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  • Weitere Quellen  (17)
  • 1
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    MESSENGER Observations of Extreme Loading and Unloading of Mercury's Magnetic Tail (2010)
    In:  CASI
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    Publikationsdatum: 2018-06-06
    Beschreibung: During MESSENGER's third flyby of Mercury, a series of 2-3 minute long enhancements of the magnetic field in the planet's magnetotail were observed. Magnetospheric substorms at Earth are powered by similar tail loading, but the amplitude is approximately 10 times less and the durations are 1 hr. These observations of extreme loading imply that the relative intensity of substorms at Mercury must be much larger than at Earth. The correspondence between the duration of tail enhancements and the calculated approximately 2 min Dungey cycle, which describes plasma circulation through Mercury's magnetosphere, suggests that such circulation determines substorm timescale. A key aspect of tail unloading during terrestrial substorms is the acceleration of energetic charged particles. Such signatures are puzzlingly absent from the MESSENGER flyby measurements.
    Schlagwort(e): Lunar and Planetary Science and Exploration
    Format: application/pdf
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  • 2
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    Elemental Composition of 433 Eros: New Calibration of the NEAR-Shoemaker XRS Data (2009)
    In:  CASI
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    Publikationsdatum: 2018-06-06
    Beschreibung: We present a new calibration of the elemental-abundance data for Asteroid 433 Fros taken by the X-ray spectrometer (XRS) aboard the NEAR-Shoemaker spacecraft. (NEAR is an acronym for "Near-Earth Asteroid Rendezvous,") Quintification of the asteroid surface elemental abundance ratios depends critically on accurate knowledge of the incident solar X-ray spectrum, which was monitored simultaneously with asteroid observations. Previously published results suffered from incompletely characterized systematic uncertainties due to an imperfect ground calibrations of the NEAR gas solar monitor. The solar monitor response function and associated uncertainties have now been characterized by cross-calibration of a large sample of NEAR solar monitor flight data against. contemporary broadband solar X-ray data from the Earth-orbiting GOES-8 (Geostationary Operational Environmental Satellite). The results have been used to analyze XRS spectra acquired from Eros during eight major solar flares (including three that have not previously been reported). The end product of this analysis is a revised set of Eros surface elemental abundance ratios with new error estimates that more accurately reflect the remaining uncertainties in the solar flare spectra: Mg/Si=.753 +0.078/-0.055, Al/Si=0.069 +/-0.055, S/Si=0.005+/-0.008, Ca/Si=0.060+0.023/-0.024, and Fe/Si= 1.578+0.338/-0.320. These revised abundance ratios are consitent within cited uncertainties with the results of Nittler et al. [Nittler, L.R., and 14 colleagues, 2001. Meteorit Planet. Sci 36, 1673-1695] and thus support the prior conclusions that 433 Eros has major-element composition simular to ordinary chondrites with the exception of a stong depletoin in sulfur, most likely caused by space weathering.
    Schlagwort(e): Solar Physics
    Materialart: Icarus; Volume 200; 129-146
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  • 3
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    Acquisition of a High Resolution Field Emission Scanning Electron Microscope for the Analysis of Returned Samples (2003)
    In:  Other Sources
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    Publikationsdatum: 2019-07-18
    Beschreibung: This grant furnished funds to purchase a state-of-the-art scanning electron microscope (SEM) to support our analytical facilities for extraterrestrial samples. After evaluating several instruments, we purchased a JEOL 6500F thermal field emission SEM with the following analytical accessories: EDAX energy-dispersive x-ray analysis system with fully automated control of instrument and sample stage; EDAX LEXS wavelength-dispersive x-ray spectrometer for high sensitivity light-element analysis; EDAX/TSL electron backscatter diffraction (EBSD) system with software for phase identification and crystal orientation mapping; Robinson backscatter electron detector; and an in situ micro-manipulator (Kleindiek). The total price was $550,000 (with $150,000 of the purchase supported by Carnegie institution matching funds). The microscope was delivered in October 2002, and most of the analytical accessories were installed by January 2003. With the exception of the wavelength spectrometer (which has been undergoing design changes) everything is working well and the SEM is in routine use in our laboratory.
