Showing papers by "Ames Research Center published in 2009"
TL;DR: The Large Area Telescope (Fermi/LAT) as mentioned in this paper is the primary instrument on the Fermi Gamma-ray Space Telescope, which is an imaging, wide field-of-view, high-energy gamma-ray telescope, covering the energy range from below 20 MeV to more than 300 GeV.
Abstract: (Abridged) The Large Area Telescope (Fermi/LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy gamma-ray telescope, covering the energy range from below 20 MeV to more than 300 GeV. This paper describes the LAT, its pre-flight expected performance, and summarizes the key science objectives that will be addressed. On-orbit performance will be presented in detail in a subsequent paper. The LAT is a pair-conversion telescope with a precision tracker and calorimeter, each consisting of a 4x4 array of 16 modules, a segmented anticoincidence detector that covers the tracker array, and a programmable trigger and data acquisition system. Each tracker module has a vertical stack of 18 x,y tracking planes, including two layers (x and y) of single-sided silicon strip detectors and high-Z converter material (tungsten) per tray. Every calorimeter module has 96 CsI(Tl) crystals, arranged in an 8 layer hodoscopic configuration with a total depth of 8.6 radiation lengths. The aspect ratio of the tracker (height/width) is 0.4 allowing a large field-of-view (2.4 sr). Data obtained with the LAT are intended to (i) permit rapid notification of high-energy gamma-ray bursts (GRBs) and transients and facilitate monitoring of variable sources, (ii) yield an extensive catalog of several thousand high-energy sources obtained from an all-sky survey, (iii) measure spectra from 20 MeV to more than 50 GeV for several hundred sources, (iv) localize point sources to 0.3 - 2 arc minutes, (v) map and obtain spectra of extended sources such as SNRs, molecular clouds, and nearby galaxies, (vi) measure the diffuse isotropic gamma-ray background up to TeV energies, and (vii) explore the discovery space for dark matter.
3,666 citations
TL;DR: Results suggest that the soil at the Phoenix landing site must have suffered alteration through the action of liquid water in geologically the recent past, and revealed an alkaline environment in contrast to that found by the Mars Exploration Rovers, indicating that many different environments have existed on Mars.
Abstract: The Wet Chemistry Laboratory on the Phoenix Mars Lander performed aqueous chemical analyses of martian soil from the polygon-patterned northern plains of the Vastitas Borealis. The solutions contained ~10 mM of dissolved salts with 0.4 to 0.6% perchlorate (ClO 4 ) by mass leached from each sample. The remaining anions included small concentrations of chloride, bicarbonate, and possibly sulfate. Cations were dominated by Mg 2+ and Na + , with small contributions from K + and Ca 2+ . A moderately alkaline pH of 7.7 ± 0.5 was measured, consistent with a carbonate-buffered solution. Samples analyzed from the surface and the excavated boundary of the ~5-centimeter-deep ice table showed no significant difference in soluble chemistry.
929 citations
TL;DR: In this article, the Fermi Large Area Telescope (Fermi LAT) was used to detect the electron spectrum up to 1 TeV using a diffusive model and a potential local extra component.
Abstract: Designed as a high-sensitivity gamma-ray observatory, the Fermi Large Area Telescope is also an electron detector with a large acceptance exceeding 2 m2 sr at 300 GeV. Building on the gamma-ray analysis, we have developed an efficient electron detection strategy which provides sufficient background rejection for measurement of the steeply falling electron spectrum up to 1 TeV. Our high precision data show that the electron spectrum falls with energy as E-3.0 and does not exhibit prominent spectral features. Interpretations in terms of a conventional diffusive model as well as a potential local extra component are briefly discussed.
890 citations
Utah State University1, Virginia Tech2, University of Arizona3, University of Maryland, College Park4, University of New Hampshire5, University of Wisconsin-Madison6, Harvard University7, Ames Research Center8, United States Geological Survey9, University of Montana10, University of Wisconsin–Milwaukee11, University of California, Berkeley12, University of Maine13, Colorado State University14
TL;DR: In this paper, the authors assess 10 start-of-spring (SOS) methods for North America between 1982 and 2006 and find that SOS estimates were more related to the first leaf and first flowers expanding phenological stages.
