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At least 739 records · Page 41
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NASA NTRS1990

Viscous effects on the instability of an axisymmetric jet

The stability characteristics of a laminar, axisymmetric jet, issuing from fully developed Poiseuille flow, are investigated. The jet preferred frequency, as inferred from surveys of u'-spectra, is found to yield a Strouhal number (St) that depends on the Reynolds number (R); St and R are based on the jet diameter (D) and the average velocity (U sub av) at the jet origin. The value of St increases with increasing R in the range 400 less than approximately R less than approximately 4000, attaining an asymptotic value of about 0.45. Flow visualization confirms that the instability is primarily in a helical mode, as predicted by stability analyses. Analyses do predict a similar St versus R variation in approximately the correct St-range. However, the R-range where this is predicted is lower than that found experimentally.

Zaman, K. B. M. Q.↗
NASA NTRS1989

Grain mantles: The impact on grain evolution and selective extinction

Depletion studies are used to infer the presence of mantles and to constrain grain evolutionary models in the diffuse interstellar medium. The presence of these mantles appears to be important in the evolution of the grains inside diffuse as well as dense clouds. In dense clouds where the element-to-element abundances sometimes differ from those found in diffuse clouds, empirical relationships are starting to emerge between gas abundances and various types of peculiar selective extinction. These peculiar extinction curves may be the results of nonvolatile mantle formation on grain cores or may reflect chemical differences due to variations in the intrinsic metalicity from one cloud to another. A simple model of the time evolution of a parcel of gas and dust as observed by the depletion of two elements is presented. Different studies of grain evolution and selective extinction are discussed and compared.

Joseph, Charles L.↗
NASA NTRS1993

Fe/Mn in olivine of carbonaceous meteorites

Olivines in primitive meteorites show a range of Fe/Mn both within one grain and among grains suggesting that they have recorded changing conditions during or after growth. Because olivine should be an early forming phase, Fe/Mn is used here to infer these earliest conditions. Initial Fe/Mn in cores of isolated, euhedral forsterite in both C2 and C3 meteorites ranges from 25 to 35 but differs at grain edge. Murchison (C2) forsterites show Fe/Mn approaching 1.0 at the grain edge while Ornans Fe/Mn is near 60 at grain edge. These values are lower than the matrix Fe/Mn for both meteorites and the distinct difference in zoning profile indicates different processes operating during and after grain growth. The Fe/Mn of bulk samples from a particular source such as the Moon is nearly constant. Individual samples show variation suggesting that there is some fractionation of Mn from Fe. Minerals have their individual ranges of Fe/Mn which has been used to recognize different types of olivine within one meteorite. Extreme values of Fe/Mn below 1.0 occur in forsterite from some IDP's, UOC matrix, and C1 meteorites. There are apparently no detailed studies of Fe/Mn variation within single olivine grains. Forsterite grains in C2 and C3 carbonaceous chondrites show complex zoning, and the nearly pure forsterites (Fo greater than approximately 99.5) have high levels of some minor elements including Ti, Al, V, and Sc. There is disagreement on the original source of these grains and both chondrule and vapor growth have been proposed. In addition, there is clear evidence that diffusion has affected the outer margins but in some cases the whole grain. Within the cores, the FeO range is limited, and if growing under constant conditions, the Fe/Mn should be near constant as there is little fractionation of Mn from Fe by forsterite. Additionally, there are apparently no co-crystallizing phases as evidenced by a lack of common inclusions in the forsterites. These observations are now followed by analyses of isolated olivine grains in C2 and C3 meteorites.

Steele, Ian M.↗
NASA NTRS2003

Early Petrologic Processes on the Ureilite Parent Body

We present a petrographic and petrologic analysis of 21 olivine-pigeonite ureilites, along with new experimental results on melt compositions predicted to be in equilibrium with ureilite compositions. We conclude that these ureilites are the residues of a partial melting/smelting event. Textural evidence preserved in olivine and pigeonite record the extent of primary smelting. In pigeonite cores, we observe fine trains of iron metal inclusions that formed by the reduction of olivine to pigeonite and metal during primary smelting. Olivine cores lack metal inclusions but the outer grain boundaries are variably reduced by a late-stage reduction event. The modal proportion of pigeonite and percentage of olivine affected by late stage reduction are inversely related and provide an estimation of the degree of primary smelting during ureilite petrogenesis. In our sample suite, this correlation holds for 16 of the 21 samples examined. Olivine-pigeonite-liquid phase equilibrium constraints are used to obtain temperature estimates for the ureilite samples examined. Inferred smelting temperatures range from approximately 1150 C to just over 1300 C and span the range of estimates published for ureilites containing two or more pyroxenes. Temperature is also positively correlated with modal percent pigeonite. Smelting temperature is inversely correlated with smelting depth--the hottest olivine-pigeonite ureilites coming from the shallowest depth in the ureilite parent body. The highest temperature samples also have oxygen isotopic signatures that fall toward the refractory inclusion-rich end of the carbonaceous chondrite-anhydrous mineral (CCAM) slope 1 mixing line. These temperature-depth variations in the ureilite parent body could have been created by a heterogeneous distribution of heat producing elements, which would indicate that isotopic heterogeneities existed in the material from which the ureilite parent body was assembled.