    Schlagwort(e): Instrumentation and Photography
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  • 4
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    Stellar Origins of C-13 and N-15-Enriched Presolar SiC Grains (2016)
    In:  CASI
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    Publikationsdatum: 2019-07-19
    Beschreibung: Extreme excesses of 13 C ( C (12 C/ 13 C〈10) and 15 N ( N (14 N/ 15 N〈 20) in rare presolar SiC 20) in rare presolar SiClar SiC grains have been considered diagnostic of an origin in classical novae [1], though an origin in core-collapse supernovae (CCSNe) has also been proposed [2]. We report multi-element isotopic data for 19 13 C- and 15 N-enriched presolar SiC grains(12 C/13 C〈16 and 14 N/ 15 N〈~150) from an acid resistant residue of the Murchison meteorite. These grains are enriched in 13 C and15 N, but with quite diverse Si isotopic signatures. Four grains with isotopic signatures. Four grains with isotopic signatures. Four grains with isotopic signatures. Four grains with isotopic signatures.
    Schlagwort(e): Astrophysics
    Materialart: JSC-CN-35396 , International Conference on Nuclei in the Cosmos (NIC-XIV 2016); Jun 19, 2016 - Jun 24, 2016; Niigata; Japan
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  • 5
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    Mineralogy of the Mercurian Surface (2016)
    In:  CASI
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    Publikationsdatum: 2019-07-13
    Beschreibung: The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft orbited Mercury for four years until April 2015, revealing its structure, chemical makeup, and compositional diversity. Data from the mission have confirmed that Mercury is a compositional end-member among the terrestrial planets. The X-Ray Spectrometer (XRS) and Gamma-Ray Spectrometer (GRS) on board MESSENGER provided the first detailed geochemical analyses of Mercury's surface. These instruments have been used in conjunction with the Neutron Spectrometer and the Mercury Dual Imaging System to classify numerous geological and geochemical features on the surface of Mercury that were previously unknown. Furthermore, the data have revealed several surprising characteristics about Mercury's surface, including elevated S abundances (up to 4 wt%) and low Fe abundances (less than 2.5 wt%). The S and Fe abundances were used to quantify Mercury's highly reduced state, i.e., between 2.6 and 7.3 log10 units below the Iron-Wustite (IW) buffer. This fO2 is lower than any of the other terrestrial planets in the inner Solar System and has important consequences for the thermal and magmatic evolution of Mercury, its surface mineralogy and geochemistry, and the petrogenesis of the planet's magmas. Although MESSENGER has revealed substantial geochemical diversity across the surface of Mercury, until now, there have been only limited efforts to understand the mineralogical and petrological diversity of the planet. Here we present a systematic and comprehensive study of the potential mineralogical and petrological diversity of Mercury.
    Schlagwort(e): Lunar and Planetary Science and Exploration
    Materialart: JSC-CN-35303 , Lunar and Planetary Science Conference; Mar 21, 2016 - Mar 25, 2016; The Woodlands, TX; United States
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  • 6
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    Transmission Electron Microscopy of an In Situ Presolar Silicon Carbide Grain (2002)
    In:  Other Sources
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    Publikationsdatum: 2019-07-18
    Beschreibung: We used a focused ion beam workstation to prepare ultra-thin sections of a presolar SiC grain. Our TEM studies indicate that the SiC formed by rapid vapor-phase condensation, trapping pre-existing graphite grains in random orientations. Additional information is contained in the original extended abstract.
    Schlagwort(e): Lunar and Planetary Science and Exploration
    Materialart: Lunar and Planetary Science XXXIII; LPI-Contrib-1109
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  • 7
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    Evidence Connecting Mercury's Magnesium Exosphere to Its Magnesium-Rich Surface Terrane (2018)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: Mercury is surrounded by a tenuous, collisionless exosphere where the surface of the planet is directly exposed to the space environment. As a consequence, impacts and space weathering processes are expected to eject atoms and molecules from the surface into the exosphere, implying a direct link between the exospheric composition and the planet's regolith material. However, observational evidence demonstrating this link has been elusive. Here we report that exospheric magnesium, a species recently discovered and systematically measured by the Mercury Surface, Space ENvironment, GEochemistry, and Ranging mission, is enhanced when observed over a portion of the planet's surface regolith rich in magnesium. These observations confirm a direct link between Mercury's magnesium exosphere and the underlying crustal surface composition, providing strong evidence supporting theoretical arguments that impact vaporization can directly supply material to the exosphere from the regolith of a rocky, airless body.
