Morphological analysis of olivine grains annealed in an iron‐nickel matrix: Experimental constraints on the origin of pallasites and on the thermal history of their parent bodies     

Kazuto SAIKI  Didier LAPORTE  Daniel VIELZEUF  Satoru NAKASHIMA  Pierre BOIVIN 《Meteoritics & planetary science》2003,38(3):427-444

Abstract— Two types of pallasites can be distinguished on the basis of the grain shape of olivine (rounded or angular). It has been suggested that these two types of textures resulted from different degrees of annealing at high temperature in the parent body. In order to characterize the kinetics of rounding of olivine grains in an Fe‐Ni matrix, we carried out a series of annealing experiments using a mixture of olivine and Fe‐Ni powder. We were able to reproduce, at a miniature scale, the range of textures in pallasites. The rate of rounding was rapid enough to be observed and measured at the scale of a few micrometers to 20 μm, even though the experiments were performed below the solidus of the Fe‐Ni metal. For instance, grains ?14 mm in diameter became nearly spherical within 7 days at 1400°C. For the morphological analysis of olivine grains, we used two independent techniques: the “critical diameter method” and the “Gaussian diffusion‐resample method,” a new technique specially developed for our study. Both techniques indicate that the rounding time scale is proportional to the cube of the grain size and that morphological adjustments in our experiments occurred by volume diffusion in the olivine lattice, not by surface diffusion along the olivine‐metal boundaries. We used our experimental data to estimate the time scales required for the development of olivine‐metal textures in natural pallasites. We determined that small scale rounding of olivine grains in a solid metal matrix can be produced within relatively short time intervals: ?100 years to produce rounded olivine grains 0.1 mm in radius at 1300–1400°C.  相似文献   

OBSERVATION OF THE UV SOLAR SPECTRAL IRRADIANCE BETWEEN 200 AND 350 nm DURING THE ATLAS I MISSION BY THE SOLSPEC SPECTROMETER     

Thuillier  Gérard  Hersé  Michel  Simon  Paul C.  Labs  Dietrich  Mandel  Holger  Gillotay  Didier 《Solar physics》1997,171(2):283-302

The SOLSPEC instrument has been built to carry out solar spectral irradiance measurements from space. It consists of three spectrometers designed to measure the solar spectral irradiance from 180 to 3000 nm. It flew for the first time in December 1983 with the SpaceLab 1 mission (SL1) and later with the ATLAS missions after significant improvement of the instrument optics and calibration procedures. For the ATLAS 1 mission in March 1992, the thermal conditions encountered during the measurements were better than those of SL1, leading to better data quality. Furthermore, other Sun spectrometers, two on the same platform and two others on board the Upper Atmosphere Research Satellite, have also carried out UV absolute spectral measurements at the same time. These opportunities allowed comparisons of solar irradiance determinations. The UV part of the measurements made during that mission is presented here as well as its calibration and accuracy analysis.  相似文献   

Subvisible CO₂ ice clouds detected in the mesosphere of Mars     

Franck Montmessin  Jean-Loup Bertaux  Oleg Korablev  François Forget  Didier Fussen  Aurélie Rébérac 《Icarus》2006,183(2):403-410

The formation of CO₂ ice clouds in the upper atmosphere of Mars has been suggested in the past on the basis of a few temperature profiles exhibiting portions colder than CO₂ frost point. However, the corresponding clouds were never observed. In this paper, we discuss the detection of the highest clouds ever observed on Mars by the SPICAM ultraviolet spectrometer on board Mars Express spacecraft. Analyzing stellar occultations, we detected several mesospheric detached layers at about 100 km in the southern winter subtropical latitudes, and found that clouds formed where simultaneous temperature measurements indicated that CO₂ was highly supersaturated and probably condensing. Further analysis of the spectra reveals a cloud opacity in the subvisible range and ice crystals smaller than 100 nm in radius. These layers are therefore similar in nature as the noctilucent clouds which appear on Earth in the polar mesosphere. We interpret these phenomena as CO₂ ice clouds forming inside supersaturated pockets of air created by upward propagating thermal waves. This detection of clouds in such an ultrararefied and supercold atmosphere raises important questions about the martian middle-atmosphere dynamics and microphysics. In particular, the presence of condensates at such high altitudes begs the question of the origin of the condensation nuclei.  相似文献   

Description of a very dense meteorite collection area in western Atacama: Insight into the long‐term composition of the meteorite flux to Earth          下载免费PDF全文

Aurore Hutzler  Jérôme Gattacceca  Pierre Rochette  Régis Braucher  Bertrand Carro  Eric J. Christensen  Cécile Cournede  Matthieu Gounelle  Nejia Laridhi Ouazaa  Rodrigo Martinez  Millarca Valenzuela  Michael Warner  Didier Bourles 《Meteoritics & planetary science》2016,51(3):468-482