Abstract: Shifts in the timing of spring phenology are a central feature of global change research. Long-term observations of plant phenology have been used to track vegetation responses to climate variability but are often limited to particular species and locations and may not represent synoptic patterns. Satellite remote sensing is instead used for continental to global monitoring. Although numerous methods exist to extract phenological timing, in particular start-of-spring (SOS), from time series of reflectance data, a comprehensive intercomparison and interpretation of SOS methods has not been conducted. Here, we assess 10 SOS methods for North America between 1982 and 2006. The techniques include consistent inputs from the 8 km Global Inventory Modeling and Mapping Studies Advanced Very High Resolution Radiometer NDVIg dataset, independent data for snow cover, soil thaw, lake ice dynamics, spring streamflow timing, over 16 000 individual measurements of ground-based phenology, and two temperature-driven models of spring phenology. Compared with an ensemble of the 10 SOS methods, we found that individual methods differed in average day-of-year estimates by � 60 days and in standard deviation by � 20 days. The ability of the satellite methods to retrieve SOS estimates was highest in northern latitudes and lowest in arid, tropical, and Mediterranean ecoregions. The ordinal rank of SOS methods varied geographically, as did the relationships between SOS estimates and the cryospheric/hydrologic metrics. Compared with ground observations, SOS estimates were more related to the first leaf and first flowers expanding phenological stages. We found no evidence for time trends in spring arrival from ground- or model-based data; using an ensemble estimate from two methods that were more closely related to ground observations than other methods, SOS
831 citations
TL;DR: In this article, the authors presented a 0.8-5 μm spectral library of 210 cool stars observed at a resolving power of R ≡ λ/Δλ ~ 2000 with the medium-resolution infrared spectrograph, SpeX, at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii.
Abstract: We present a 0.8-5 μm spectral library of 210 cool stars observed at a resolving power of R ≡ λ/Δλ ~ 2000 with the medium-resolution infrared spectrograph, SpeX, at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The stars have well-established MK spectral classifications and are mostly restricted to near-solar metallicities. The sample not only contains the F, G, K, and M spectral types with luminosity classes between I and V, but also includes some AGB, carbon, and S stars. In contrast to some other spectral libraries, the continuum shape of the spectra is measured and preserved in the data reduction process. The spectra are absolutely flux calibrated using the Two Micron All Sky Survey photometry. Potential uses of the library include studying the physics of cool stars, classifying and studying embedded young clusters and optically obscured regions of the Galaxy, evolutionary population synthesis to study unresolved stellar populations in optically obscured regions of galaxies and synthetic photometry. The library is available in digital form from the IRTF Web site.
795 citations
TL;DR: Models of electrochemical processes in the form of equivalent electric circuit parameters were combined with statistical models of state transitions, aging processes, and measurement fidelity in a formal framework to assess the remaining useful life of complex systems.
Abstract: This paper explores how the remaining useful life (RUL) can be assessed for complex systems whose internal state variables are either inaccessible to sensors or hard to measure under operational conditions. Consequently, inference and estimation techniques need to be applied on indirect measurements, anticipated operational conditions, and historical data for which a Bayesian statistical approach is suitable. Models of electrochemical processes in the form of equivalent electric circuit parameters were combined with statistical models of state transitions, aging processes, and measurement fidelity in a formal framework. Relevance vector machines (RVMs) and several different particle filters (PFs) are examined for remaining life prediction and for providing uncertainty bounds. Results are shown on battery data.
692 citations
TL;DR: The Gamma-ray Burst Monitor and Large Area Telescope onboard the Fermi Observatory together record GRBs over a broad energy range spanning about 7 decades of gammaray energy, with the largest apparent energy release yet measured.
Abstract: Gamma-ray bursts (GRBs) are highly energetic explosions signaling the death of massive stars in distant galaxies. The Gamma-ray Burst Monitor and Large Area Telescope onboard the Fermi Observatory together record GRBs over a broad energy range spanning about 7 decades of gamma-ray energy. In September 2008, Fermi observed the exceptionally luminous GRB 080916C, with the largest apparent energy release yet measured. The high-energy gamma rays are observed to start later and persist longer than the lower energy photons. A simple spectral form fits the entire GRB spectrum, providing strong constraints on emission models. The known distance of the burst enables placing lower limits on the bulk Lorentz factor of the outflow and on the quantum gravity mass.
651 citations
TL;DR: In this article, the Substellar and Planetary Atmospheric Radiation and Circulation model (SARIMA) was used to simulate the atmospheric dynamics of HD 189733b and HD 209458b and provided a realistic representation of nongray cloud-free radiative transfer.
Abstract: We present global, three-dimensional numerical simulations of HD 189733b and HD 209458b that couple the atmospheric dynamics to a realistic representation of nongray cloud-free radiative transfer. The model, which we call the Substellar and Planetary Atmospheric Radiation and Circulation model, adopts the MITgcm for the dynamics and uses the radiative model of McKay, Marley, Fortney, and collaborators for the radiation. Like earlier work with simplified forcing, our simulations develop a broad eastward equatorial jet, mean westward flow at higher latitudes, and substantial flow over the poles at low pressure. For HD 189733b, our simulations without TiO and VO opacity can explain the broad features of the observed 8 and 24 ?m light curves, including the modest day-night flux variation and the fact that the planet/star flux ratio peaks before the secondary eclipse. Our simulations also provide reasonable matches to the Spitzer secondary-eclipse depths at 4.5, 5.8, 8, 16, and 24 ?m and the ground-based upper limit at 2.2 ?m. However, we substantially underpredict the 3.6 ?m secondary-eclipse depth, suggesting that our simulations are too cold in the 0.1-1 bar region. Predicted temporal variability in secondary-eclipse depths is ~1% at Spitzer bandpasses, consistent with recent observational upper limits at 8 ?m. We also show that nonsynchronous rotation can significantly alter the jet structure. For HD 209458b, we include TiO and VO opacity; these simulations develop a hot (>2000 K) dayside stratosphere whose horizontal dimensions are small at depth but widen with altitude. Despite this stratosphere, we do not reproduce current Spitzer photometry of this planet. Light curves in Spitzer bandpasses show modest phase variation and satisfy the observational upper limit on day-night phase variation at 8 ?m.
