk energy is deposited in the transition region. These shocks caused by electron beams can explain the observations of seismic emission by time?–?distance (TD) diagrams and the holographic method in M- and C-class flares, whereas to account for the quakes in X-class flares, high-energy quasi-thermal protons or power-law proton beams either by themselves or blended with electron beams are the most likely agents. Nonthermal ionization and excitation of lower atmospheric levels during the beam injection followed by thermo-conductive heating after the beam is stopped can contribute to the seismic signatures observed with the holographic technique caused by strong nonthermal ionization and back-warming heating occurring in the shock while it loses its energy by optically-thick radiation in the photospheric lines and continua.     

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Three specimens from the pristine suite of the Tagish Lake meteorite, showing variability in texture, from relatively chondrule‐rich (specimen 5b, upper left) to chondrule‐poor (specimen 11i, right) to intermediate (specimen 11h, lower left). The textural variation is due to differences in petrographic character, as described by Blinova et al. (this issue); differences in organic matter correlate with the textural variation (Alexander et al. and Hilts et al., this issue), providing insights into parent body processing. Specimens from the University of Alberta Meteorite Collection (MET11611/P?5b, ?11i and ?11h). Scale bar applies to all three images. Photography by C. D. K. Herd and R. K. Herd.     

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Shock features (right) identifi ed in the core (left) taken at a depth of 120 m from the Saqqar structure in Saudi Arabia prove the structure was formed by a hypervelocity impact event. Thomas Kenkmann et al. discuss the structure in their article on pp. 1925–1940. Images courtesy of T. Kenkmann.     

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Portions of the Gao‐Guenie H5 chondrite that fell in Burkina Faso in 1960 are impact‐melted. Left: high‐resolution color scan of thin section; right: refl ected light image (mosaic) of thin section. On pp. 1022–1045, Martin Schmieder et al. interpret the Gao‐Guenie impact melt breccia (image) as a sample from a melt injection dike in the basement of an impact crater, generated during an impact event on an H chondrite asteroid ~300 million years ago. Images courtesy of M. Schmieder.     

Origin and emplacement of the impact formations at Chicxulub,Mexico, as revealed by the ICDP deep drilling at Yaxcopoil‐1 and by numerical modeling

We present and interpret results of petrographic, mineralogical, and chemical analyses of the 1511 m deep ICDP Yaxcopoil‐1 (Yax‐1) drill core, with special emphasis on the impactite units. Using numerical model calculations of the formation, excavation, and dynamic modification of the Chicxulub crater, constrained by laboratory data, a model of the origin and emplacement of the impact formations of Yax‐1 and of the impact structure as a whole is derived. The lower part of Yax‐1 is formed by displaced Cretaceous target rocks (610 m thick), while the upper part comprises six suevite‐type allochthonous breccia units (100 m thick). From the texture and composition of these lithological units and from numerical model calculations, we were able to link the seven distinct impact‐induced units of Yax‐1 to the corresponding successive phases of the crater formation and modification, which are as follows: 1) transient cavity formation including displacement and deposition of Cretaceous “megablocks;” 2) ground surging and mixing of impact melt and lithic clasts at the base of the ejecta curtain and deposition of the lower suevite right after the formation of the transient cavity; 3) deposition of a thin veneer of melt on top of the lower suevite and lateral transport and brecciation of this melt toward the end of the collapse of the transient cavity (brecciated impact melt rock); 4) collapse of the ejecta plume and deposition of fall‐back material from the lower part of the ejecta plume to form the middle suevite near the end of the dynamic crater modification; 5) continued collapse of the ejecta plume and deposition of the upper suevite; 6) late phase of the collapse and deposition of the lower sorted suevite after interaction with the inward flowing atmosphere; 7) final phase of fall‐back from the highest part of the ejecta plume and settling of melt and solid particles through the reestablished atmosphere to form the upper sorted suevite; and 8) return of the ocean into the crater after some time and minor reworking of the uppermost suevite under aquatic conditions. Our results are compatible with: a) 180 km and 100 km for the diameters of the final crater and the transient cavity of Chicxulub, respectively, as previously proposed by several authors, and b) the interpretation of Chicxulub as a peak‐ring impact basin that is at the transition to a multi‐ring basin.     