We describe the geological, morphological, and climatic settings of two new meteorite collections from Atacama (Chile). The “El Médano collection” was recovered by systematic on‐foot search in El Médano and Caleta el Cobre dense collection areas and is composed of 213 meteorites before pairing, 142 after pairing. The “private collection” has been recovered by car by three private hunters and consists of 213 meteorites. Similar to other hot desert finds, and contrary to the falls and Antarctica finds, both collections show an overabundance of H chondrites. A recovery density can be calculated only for the El Médano collection and gives 251 and 168 meteorites larger than 10 g km^?2, before and after pairing, respectively. It is by far the densest collection area described in hot deserts. The Atacama Desert is known to have been hyperarid for a long period of time and, based on cosmic‐ray exposure ages on the order of 1–10 Ma, to have been stable over a period of time of several million years. Such a high meteorite concentration might be explained invoking either a yet unclear concentration mechanism (possibly related to downslope creeping) or a previously underestimated meteorite flux in previous studies or an average terrestrial age over 2 Myr. This last hypothesis is supported by the high weathering grade of meteorites and by the common terrestrial fragmentation (with fragments scattered over a few meters) of recovered meteorites.  相似文献   

基于静稳天气下气球探空实验的中国沙漠地区沙尘气溶胶的垂直分布（英文）          下载免费PDF全文

HABIB Ammara  CHEN Bin  SHI Guangyu  IWASAKA Yasunobu  NATH Debashis  KHALID Bushra  TAN Saichun  MAHMOOD Tariq  JIAO Reguang  NTWALI Didier 《大气和海洋科学快报》2019,(1):12-20

塔克拉玛干沙漠是亚洲沙尘气溶胶的重要源地。为探讨塔克拉玛干地区沙尘气溶胶的理化特性与时空变化,研究其环境与气候效应,本文分析了四个季节在中国敦煌(塔克拉玛干沙漠内)取得的探空气球观测数据,包括气溶胶的数浓度、粒径分布、质量浓度及在西风主导下的水平输送通量。气溶胶数浓度的垂直廓线显示,来自沙漠地区的矿物粒子对局地环境与气候有重要影响,且所有季节都存在长距离输送。粒子谱分布显示局地有大量粗粒子输入。结果说明,源于塔克拉玛干沙漠的沙尘气溶胶的背景输送有着重要的科学意义,需进一步研究其对东亚和西太平洋地区环境与气候的影响。  相似文献   

Disturbance versus preservation of U–Th–Pb ages in monazite during fluid–rock interaction: textural, chemical and isotopic in situ study in microgranites (Velay Dome, France)     

A. Didier  V. Bosse  P. Boulvais  J. Bouloton  J.-L. Paquette  J.-M. Montel  J.-L. Devidal 《Contributions to Mineralogy and Petrology》2013,165(6):1051-1072

Monazite is extensively used to date crustal processes and is usually considered to be resistant to diffusive Pb loss. Nevertheless, fluid-assisted recrystallisation is known to be capable of resetting the monazite chronometer. This study focuses on chemical and isotopic disturbances in monazite grains from two microgranite intrusions in the French Central Massif (Charron and Montasset). Petrologic data and oxygen isotopes suggest that both intrusions have interacted with alkali-bearing hydrothermal-magmatic fluids. In the Charron intrusion, regardless of their textural location, monazite grains are sub-euhedral and cover a large domain of compositions. U–Pb chronometers yield a lower intercept age of 297 ± 4 Ma. An inherited component at 320 Ma is responsible for the scattering of the U–Th–Pb ages. The Montasset intrusion was later affected by an additional F-rich crustal fluid with crystallisation of Ca-REE-fluorocarbonates, fluorite, calcite and chloritisation. Pristine monazite is chemically homogeneous and displays ²⁰⁸Pb/²³²Th and ²⁰⁶Pb/²³⁸U concordant ages at 307 ± 2 Ma. By contrast, groundmass monazite shows dissolution-recrystallisation features associated with apatite and thorite precipitation (Th-silicate) and strong chemical reequilibration. ²⁰⁸Pb/²³²Th ages are disturbed and range between 270 and 690 Ma showing that the Th/Pb ratio is highly fractionated during the interaction with fluids. Apparent U–Pb ages are older due to common Pb incorporation yielding a lower intercept age at 312 ± 10 Ma, the age of the pristine monazite. These results show that F-rich fluids are responsible for Th mobility and incorporation of excess Pb, which thus strongly disturbed the U–Th–Pb chronometers in the monazite.  相似文献   

An empirical line list for methane in the 1.26–1.71 μm region for planetary investigations (T = 80–300 K). Application to Titan     

Alain Campargue  Le Wang  Didier Mondelain  Samir Kassi  Bruno Bézard  Emmanuel Lellouch  Athena Coustenis  Catherine de Bergh  Mathieu Hirtzig  Pierre Drossart 《Icarus》2012,219(1):110-128

  相似文献   

GRAVITAS: general relativistic astrophysics via timing and spectroscopy     

Kirpal Nandra  Didier Barret  Andy Fabian  Lothar Strueder  Richard Willingale  Mike Watson  Peter Jonker  Hideyo Kunieda  Giovanni Miniutti  Christian Motch  Peter Predehl 《Experimental Astronomy》2012,34(2):445-462