600 citations
TL;DR: In this article, the authors used the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) to detect alteration minerals in the Noachian terrain west of the Isidis basin.
Abstract: The Noachian terrain west of the Isidis basin hosts a diverse collection of alteration minerals in rocks comprising varied geomorphic units within a 100,000 km2 region in and near the Nili Fossae. Prior investigations in this region by the Observatoire pour l'Mineralogie, l'Eau, les Glaces, et l'Activite (OMEGA) instrument on Mars Express revealed large exposures of both mafic minerals and iron magnesium phyllosilicates in stratigraphic context. Expanding on the discoveries of OMEGA, the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) has found more spatially widespread and mineralogically diverse alteration minerals than previously realized, which represent multiple aqueous environments. Using CRISM near-infrared spectral data, we detail the basis for identification of iron and magnesium smectites (including both nontronite and more Mg-rich varieties), chlorite, prehnite, serpentine, kaolinite, potassium mica (illite or muscovite), hydrated (opaline) silica, the sodium zeolite analcime, and magnesium carbonate. The detection of serpentine and analcime on Mars is reported here for the first time. We detail the geomorphic context of these minerals using data from high-resolution imagers onboard MRO in conjunction with CRISM. We find that the distribution of alteration minerals is not homogeneous; rather, they occur in provinces with distinctive assemblages of alteration minerals. Key findings are (1) a distinctive stratigraphy, in and around the Nili Fossae, of kaolinite and magnesium carbonate in bedrock units always overlying Fe/Mg smectites and (2) evidence for mineral phases and assemblages indicative of low-grade metamorphic or hydrothermal aqueous alteration in cratered terrains. The alteration minerals around the Nili Fossae are more typical of those resulting from neutral to alkaline conditions rather than acidic conditions, which appear to have dominated much of Mars. Moreover, the mineralogic diversity and geologic context of alteration minerals found in the region around the Nili Fossae indicates several episodes of aqueous activity in multiple distinct environments.
598 citations
TL;DR: The detection of emission up to ∼31 GeV from the distant and short GRB, and no evidence for the violation of Lorentz invariance is found, which disfavour quantum-gravity theories in which the quantum nature of space–time on a very small scale linearly alters the speed of light.
Abstract: A cornerstone of Einstein's special relativity is Lorentz invariance-the postulate that all observers measure exactly the same speed of light in vacuum, independent of photon-energy. While special relativity assumes that there is no fundamental length-scale associated with such invariance, there is a fundamental scale (the Planck scale, l(Planck) approximate to 1.62 x 10(-33) cm or E(Planck) = M(Planck)c(2) approximate to 1.22 x 10(19) GeV), at which quantum effects are expected to strongly affect the nature of space-time. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy(1-7). Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in gamma-ray burst (GRB) light-curves(2). Here we report the detection of emission up to similar to 31GeV from the distant and short GRB090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2E(Planck) on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of l(Planck)/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories(3,6,7) in which the quantum nature of space-time on a very small scale linearly alters the speed of light.
586 citations
Johns Hopkins University Applied Physics Laboratory1, Brown University2, Jet Propulsion Laboratory3, Search for extraterrestrial intelligence4, University of California, Santa Cruz5, Washington University in St. Louis6, Cornell University7, United States Geological Survey8, Ames Research Center9, University of Paris-Sud10, University of Arizona11
TL;DR: Martian aqueous mineral deposits have been examined and characterized using data acquired during Mars Reconnaissance Orbiter's (MRO) primary science phase, including Compact Reconnaissance Imaging Spectrometer for Mars hyperspectral images covering the 0.4-3.9 μm wavelength range, coordinated with higher-spatial resolution HiRISE and Context Imager images as discussed by the authors.