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Cover: Upper left: High‐resolution mosaic of section ALH 84001,82 (~30 μm thick) from Martian meteorite Allan Hills 84001. Several highly detailed transmitted light images, obtained with a Zeiss Scope petrographic microscope at magnification of 500× and with crossed nicols (polarizers), were merged to create this mosaic. A grid was superimposed onto the image (square size is 1 mm²) to navigate around the section and (re)locate regions of interest. D5 and J7 are regions that contain a particularly high concentration of carbonates. The images were obtained at the Institute of Space Sciences (ICE, IEEC/CSIC) by Carles E. Moyano‐Cambero, who also created the mosaic. For details see the article on p. 1030. Upper right: Detail of region D5 in the mosaic. Cathodoluminescence image, obtained with a Nikon Eclipse LV100NPol petrographic microscope at 100× magnification, after applying electron bombardment in the sample with a Cambridge Image Technology Ltd. (CITL) Technosyn cold cathodoluminescent MK4 operated at a voltage of 20–24 kV and an intensity of 350–400 mA. The light emitted during electron bombardment of a mineral has intensity and wavelength that depends on its chemical composition and crystallographic structure, and even trace amounts of some elements can act as activators or quenchers of cathodoluminescence. Since iron is a typical quencher, iron‐free minerals can be particularly luminescent. In this case, bright red areas correspond to very Fe‐poor and Mn‐rich layers in the carbonates. This and the following images were obtained at the Universidad Complutense de Madrid by M. Isabel Benito. Lower left: Detail of the region D5 of the mosaic. Transmitted light image, obtained with a Nikon Eclipse LV100NPol petrographic microscope at 100× magnification and with crossed nicols. Carbonates are the brown‐to‐orange areas, while the large dark blue area is maskelynite (shock modified plagioclase). Lower right: BSE image of one of the spherical carbonates in D5 obtained with a JEOL JSM7600F SEM with a BSED (magnification 1,100×, voltage of 15 kV). The SEM was used to select regions and points of interest for subsequent chemical analysis by electron microprobe. The Fe‐rich and Mg‐rich rims of the carbonate can be clearly observed as bright and dark rims, respectively.     

Heating effects of the matrix of experimentally shocked Murchison CM chondrite: Comparison with micrometeorites

Abstract— Micrometeorites have been significantly altered or melted by heating, which has been mainly ascribed to aerodynamic drag during atmospheric entry. However, if a major fraction of micrometeorites are produced by impacts on porous asteroids, they may have experienced shock heating before contact with the Earth's atmosphere (Tomeoka et al. 2003). A transmission electron microscope (TEM) study of the matrix of Murchison CM chondrite experimentally shocked at pressures of 10–49 GPa shows that its mineralogy and texture change dramatically, mainly due to shock heating, with the progressive shock pressures. Tochilinite is completely decomposed to an amorphous material at 10 GPa. Fe‐Mg serpentine is partially decomposed and decreases in amount with increasing pressure from 10 to 30 GPa and is completely decomposed at 36 GPa. At 49 GPa, the matrix is extensively melted and consists mostly of aggregates of equigranular grains of Fe‐rich olivine and less abundant low‐Ca pyroxene embedded in Si‐rich glass. The mineralogy and texture of the shocked samples are similar to those of some types of micrometeorites. In particular, the samples shocked at 10 and 21 GPa are similar to the phyllosilicate (serpentine)‐rich micrometeorites, and the sample shocked at 49 GPa is similar to the olivine‐rich micrometeorites. The shock heating effects also resemble the effects of pulse‐heating experiments on the CI and CM chondrite matrices that were conducted to simulate atmospheric entry heating. We suggest that micrometeorites derived from porous asteroids are likely to go through both shock and atmospheric‐entry heating processes.     

Review of Hinode results

Hinode is an observatory‐style satellite, carrying three advanced instruments being designed and built to work together to explore the physical coupling between the photosphere and the upper layers for understanding the mechanism of dynam‐ ics and heating. The three instruments aboard are the Solar Optical Telescope (SOT), which can provide high‐precision photometric and polarimetric data of the lower atmosphere in the visible light (388–668 nm) with a spatial resolution of 0.2–0.3 arcseconds, the X‐Ray Telescope (XRT) which takes a wide field of full sun coverage X‐ray images being capable of diagnosing the physical condition of coronal plasmas, and the EUV Imaging Spectrometer (EIS) which observes the upper transition region and coronal emission lines in the wavelength ranges of 17–21 nm and 25–29 nm. Since first‐light observations in the end of October 2006, Hinode has been continuously providing unprecedented high‐quality solar data. We will present some new findings of the sun with Hinode, focusing on those from SOT (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Variations in ground surface temperature histories in the Thompson Belt, Manitoba, Canada: environment and climate changes