GRAVITAS is an X-ray observatory, designed and optimised to address the ESA Cosmic Vision theme of ??Matter under extreme conditions??. It was submitted as a response to the call for M3 mission proposals. The concept centres around an X-ray telescope of unprecedented effective area, which will focus radiation emitted from close to the event horizon of black holes or the surface of neutron stars. To reveal the nature and behaviour of matter in the most extreme astrophysical environments, GRAVITAS targets a key feature in the X-ray spectra of compact objects: the iron K?? line at ~6.5?keV. The energy, profile, and variability of this emission line, and the properties of the surrounding continuum emission, shaped by General Relativity (GR) effects, provide a unique probe of gravity in its strong field limit. Among its prime targets are hundreds of supermassive black holes in bright Active Galactic Nuclei (AGN), which form the perfect laboratory to help understand the physical processes behind black hole growth. Accretion plays a fundamental role in the shaping of galaxies throughout cosmic time, via the process of feedback. Modest (~sub-arcmin) spatial resolution would deliver the necessary sensitivity to extend high quality X-ray spectroscopy of AGN to cosmologically-relevant distances. Closer to home, ultra-high count rate capabilities and sub-millisecond time resolution enable the study of GR effects and the equation of state of dense matter in the brightest X-ray binaries in our own Galaxy, using multiple probes, such as the broad iron line, the shape of the disk continuum emission, quasi-periodic oscillations, reverberation mapping, and X-ray burst oscillations. The enormous advance in spectral and timing capability compared to current or planned X-ray observatories would enable a vast array of additional scientific investigations, spanning the entire range of contemporary astrophysics from stars to distant galaxy clusters. Despite its breakthrough capabilities, all enabling technologies for GRAVITAS are already in a high state of readiness. It is based on ultra light-weight X-ray optics and a focal plane detector using silicon technology. The baseline launcher would be a Soyuz?CFregat to place GRAVITAS into a zero inclination, low-earth orbit, providing low and relatively stable background.  相似文献   

SPICES: spectro-polarimetric imaging and characterization of exoplanetary systems     

Anthony Boccaletti  Jean Schneider  Wes Traub  Pierre-Olivier Lagage  Daphne Stam  Raffaele Gratton  John Trauger  Kerri Cahoy  Frans Snik  Pierre Baudoz  Raphael Galicher  Jean-Michel Reess  Dimitri Mawet  Jean-Charles Augereau  Jenny Patience  Marc Kuchner  Mark Wyatt  Eric Pantin  Anne-Lise Maire  Christophe Vérinaud  Samuel Ronayette  Didier Dubreuil  Michiel Min  Michiel Rodenhuis  Dino Mesa  Russ Belikov  Olivier Guyon  Motohide Tamura  Naoshi Murakami  Ingrid Mary Beerer 《Experimental Astronomy》2012,34(2):355-384

SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems) is a five-year M-class mission proposed to ESA Cosmic Vision. Its purpose is to image and characterize long-period extrasolar planets and circumstellar disks in the visible (450?C900 nm) at a spectral resolution of about 40 using both spectroscopy and polarimetry. By 2020/2022, present and near-term instruments will have found several tens of planets that SPICES will be able to observe and study in detail. Equipped with a 1.5 m telescope, SPICES can preferentially access exoplanets located at several AUs (0.5?C10?AU) from nearby stars (<25 pc) with masses ranging from a few Jupiter masses to Super Earths (??2 Earth radii, ??10 M_??) as well as circumstellar disks as faint as a few times the zodiacal light in the Solar System.  相似文献   

Frequency ratio of twin kHz quasi-periodic oscillations: The case of 4U1820-303     

Didier Barret  Martin Boutelier 《New Astronomy Reviews》2008,51(10-12):835

By analyzing all archival Rossi X-ray timing explorer (RXTE) data of the neutron star low mass X-ray binary 4U1820-303, we investigate the detectability of simultaneous twin kHz quasi-periodic oscillations (QPOs) as a function of their frequency, width and root mean squared (RMS) amplitudes. In a blind search over the whole data set (spanning over about 10 years), we show that in continuous time intervals (2000–3000 s), twin QPOs are preferentially detected over narrow range of frequencies (100 Hz wide), leading to a clustering in the distribution of frequency ratios, and a gap around the point (600, 900) Hz in the correlation line that links the upper QPO frequency to the lower QPO frequency. A deficit of lower QPOs around 600 Hz had already been noticed by Belloni et al. [Belloni, T., Méndez, M., Homan, J., 2005. A&A 437, 209], it is now confirmed using a much larger data set. We show that the lack of twin QPOs within the gap is not due to a lack of sensitivity for QPO detection, if the parameters of the QPOs (RMS and width) can be interpolated within the gap, using values measured before and after. Since as previously noticed, the gap cannot be attributed to an incomplete sampling of the source states, it thus implies a sudden change of the QPO properties within the gap, either a loss of coherence or an amplitude decrease, or alternatively, that there may be forbidden frequencies in this system.  相似文献