Abstract: Martian aqueous mineral deposits have been examined and characterized using data acquired during Mars Reconnaissance Orbiter's (MRO) primary science phase, including Compact Reconnaissance Imaging Spectrometer for Mars hyperspectral images covering the 0.4–3.9 μm wavelength range, coordinated with higher–spatial resolution HiRISE and Context Imager images. MRO's new high-resolution measurements, combined with earlier data from Thermal Emission Spectrometer; Thermal Emission Imaging System; and Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitie on Mars Express, indicate that aqueous minerals are both diverse and widespread on the Martian surface. The aqueous minerals occur in 9–10 classes of deposits characterized by distinct mineral assemblages, morphologies, and geologic settings. Phyllosilicates occur in several settings: in compositionally layered blankets hundreds of meters thick, superposed on eroded Noachian terrains; in lower layers of intracrater depositional fans; in layers with potential chlorides in sediments on intercrater plains; and as thousands of deep exposures in craters and escarpments. Carbonate-bearing rocks form a thin unit surrounding the Isidis basin. Hydrated silica occurs with hydrated sulfates in thin stratified deposits surrounding Valles Marineris. Hydrated sulfates also occur together with crystalline ferric minerals in thick, layered deposits in Terra Meridiani and in Valles Marineris and together with kaolinite in deposits that partially infill some highland craters. In this paper we describe each of the classes of deposits, review hypotheses for their origins, identify new questions posed by existing measurements, and consider their implications for ancient habitable environments. On the basis of current data, two to five classes of Noachian-aged deposits containing phyllosilicates and carbonates may have formed in aqueous environments with pH and water activities suitable for life.
University of Arizona1, California Institute of Technology2, Washington University in St. Louis3, Optech4, University of Bristol5, University of Washington6, Space Science Institute7, Dalhousie University8, University of Ottawa9, Max Planck Society10, Aarhus University11, Canadian Space Agency12, University of Texas at Dallas13, Tufts University14, University of Alberta15, Texas A&M University16, University of Copenhagen17, Search for extraterrestrial intelligence18, Ames Research Center19, University of Colorado Boulder20, University of Michigan21, Imperial College London22, Delft University of Technology23, York University24
TL;DR: The analysis of the data from the Phoenix mission revealed an alkaline environment, in contrast to that found by the Mars Exploration Rovers, indicating that many different environments have existed on Mars.
Abstract: The Phoenix mission investigated patterned ground and weather in the northern arctic region of Mars for 5 months starting 25 May 2008 (solar longitude between 76.5° and 148°). A shallow ice table was uncovered by the robotic arm in the center and edge of a nearby polygon at depths of 5 to 18 centimeters. In late summer, snowfall and frost blanketed the surface at night; H2O ice and vapor constantly interacted with the soil. The soil was alkaline (pH = 7.7) and contained CaCO3, aqueous minerals, and salts up to several weight percent in the indurated surface soil. Their formation likely required the presence of water.
TL;DR: The LAT Bright AGN Sample (LBAS) as discussed by the authors contains two radio galaxies, namely Centaurus A and NGC 1275, and 104 blazars consisting of 57 flat spectrum radio quasars (FSRQs), 42 BL Lac objects, and 5 BLazars with uncertain classification.
Abstract: The first three months of sky-survey operation with the Fermi Gamma Ray Space Telescope (Fermi) Large Area Telescope (LAT) reveals 132 bright sources at |b|>10 deg with test statistic greater than 100 (corresponding to about 10 sigma). Two methods, based on the CGRaBS, CRATES and BZCat catalogs, indicate high-confidence associations of 106 of these sources with known AGNs. This sample is referred to as the LAT Bright AGN Sample (LBAS). It contains two radio galaxies, namely Centaurus A and NGC 1275, and 104 blazars consisting of 57 flat spectrum radio quasars (FSRQs), 42 BL Lac objects, and 5 blazars with uncertain classification. Four new blazars were discovered on the basis of the LAT detections. Remarkably, the LBAS includes 10 high-energy peaked BL Lacs (HBLs), sources which were so far hard to detect in the GeV range. Another 10 lower-confidence associations are found. Only thirty three of the sources, plus two at |b|>10 deg, were previously detected with EGRET, probably due to the variable nature of these sources. The analysis of the gamma-ray properties of the LBAS sources reveals that the average GeV spectra of BL Lac objects are significantly harder than the spectra of FSRQs. No significant correlation between radio and peak gamma-ray fluxes is observed. Blazar log N - log S and luminosity functions are constructed to investigate the evolution of the different blazar classes, with positive evolution indicated for FSRQs but none for BLLacs. The contribution of LAT-blazars to the total extragalactic gamma-ray intensity is estimated.
TL;DR: In this article, the authors presented the initial results for energies above 100 MeV for the 205 most significant (statistical significance greater than 10-sigma) gamma-ray sources in early-mission data.
Abstract: Following its launch in June 2008, the Fermi Gamma-ray Space Telescope (Fermi) began a sky survey in August. The Large Area Telescope (LAT) on Fermi in 3 months produced a deeper and better-resolved map of the gamma-ray sky than any previous space mission. We present here initial results for energies above 100 MeV for the 205 most significant (statistical significance greater than 10-sigma) gamma-ray sources in these data. These are the best-characterized and best-localized gamma-ray sources in the early-mission data.