Several temperature–depth profiles recorded at Pipe Mine, 32 km southwest of Thompson, Manitoba, in central Canada, exhibit a marked departure from the equilibrium gradient. These profiles could be interpreted as indicating strong warming (up to 4.5 K) of the ground surface during the last 200 years. All the temperature profiles at Pipe Mine show perturbations stronger than at the others sites in the Thompson Nickel Belt. Temperature profiles recorded near the town of Thompson show a moderate warming (≈1–2 K) trend, while temperature profiles at Soab, 45 km southwest of Pipe Mine, indicate very moderate cooling (≈0.5 K). There was little human activity in this part of Manitoba before the development of the mining camp of Thompson in the late 1950s. Our study shows the variability of ground surface temperature histories at a very local scale (i.e. <1 km) with much stronger signals at some of the Pipe Mine drill holes than at others. These holes are located within 500 m of the highway and a power line built after 1955, at ≈3 km from the now abandoned open pit mine. The ground surface temperature history (GSTH) obtained by the inversion of Pipe Mine temperature profiles suggests that a recent (50 years) and strong (≈1–2 K) ground surface warming is superimposed on a 1–2 K warming trend that started 200 years ago, without any indication of a cold (little ice ages) episode before. The recent warming (40 years) at Pipe Mine is only a local effect and is likely to be related to the presence of the highway. Before 1960, the ground surface temperature history for Pipe is similar to other sites in the Thompson region. Ground surface temperature histories from other profiles within and near the city of Thompson seem less affected by environmental perturbations and their trends are parallel to that of the meteorological records in the Canadian Prairies.     

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Brecciated and hydrothermally altered orthogneiss‐amphibolite complex in the Maniitsoq structure, West Greenland. Inset: variably hydrothermally altered zircon with reset U‐Pb systematics, yielding a hydrothermal age of 3000.9 ± 1.9 Ma. The hydrothermal alteration of regional extent took place at upper amphibolite facies, and is interpreted as due to infl ux of (sea?) water and development of a crustal‐scale hydrothermal convection cell in a now deeply exhumed Archaean impact structure. A. Scherstén and A. A. Garde discuss the details in their paper on pp. 1472–1498. (Images courtesy of Adam Garde and Leif Johansson.)     

In situ experimental formation and growth of Fe nanoparticles and vesicles in lunar soil

We report the results of the first dynamic, in situ heating of lunar soils to simulate micrometeorite impacts on the lunar surface. We performed slow‐ and rapid‐heating experiments inside the transmission electron microscope to understand the chemical and microstructural changes in surface soils resulting from space‐weathering processes. Our slow‐heating experiments show that the formation of Fe nanoparticles begins at ~575 °C. These nanoparticles also form as a result of rapid‐heating experiments, and electron energy‐loss spectroscopy measurements indicate the Fe nanoparticles are composed entirely of Fe⁰, suggesting this simulation accurately mimics micrometeorite space‐weathering processes occurring on airless body surfaces. In addition to Fe nanoparticles, rapid‐heating experiments also formed vesiculated textures in the samples. Several grains were subjected to repeated thermal shocks, and the measured size distribution and number of Fe nanoparticles evolved with each subsequent heating event. These results provide insight into the formation and growth mechanisms for Fe nanoparticles in space‐weathered soils and could provide a new methodology for relative age dating of individual soil grains from within a sample population.     

What metal‐troilite textures can tell us about post‐impact metamorphism in chondrite meteorites