Singletary, S. J.↗
NASA NTRS2012

The Ice, Cloud, and land Elevation Satellite (ICESat) Summary Mission Timeline and Performance Relative to Pre-Launch Mission Success Criteria

The Ice, Cloud and land Elevation Satellite (ICESat) mission was conceived, primarily, to quantify the spatial and temporal variations in the topography of the Greenland and Antarctic ice sheets. It carried on board the Geoscience Laser Altimeter System (GLAS), which measured the round-trip travel time of a laser pulse emitted from the satellite to the surface of the Earth and back. Each range derived from these measurements was combined with precise, concurrent orbit and pointing information to determine the location of the laser spot centroid on the Earth. By developing a time series of precise topographic maps for each ice sheet, changes in their surface elevations can be used to infer their mass balances.

Webb, Charles E.↗
NASA NTRS2017

Oxychlorine Species in Gale Crater and Broader Implications for Mars

Of 15 samples analyzed to date, the Sample Analysis at Mars (SAM) instrument on the Mars Science Laboratory (MSL) has detected oxychlorine compounds (perchlorate or chlorate) in 12 samples. The presence of oxychlorine species is inferred from the release of oxygen at temperatures less than 600degC and HCl between 350-850degC when a sample is heated to 850degC. The O2 release temperature varies with sample, likely caused by different cations, grain size differences, or catalytic effects of other minerals. In the oxychlorine-containing samples, perchlorate abundances range from 0.06 +/- 0.03 to 1.15 +/- 0.5 wt% Cl2O7 equivalent. Comparing these results to the elemental Cl concentration measured by the Alpha Particle X-ray Spectrometer (APXS) instrument, oxychlorine species account for 5-40% of the total Cl present. The variation in oxychlorine abundance has implications for their production and preservation over time. For example, the John Klein (JK) and Cumberland (CB) samples were acquired within a few meters of each other and CB contained approximately1.2 wt% Cl2O7 equivalent while JK had approximately 0.1 wt%. One difference between the two samples is that JK has a large number of veins visible in the drill hole wall, indicating more post-deposition alteration and removal. Finally, despite Cl concentrations similar to previous samples, the last three Murray formation samples (Oudam, Marimba, and Quela) had no detectable oxygen released during pyrolysis. This could be a result of oxygen reacting with other species in the sample during pyrolysis. Lab work has shown this is likely to have occurred in SAM but it is unlikely to have consumed all the O2 released. Another explanation is that the Cl is present as chlorides, which is consistent with data from the ChemCam (Chemical Camera) and CheMin (Chemistry and Mineralogy) instruments on MSL. For example, the Quela sample has approximately1 wt% elemental Cl detected by APXS, had no detectable O2 released, and halite (NaCl) has been tentatively identified in CheMin X-ray diffraction data. These data show that oxychlorines are likely globally distributed on Mars but the distribution is heterogenous depending on the perchlorate formation mechanism (production rate), burial, and subsequent diagenesis

Ming, Douglas W.↗
NASA NTRS2005

Changes in the Temperature of Saturn's Stratosphere from 2002 to 2004 and Direct Evidence of a Mesopause

The state of a planetary stratosphere is intricately linked to the radiation it receives from its host star. The stellar flux impinging on the the planetary stratosphere affects the temperature and chemistry occurring there. The revolution of the planet about its orbit coupled to the planets axial tilt can cause variations in the amount of flux captured by the planet at a given latitude over the period of a year even if the flux output of the star is constant. This variation of stellar flux at a given latitude during the orbit of a planet is what causes seasons. Saturn, with its axial tilt of 27 degrees, experiences seasonal forcing much like the Earth, albeit with far less solar flux and a year equal to 30 Earth years. In an attempt to measure the effect seasonal forcing has on the temperature structure of Saturn's stratosphere, we have started a long term observing program. By observing Saturn with a high-resolution mid-infrared spectrograph in the v(sub 4) band of methane, we can infer the stratospheric temperature between the pressure levels of 10 and 0.01 mbar. We present latitudinally resolved observations of Saturn's southern hemisphere from 2002 and 2004. We have analyzed the data using a line-by-line radiative transfer model and inferred stratospheric temperatures and winds. We compare the temperatures from the two observing runs and the predictions of a stratospheric seasonal climate model. The zonal winds in the southern hemisphere, between the 10 and 0.01 mbar pressure levels, will be shown and discussed.