TL;DR: In this article, the Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) instruments on-board the Fermi observatory were used to observe the long gamma-ray burst, GRB 090902B.
Abstract: We report on the observation of the bright, long gamma-ray burst (GRB), GRB 090902B, by the Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) instruments on-board the Fermi observatory. ...
TL;DR: In this article, the authors presented new 5.2-14.5 {mu}m low-resolution spectra of 14 mid-L to mid-T dwarfs and compared these spectra to those generated from the model atmospheres of Saumon and Marley.
Abstract: We present new 5.2-14.5 {mu}m low-resolution spectra of 14 mid-L to mid-T dwarfs. We also present new 3.0-4.1 {mu}m spectra for five of these dwarfs. These data are supplemented by existing red and near-infrared spectra ({approx}0.6-2.5 {mu}m), as well as red through mid-infrared spectroscopy of seven other L and T dwarfs presented by Cushing et al. We compare these spectra to those generated from the model atmospheres of Saumon and Marley. The models reproduce the observed spectra well, except in the case of one very red L3.5 dwarf, 2MASS J22244381-0158521. The broad wavelength coverage allows us to constrain almost independently the four parameters used to describe these photospheres in our models: effective temperature (T {sub eff}), surface gravity, grain sedimentation efficiency (f{sub sed}), and vertical gas transport efficiency (K{sub zz} ). The CH{sub 4} bands centered at 2.2, 3.3, and 7.65 {mu}m and the CO band at 2.3 {mu}m are sensitive to K{sub zz} , and indicates that chemical mixing is important in all L and T dwarf atmospheres. The sample of L3.5 to T5.5 dwarfs spans the range 1800 K{approx}> T{sub eff} {approx}>1000 K, with an L-T transition (spectral types L7 to T4) that lies between 1400 and 1100more » K for dwarfs with typical near-infrared colors; bluer and redder dwarfs can be 100 K warmer or cooler, respectively, when using infrared spectral types. When using optical spectral types, the bluer dwarfs have more typical T {sub eff} values as they tend to have earlier optical spectral types. In this model analysis, f {sub sed} increases rapidly between types T0 and T4, indicating that increased sedimentation can explain the rapid disappearance of clouds at this stage of brown dwarf evolution. There is a suggestion that the transition to dust-free atmospheres happens at lower temperatures for lower gravity dwarfs.« less
TL;DR: In this article, the authors model the temperature and chemical structure of molecular clouds as a function of depth into the cloud, assuming a cloud of constant density n illuminated by an external far-ultraviolet (FUV) flux G 0 (scaling factor in multiples of the local interstellar field).
Abstract: We model the temperature and chemical structure of molecular clouds as a function of depth into the cloud, assuming a cloud of constant density n illuminated by an external far-ultraviolet (FUV; 6 eV
TL;DR: Batteries were chosen as an example of a complex system whose internal state variables are either inaccessible to sensors or hard to measure under operational conditions, where battery performance is strongly influenced by ambient environmental and load conditions and the Bayesian theory of uncertainty management provides a way to contain these problems.
Abstract: The estimation of remaining useful life (RUL) of a faulty component is at the centre of system prognostics and health management. It gives operators a potent tool in decision making by quantifying ...
TL;DR: In this paper, the authors calculate the rate of photoevaporation of a circumstellar disk by energetic radiation (far-UV), 6 eV 0.1 keV) from its central star.
Abstract: We calculate the rate of photoevaporation of a circumstellar disk by energetic radiation (far-UV (FUV), 6 eV 0.1 keV) from its central star. We focus on the effects of FUV and X-ray photons since EUV photoevaporation has been treated previously, and consider central star masses in the range 0.3-7 M ?. Contrary to the EUV photoevaporation scenario, which creates a gap at about rg ~ 7(M */M ?) AU and then erodes the outer disk from inside out, we find that FUV photoevaporation predominantly removes less bound gas from the outer disk. Heating by FUV photons can cause significant erosion of the outer disk where most of the mass is typically located. X-rays indirectly increase the mass-loss rates (by a factor of ~2) by ionizing the gas, thereby reducing the positive charge on grains and polycyclic aromatic hydrocarbons and enhancing FUV-induced grain photoelectric heating. FUV and X-ray photons may create a gap in the disk at ~10 AU under favorable circumstances. Photoevaporation timescales for M * ~ 1 M ? stars are estimated to be ~106 years, after the onset of disk irradiation by FUV and X-rays. Disk lifetimes do not vary much for stellar masses in the range 0.3-3 M ?. More massive stars (7 M ?) lose their disks rapidly (in ~105 years) due to their high EUV and FUV fields. Disk lifetimes are shorter for shallow surface density distributions and when the dust opacity in the disk is reduced by processes such as grain growth or settling. The latter suggests that the photoevaporation process may accelerate as the dust disk evolves.