    Schlagwort(e): Lunar and Planetary Science and Exploration
    Materialart: GSFC-E-DAA-TN59103 , Geophysical Research Letters (ISSN 0094-8276) (e-ISSN 1944-8007); 45; 14; 6790-6797
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  • 8
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    Origin and Evolution of Prebiotic Organic Matter as Inferred from the Tagish Lake Meteorite (2011)
    In:  CASI
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    Publikationsdatum: 2019-08-24
    Beschreibung: The complex suite of organic materials in carbonaceous chondrite meteorites probably originally formed in the interstellar medium and/or the solar protoplanetary disk, but was subsequently modified in the meteorites' asteroidal parent bodies. The mechanisms of formation and modification are still very poorly understood. We carried out a systematic study of variations in the mineralogy, petrology, and soluble and insoluble organic matter in distinct fragments of the Tagish Lake meteorite. The variations correlate with indicators of parent body aqueous alteration and at least some molecules of pre-biotic importance formed during the alteration.
    Schlagwort(e): Lunar and Planetary Science and Exploration
    Materialart: GSFC.JA.4458.2011
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  • 9
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    Transmission Electron Microscopy of Non-Etched Presolar Silicon Carbide (2003)
    In:  CASI
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    Publikationsdatum: 2019-07-10
    Beschreibung: Our solar system formed from nuclei produced in earlier generations of stars. Mixing in the proto-solar nebula isotopically homogenized most of this material, but some grains, called presolar grains, retain their original isotopic composition. The isotopic properties of presolar SiC grains indicate that most of the grains formed in the outflows of carbon-rich Asymptotic Giant Branch (AGB) stars. The microstructure of these presolar grains reflects the conditions of the dust formation and subsequent alteration. Early microstructural studies of SiC grains obtained by acid dissolution from meteorites show that most isotopically anomalous SiC grains have the face-centered cubic b- SiC structure. However, Daulton et al. have shown that a small fraction of sub-micron presolar SiC grains are of the hexagonal 2H polytype (a-SiC). Although the harsh chemical treatments of these grains does not alter their crystal structure, significant alteration of the surface morphology of the grains due to the acid treatments has been observed. In addition, the acid treatments may preferentially remove cracked or fissured grains, and possible sub-grains, such as graphite. By studying SiC grains isolated by physical separation and found in situ, we attempt to obtain a more complete analysis of presolar SiC microstructures, including the surface morphology, in order to address the formation and processing history of the grains. In our prior work, we reported on one in situ SiC grain (hereafter CBIS1). Here we present results from two additional grains, one in situ, and one prepared as a physical separate.
    Schlagwort(e): Lunar and Planetary Science and Exploration
    Materialart: Lunar and Planetary Science XXXIV; LPI-Contrib-1156
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  • 10
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    Bulk Compositional Trends in Meteorites: A Guide for Analysis and Interpretation of NEAR XGRS Data from Asteroid 433 EROS (1999)
    In:  CASI
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    Publikationsdatum: 2019-07-13
    Beschreibung: The Near Earth Asteroid Rendezvous (NEAR) spacecraft is to orbit the S-class asteroid 433 Eros for about one year beginning on February 14, 2000. The X-ray/gamma-ray O, Mg, Al, Si, Fe, and K; possibly H, Ca, S, Ti, and Th) of Eros with a spatial resolution ranging from a few km for X-rays to approx. 25% of the asteroid's surface for gamma-rays. The major scientific goals for the NEAR XGRS are to relate the composition of Eros to known classes of meteorites, to assess compositional heterogeneity and to identify geological processes that have occurred on the asteroid. Comparing remote-sensing data from asteroids to laboratory data from meteorites requires that the latter be well determined and understood. How well particular classes of meteorites can be identified as analogues of Eros depends not only on the error of the XGRS measurement, but also on the spread in abundances observed among different members of a given meteorite class. To prepare for the return of XGRS data from Eros, we have compiled a large database of bulk elemental compositions of meteorites, using data from a wide variety of published and unpublished sources. Custom software was developed to easily extract statistical information and make plots of data from different meteorite classes. Here, we use the meteorite compositional database to investigate which abundances and abundance ratios, of those measureable by the NEAR XGRS, are most diagnostic for distinguishing meteorite classes and identifying geological processes that have occurred on the samples' parent asteroids.
    Schlagwort(e): Astrophysics
    Materialart: 31th Lunar and Planetary Science Conference; Mar 13, 1999 - Mar 17, 1999; Houston, TX; United States
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