Abstract— Metal‐troilite textures are examined in metamorphosed and impact‐affected ordinary chondrites to examine the response of these phases to rapid changes in temperature. Complexly intergrown metal‐troilite textures are shown to form in response to three different impact‐related processes. (1) During impacts, immiscible melt emulsions form in response to spatially focused heating. (2) Immediately after impact events, re‐equilibration of heterogeneously distributed heat promotes metamorphism adjacent to zones of maximum impact heating. Where temperatures exceed ～850 ° C, this post‐impact metamorphism results in melting of conjoined metal‐troilite grains in chondrites that were previously equilibrated through radiogenic metamorphism. When the resulting Fe‐Ni‐S melt domains crystallize, a finely intergrown mixture of troilite and metal forms, which can be zoned with kamacite‐rich margins and taenite‐rich cores. (3) At lower temperatures, post‐impact metamorphism can also cause liberation of sulfur from troilite, which migrates into adjacent Fe‐Ni metal, allowing formation of troilite and occasionally copper within the metal during cooling. Because impact events cause heating within a small volume, post‐impact metamorphism is a short duration event (days to years) compared with radiogenic metamorphism (>10⁶ years). The fast kinetics of metal‐sulfide reactions allows widespread textural changes in conjoined metal‐troilite grains during post‐impact metamorphism, whereas the slow rate of silicate reactions causes these to be either unaffected or only partially annealed, except in the largest impact events. Utilizing this knowledge, information can be gleaned as to whether a given meteorite has suffered a post‐impact thermal overprint, and some constraints can be placed on the temperatures reached and duration of heating.     

Meteorite 3‐D synchrotron microtomography: Methods and applications

Abstract— Methods of synchrotron X‐ray computed microtomography (XRCMT) are described, which allow nondestructive, high spatial and contrast resolution imaging of the density structures of meteorites and their components in three dimensions. Images of bulk chondrites (to one cubic centimeter in size) reveal compound chondrules, chondrule/matrix volumetric ratios, metal and sulfide distribution, petrofabrics, and 3‐D chondrule and calcium‐aluminum inclusion (CAI) sizes and shapes. Images of separated chondrules and CAIs reveal void spaces, mineral intergrowth textures, and the true locations of crystal rims and cores, at resolutions to <8 cubic micron/volume element. Images of achondrites reveal mineral fabrics and crystal zoning. Lunar glass spherules can be searched for phenocrysts bearing deeply sourced melt inclusions. A companion DVD and URL contain images for classroom and research use. Numerical techniques for quantification of X‐ray computed microtomography (XRCMT) data and its potential applications are discussed. Three‐dimensional X‐ray images of meteorites provide a way to discover components of interest and to precisely slice samples to expose these components with minimal damage and loss of material. Three‐dimensional studies of petrographic features (size, shape, texture, and modal abundance) of chondrites and their components, as well as other meteorites, have definite advantages over standard 2‐D studies using randomly sliced thin sections.     

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Scanning electron microscope backscatter electron image of a small magnetite‐bearing brecciated rock fragment located in Apollo 16 regolith breccia sample 60016, as discussed in Joy et al. (pp. 1157–1172). The identifi cation of magnetite in a lunar sample provides direct evidence for oxidized conditions on the Moon. The lower color image shows the location of different mineral phases in the sample noted in the color scale at left (black areas represent pores, fractures, or phases that cannot be identifi ed). Image courtesy of Katherine Joy.     

Radiative hydrodynamic simulations of the spectral characteristics of solar white-light flares

As one of the most violent activities in the solar atmosphere,white-light flares(WLFs)are generally known for their enhanced white-light(or continuum)emission,which primarily originates in the solar lower atmosphere.However,we know little about how white-light emission is produced.In this study,we aim to investigate the response of the continua at 3600?and 4250?and also the Hαand Lyαlines during WLFs modeled using radiative hydrodynamic simulations.We take non-thermal electron beams as the energy source for the WLFs in two different initial atmospheres and vary their parameters.Our results show that the model with non-thermal electron beam heating clearly shows enhancements in the continua at 3600?and 4250?as well as in the Hαand Lyαlines.A larger electron beam flux,a smaller spectral index,or an initial penumbral atmosphere leads to a stronger emission increase at 3600?,4250?and in the Hαline.The Lyαline,however,is more obviously enhanced in a quiet-Sun initial atmosphere with a larger electron beam spectral index.It is also notable that the continua at 3600?and 4250?and the Hαline exhibit a dimming at the start of heating and reach their peak emissions after the peak time of the heating function,while the Lyαline does not show such behaviors.These results can serve as a reference for the analysis of future WLF observations.     