TL;DR: A decline in the molar nickel to iron ratio recorded in banded iron formations about 27 Gyr ago was attributed to a reduced flux of nickel to the oceans, a consequence of cooling upper-mantle temperatures and decreased eruption of nickel-rich ultramafic rocks at the time as mentioned in this paper.
Abstract: It has been suggested that a decrease in atmospheric methane levels triggered the progressive rise of atmospheric oxygen, the so-called Great Oxidation Event, about 24 Gyr ago(1) Oxidative weathering of terrestrial sulphides, increased oceanic sulphate, and the ecological success of sulphate-reducing microorganisms over methanogens has been proposed as a possible cause for the methane collapse(1), but this explanation is difficult to reconcile with the rock record(2,3) Banded iron formations preserve a history of Precambrian oceanic elemental abundance and can provide insights into our understanding of early microbial life and its influence on the evolution of the Earth system(4,5) Here we report a decline in the molar nickel to iron ratio recorded in banded iron formations about 27 Gyr ago, which we attribute to a reduced flux of nickel to the oceans, a consequence of cooling upper-mantle temperatures and decreased eruption of nickel-rich ultramafic rocks at the time We measured nickel partition coefficients between simulated Precambrian sea water and diverse iron hydroxides, and subsequently determined that dissolved nickel concentrations may have reached similar to 400nM throughout much of the Archaean eon, but dropped below similar to 200nM by 25 Gyr ago and to modern day values(6) (similar to 9 nM) by similar to 550 Myr ago Nickel is a key metal cofactor in several enzymes of methanogens(7) and we propose that its decline would have stifled their activity in the ancient oceans and disrupted the supply of biogenic methane A decline in biogenic methane production therefore could have occurred before increasing environmental oxygenation and not necessarily be related to it The enzymatic reliance of methanogens on a diminishing supply of volcanic nickel links mantle evolution to the redox state of the atmosphere
TL;DR: The Fermi Large Area Telescope makes it possible to pinpoint neutron stars through their gamma-ray pulsations, enabling studies of emission mechanisms, population statistics, and the energetics of pulsar wind nebulae and supernova remnants.
Abstract: Pulsars are rapidly rotating, highly magnetized neutron stars emitting radiation across the electromagnetic spectrum. Although there are more than 1800 known radio pulsars, until recently only seven were observed to pulse in gamma rays, and these were all discovered at other wavelengths. The Fermi Large Area Telescope (LAT) makes it possible to pinpoint neutron stars through their gamma-ray pulsations. We report the detection of 16 gamma-ray pulsars in blind frequency searches using the LAT. Most of these pulsars are coincident with previously unidentified gamma-ray sources, and many are associated with supernova remnants. Direct detection of gamma-ray pulsars enables studies of emission mechanisms, population statistics, and the energetics of pulsar wind nebulae and supernova remnants.
TL;DR: Results suggest that the soil at the Phoenix landing site must have suffered alteration through the action of liquid water in geologically the recent past, and an alkaline environment was revealed, in contrast to that found by the Mars Exploration Rovers, indicating that many different environments have existed on Mars.
Abstract: Carbonates are generally products of aqueous processes and may hold important clues about the history of liquid water on the surface of Mars Calcium carbonate (approximately 3 to 5 weight percent) has been identified in the soils around the Phoenix landing site by scanning calorimetry showing an endothermic transition beginning around 725°C accompanied by evolution of carbon dioxide and by the ability of the soil to buffer pH against acid addition Based on empirical kinetics, the amount of calcium carbonate is most consistent with formation in the past by the interaction of atmospheric carbon dioxide with liquid water films on particle surfaces
01 Jan 2009
TL;DR: In this article, an empirical model to describe battery behavior during individual discharge cycles as well as over its cycle life is presented, which is linked to the internal processes of the battery and validated using experimental data.
Abstract: This paper presents an empirical model to describe battery behavior during individual discharge cycles as well as over its cycle life. The basis for the form of the model has been linked to the internal processes of the battery and validated using experimental data. Subsequently, the model has been used in a Particle Filtering framework to make predictions of remaining useful life for individual discharge cycles as well as for cycle life. The prediction performance was found to be satisfactory as measured by performance metrics customized for prognostics. The work presented here provides initial steps towards a comprehensive health management solution for energy storage devices.
TL;DR: In this paper, the authors model the growth of Jupiter via core nucleated accretion, applying constraints from hydrodynamical processes that result from the disk-planet interaction, and compute the planet's internal structure using a well tested planetary formation code that is based upon a Henyey-type stellar evolution code.
01 Jun 2009
TL;DR: In this article, a decline in the molar nickel to iron ratio recorded in banded iron formations about 2.7 Gyr ago was attributed to a reduced flux of nickel to the oceans, a consequence of cooling upper-mantle temperatures and decreased eruption of nickel-rich ultramafic rocks at the time.
Abstract: It has been suggested that a decrease in atmospheric methane levels triggered the progressive rise of atmospheric oxygen, the so-called Great Oxidation Event, about 2.4 Gyr ago(1). Oxidative weathering of terrestrial sulphides, increased oceanic sulphate, and the ecological success of sulphate-reducing microorganisms over methanogens has been proposed as a possible cause for the methane collapse(1), but this explanation is difficult to reconcile with the rock record(2,3). Banded iron formations preserve a history of Precambrian oceanic elemental abundance and can provide insights into our understanding of early microbial life and its influence on the evolution of the Earth system(4,5). Here we report a decline in the molar nickel to iron ratio recorded in banded iron formations about 2.7 Gyr ago, which we attribute to a reduced flux of nickel to the oceans, a consequence of cooling upper-mantle temperatures and decreased eruption of nickel-rich ultramafic rocks at the time. We measured nickel partition coefficients between simulated Precambrian sea water and diverse iron hydroxides, and subsequently determined that dissolved nickel concentrations may have reached similar to 400nM throughout much of the Archaean eon, but dropped below similar to 200nM by 2.5 Gyr ago and to modern day values(6) (similar to 9 nM) by similar to 550 Myr ago. Nickel is a key metal cofactor in several enzymes of methanogens(7) and we propose that its decline would have stifled their activity in the ancient oceans and disrupted the supply of biogenic methane. A decline in biogenic methane production therefore could have occurred before increasing environmental oxygenation and not necessarily be related to it. The enzymatic reliance of methanogens on a diminishing supply of volcanic nickel links mantle evolution to the redox state of the atmosphere.
TL;DR: The facility of designing optical biosensors based on fluorophore conjugates using 8 environmentally sensitive fluorophores and 11 bPBPs specific for diverse ligands, including sugars, amino acids, anions, cations, and dipeptides is demonstrated.
Abstract: Bacterial periplasmic binding proteins (bPBPs) are specific for a wide variety of small molecule ligands. bPBPs undergo a large, ligand-mediated conformational change that can be linked to reporter functions to monitor ligand concentrations. This mechanism provides the basis of a general system for engineering families of reagentless biosensors that share a common physical signal transduction functionality and detect many different analytes. We demonstrate the facility of designing optical biosensors based on fluorophore conjugates using 8 environmentally sensitive fluorophores and 11 bPBPs specific for diverse ligands, including sugars, amino acids, anions, cations, and dipeptides. Construction of reagentless fluorescent biosensors relies on identification of sites that undergo a local conformational change in concert with the global, ligand-mediated hinge-bending motion. Construction of cysteine mutations at these locations then permits site-specific coupling of environmentally sensitive fluorophores that report ligand binding as changes in fluorescence intensity. For 10 of the bPBPs presented in this study, the three-dimensional receptor structure was used to predict the location of reporter sites. In one case, a bPBP sensor specific for glutamic and aspartic acid was designed starting from genome sequence information and illustrates the potential for discovering novel binding functions in the microbial genosphere using bioinformatics.
TL;DR: In this paper, the gamma-ray emission from three radio-loud narrow-line Seyfert 1 galaxies was detected with Fermi/LAT, and they may form an emerging new class of gamma ray active galactic nuclei (AGN).
Abstract: We report the discovery with Fermi/LAT of gamma-ray emission from three radio-loud narrow-line Seyfert 1 galaxies: PKS 1502+036 (z=0.409), 1H 0323+342 (z=0.061) and PKS 2004-447 (z=0.24). In addition to PMN J0948+0022 (z=0.585), the first source of this type to be detected in gamma rays, they may form an emerging new class of gamma-ray active galactic nuclei (AGN). These findings can have strong implications on our knowledge about relativistic jets and the unified model of AGN.
Search for extraterrestrial intelligence1, University of Khartoum2, University of Juba3, Fordham University4, American Museum of Natural History5, Carnegie Institution for Science6, California Institute of Technology7, Queen's University Belfast8, Armagh Observatory9, University of Western Ontario10, Sandia National Laboratories11, Academy of Sciences of the Czech Republic12, Czech Hydrometeorological Institute13, Institut de mécanique céleste et de calcul des éphémérides14, Meteor15, Ames Research Center16
TL;DR: A dedicated search along the approach trajectory recovered 47 meteorites, fragments of a single body named Almahata Sitta, with a total mass of 3.95 kg, identifying the asteroid as F class, now firmly linked to dark carbon-rich anomalous ureilites, a material so fragile it was not previously represented in meteorite collections.