Fine‐grained precursors dominate the micrometeorite flux

Abstract– We optically classified 5682 micrometeorites (MMs) from the 2000 South Pole collection into textural classes, imaged 2458 of these MMs with a scanning electron microscope, and made 200 elemental and eight isotopic measurements on those with unusual textures or relict phases. As textures provide information on both degree of heating and composition of MMs, we developed textural sequences that illustrate how fine‐grained, coarse‐grained, and single mineral MMs change with increased heating. We used this information to determine the percentage of matrix dominated to mineral dominated precursor materials (precursors) that produced the MMs. We find that at least 75% of the MMs in the collection derived from fine‐grained precursors with compositions similar to CI and CM meteorites and consistent with dynamical models that indicate 85% of the mass influx of small particles to Earth comes from Jupiter family comets. A lower limit for ordinary chondrites is estimated at 2–8% based on MMs that contain Na‐bearing plagioclase relicts. Less than 1% of the MMs have achondritic compositions, CAI components, or recognizable chondrules. Single mineral MMs often have magnetite zones around their peripheries. We measured their isotopic compositions to determine if the magnetite zones demarcate the volume affected by atmospheric exchange during entry heating. Because we see little gradient in isotopic composition in the olivines, we conclude that the magnetites are a visual marker that allows us to select and analyze areas not affected by atmospheric exchange. Similar magnetite zones are seen in some olivine and pyroxene relict grains contained within MMs.     

Hydrodynamic response of the solar chromosphere to an elementary flare burst

Impulsive heating of the upper chromosphere by a very powerful thermal flux is studied as the cause of hard X-rays during a solar flare. The electron temperature at the boundary between the corona and chromosphere is assumed to change in accordance with the hard X-ray intensity in an elementary flare burst (EFB). A maximum value of about 10⁸ K is reached after 5 s, after which the boundary temperature decreases. These high-temperature changes lead to fast propagation of heat into the chromosphere. Numerical solution of the hydrodynamic equations, which take into account all essential dissipative processes, shows that classical heat conduction is not valid due to heat flux saturation in the case of impulsive heating from a high-temperature source. The saturation effect and hydrodynamic flow along a magnetic field lead to electron temperature and density distributions such that the thermal X-ray spectrum of a high-temperature plasma can be well enough approximated by an exponential law or by two power-law spectra. According to this dissipative thermal model for the source of hard X-rays, the emission measure of the high-temperature plasma increases monotonously during the whole EFB even after the temperature maximum. Some results for the low-temperature region are discussed in connection with short-lived chromospheric bright points.     

Transmission electron microscopy of minerals in the martian meteorite Allan Hills 84001

Abstract— We have studied carbonate and associated oxides and glasses in a demountable section of Allan Hills 84001 (ALH 84001) using optical, scanning, and transmission electron microscopy (TEM) to elucidate their origins and the shock history of the rock. Massive, fracture‐zone, and fracture‐filling carbonates in typical locations were characterized by TEM, X‐ray microanalysis, and electron diffraction in a comprehensive study that preserved textural and spatial relationships. Orthopyroxene is highly deformed, fractured, partially comminuted, and essentially unrecovered. Lamellae of diaplectic glass and other features indicate shock pressures >30 GPa. Bridging acicular crystals and foamy glass at contacts of orthopyroxene fragments indicate localized melting and vaporization of orthopyroxene. Carbonate crystals are >5 mm in size, untwinned, and very largely exhibit the R3c calcite structure. Evidence of plastic deformation is generally found mildly only in fracture‐zone and fracture‐filling carbonates, even adjacent to highly deformed orthopyroxene, and appears to have been caused by low‐stress effects including differential shrinkage. High dislocation densities like those observed in moderately shocked calcite are absent. Carbonate contains impactderived glasses of plagioclase, silica, and orthopyroxene composition indicating brief localized impact heating. Stringers and lenses of orthopyroxene glass in fracture‐filling carbonate imply flow of carbonates and crystallization during an impact. Periclase (MgO) occurs in magnesite as 30–50 nm crystals adjacent to voids and negative crystals and as ?1 μm patches of 3 nm crystals showing weak preferred orientation consistent with (111)_MgO//(0001)_carb, as observed in the thermal decomposition of CaCO₃ to CaO. Magnetite crystals that are epitaxially oriented at voids, negative crystals, and microfractures clearly formed in situ. Fully embedded, faceted magnetites are topotactically oriented, in general with (111)_mag//(0001)_carb, so that their oxygen layers are aligned. In optically opaque rims, magnetites are more irregularly shaped and, except for the smallest crystals, poorly aligned. All magnetite and periclase crystals probably formed by exsolution from slightly non‐stoichiometric, CO₂‐poor carbonate following impact‐induced thermal decomposition. Any magnetites that existed in the rock before shock heating could not have preserved evidence for biogenic activity.