Abstract: On 6 October 2008, a small Earth-bound asteroid designated 2008 TC3 was discovered by the Catalina Sky Survey. Some 19 hours — and many astronomical observations — later it entered the atmosphere and disintegrated at 37 km altitude. No macroscopic fragments were expected to have survived but a dedicated search along the approach trajectory in a desert in northern Sudan has recovered 47 meteorites, fragments of a single body named Almahata Sitta, with a total mass of 3.95 kg. The asteroid and meteorite reflectance spectra identify the asteroid as surface matter from a class 'F' asteroid, material so fragile that it was not previously represented in meteorite collections. To have recovered meteorites from a known class of asteroids is a coup on a par with a successful spacecraft sample-return mission — without the rocket science. On 6 October 2008, a small asteroid designated 2008 TC3 hit the Earth in northern Sudan. Jenniskens et al. searched along the approach trajectory and luckily found 47 bits of a meteorite named Almahata Sitta. Analysis reveals it to be a porous achondrite and a polymict ureilite, and so the asteroid was F-class (dark carbon-rich anomalous ureilites). In the absence of a firm link between individual meteorites and their asteroidal parent bodies, asteroids are typically characterized only by their light reflection properties, and grouped accordingly into classes1,2,3. On 6 October 2008, a small asteroid was discovered with a flat reflectance spectrum in the 554–995 nm wavelength range, and designated 2008 TC3 (refs 4–6). It subsequently hit the Earth. Because it exploded at 37 km altitude, no macroscopic fragments were expected to survive. Here we report that a dedicated search along the approach trajectory recovered 47 meteorites, fragments of a single body named Almahata Sitta, with a total mass of 3.95 kg. Analysis of one of these meteorites shows it to be an achondrite, a polymict ureilite, anomalous in its class: ultra-fine-grained and porous, with large carbonaceous grains. The combined asteroid and meteorite reflectance spectra identify the asteroid as F class3, now firmly linked to dark carbon-rich anomalous ureilites, a material so fragile it was not previously represented in meteorite collections.
TL;DR: The discovery of bright gamma-ray emission coincident with supernova remnant (SNR) W51C is reported using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope as discussed by the authors.
Abstract: The discovery of bright gamma-ray emission coincident with supernova remnant (SNR) W51C is reported using the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. W51C is a middle-aged remnant (~10^4 yr) with intense radio synchrotron emission in its shell and known to be interacting with a molecular cloud. The gamma-ray emission is spatially extended, broadly consistent with the radio and X-ray extent of SNR W51C. The energy spectrum in the 0.2-50 GeV band exhibits steepening toward high energies. The luminosity is greater than 1x10^{36} erg/s given the distance constraint of D>5.5 kpc, which makes this object one of the most luminous gamma-ray sources in our Galaxy. The observed gamma-rays can be explained reasonably by a combination of efficient acceleration of nuclear cosmic rays at supernova shocks and shock-cloud interactions. The decay of neutral pi-mesons produced in hadronic collisions provides a plausible explanation for the gamma-ray emission. The product of the average gas density and the total energy content of the accelerated protons amounts to 5x10^{51}(D/6kpc)^2 erg/cm^3. Electron density constraints from the radio and X-ray bands render it difficult to explain the LAT signal as due to inverse Compton scattering. The Fermi LAT source coincident with SNR W51C sheds new light on the origin of Galactic cosmic rays.
TL;DR: In this article, a new one-dimensional photochemical kinetics code was developed to address stratospheric chemistry and stratosphere heating in hot Jupiters, where the authors used molecular theory to compute an HS absorption spectrum from sparse available data and found that HS should absorb strongly between 300 and 460 nm.
Abstract: We develop a new one-dimensional photochemical kinetics code to address stratospheric chemistry and stratospheric heating in hot Jupiters. Here we address optically active S-containing species and CO2 at 1200 ≤ T ≤ 2000 K. HS (mercapto) and S2 are highly reactive species that are generated photochemically and thermochemically from H2S with peak abundances between 1 and 10 mbar. S2 absorbs UV between 240 and 340 nm and is optically thick for metallicities [S/H]>0 at T ≥ 1200 K. HS is probably more important than S2, as it is generally more abundant than S2 under hot Jupiter conditions and it absorbs at somewhat redder wavelengths. We use molecular theory to compute an HS absorption spectrum from sparse available data and find that HS should absorb strongly between 300 and 460 nm, with absorption at the longer wavelengths being temperature sensitive. When the two absorbers are combined, radiative heating (per kg of gas) peaks at 100 μbars, with a total stratospheric heating of ~8 × 104 W m–2 for a jovian planet orbiting a solar-twin at 0.032 AU. Total heating is insensitive to metallicity. The CO2 mixing ratio is a well behaved quadratic function of metallicity, ranging from 1.6 × 10–8 to 1.6 × 10–4 for –0.3 < [M/H] < 1.7. CO2 is insensitive to insolation, vertical mixing, temperature (1200 < T < 2000), and gravity. The photochemical calculations confirm that CO2 should prove a useful probe of planetary metallicity.