Greathouse, Thomas K.↗
NASA NTRS2020

Mg Isotopic Compositions of Fine-Grained Ca-Al-Rich Inclusions from Reduced CV3 Chondrites and Implications on the Timescale of Nebular Condensation

Fine-grained Ca-Al-rich inclusions (FGIs) in carbonaceous chondrites are interpreted as aggregates of nebular gas-solid condensates that escaped significant melting [1]. Recent Al-Mg isotopic studies of small FGIs (<200 µm in size) from pristine CO3 chondrites have revealed significant variations in initial 26Al/27Al ratio, (26Al/27Al)(sub 0), spanning from 5.73×10(exp ˗5) to 8×10(exp ˗6) [2,3]. Importantly, [3] found a main CAI population characterized by (26Al/27Al)(sub 0) = 5.4×10(exp ˗5) that likely record the onset of dust condensation and initial coagulation events, followed by multiple thermal events over an extended time period. Thus far, a small number of large FGIs from CV3 chondrites have been analyzed, showing a narrower range of ( 26Al/27Al)(sub 0) = 5.27×10(exp ˗5) to 3.35×10(exp ˗5) [4-6], similar to those inferred from coarse-grained, igneous CAIs (CGIs) from CV3 chondrites [7]. These observed different ratios may raise a question if large FGIs in CV3 chondrites record a time interval from primary direct condensation of refractory solids to their aggregation and growth into larger inclusions. In this study, we present Al-Mg isotopic compositions of FGIs from reduced CV3 chondrites to better constrain their formation timescale relative to smaller FGIs in CO3 chondrites, as well as to CGIs in CV3 chondrites. This isotopic study is coordinated with a microstructural study to elucidate the origin and nature of FGIs in the petrologic and mineralogical context.

Han, J.↗
NASA NTRS2014

Variation of a Lightning NOx Indicator for National Climate Assessment

In support of the National Climate Assessment (NCA) program, satellite Lightning Imaging Sensor (LIS) data is used to estimate lightning nitrogen oxides (LNOx) production over the southern portion of the conterminous US. The total energy of each flash is estimated by analyzing the LIS optical event data associated with each flash (i.e., event radiance, event footprint area, and derivable event range). The LIS detects an extremely small fraction of the total flash energy; this fraction is assumed to be constant apart from the variability associated with the flash optical energy detected across the narrow (0.909 nm) LIS band. The estimate of total energy from each flash is converted to moles of LNOx production by assuming a chemical yield of 10(17) molecules Joule(-1). The LIS-inferred variable LNOx production from each flash is summed to obtain total LNOx production, and then appropriately enhanced to account for LIS detection efficiency and LIS view time. Annual geographical plots and time series of LNOx production are provided for a 16 year period (1998-2013).

Koshak, W. J.↗
NASA NTRS1977

Terrestrial kilometric radiation. III - Average spectral properties

The spectral properties of terrestrial kilometric radiation (TKR) derived from observations made during radio-astronomy experiments on board the Imp 6 and Radio Astronomy Explorer 2 spacecraft are studied. As viewed from near the equatorial plane, TKR is most intense and most often observed in the 2100-2400 LT zone and is rarely seen in the 0900-1200 LT zone. The absolute flux levels in the 100- to 600-kHz TKR band increase significantly with increasing substorm activity as inferred from the auroral electrojet index (AE). In the late-evening sector the median power increases by about 3 orders of magnitude between quiet periods (AE less than 75 gammas) and disturbed periods (AE above 200 gammas). The peak flux density usually occurs near 250 kHz, although the frequency of the peak in the flux spectrum appears to vary inversely with AE from a maximum near 300 kHz during very quiet times to a minimum below 200 kHz during very disturbed times. The half-power bandwidth is typically 100% of the peak frequency. The variation of TKR flux density with apparent source altitude indicates that source strength decreases more rapidly than the inverse square of distance.

Kaiser, M. L.↗
NASA NTRS1986

On transient rheology and glacial isostasy

The effect of transient creep on the inference of long-term mantle viscosity is investigated using theoretical predictions from self-gravitating, layered earth models with Maxwell, Burgers' body, and standard linear solid rheologies. The interaction between transient and steady-state rheologies is studied. The responses of the standard linear solid and Burgers' body models to transient creep in the entire mantle, and of the Burgers' body and Maxwell models to creep in the lower mantle are described. The models' responses are examined in terms of the surface displacement, free air gravity anomaly, wander of the rotation pole, and the secular variation of the degree 2 zonal coefficient of the earth's gravitational potential field. The data reveal that transient creep cannot operate throughout the entire mantle.

Yuen, David A.↗
NASA NTRS1989

Formation of Fourier phase shifts in the solar Ni I 6768 A line

A formalism is developed to understand better how Doppler shifts of spectrum lines as inferred from phase shifts in the Fourier transforms of line profiles are related to the underlying velocity structures which they are intended to measure. With a standard model atmosphere and a simplified, quasi-LTE treatment of line formation, the formalism is applied to the Ni I 6768 A line, which has been selected for use with a network of imaging interferometers under development by the Global Oscillations Network Group for research in helioseismology. Fourier phase shifts are found to be a remarkably linear measure of velocity even in the presence of gradients and unresolved lateral variations in the assumed velocity field. An assumed outward increase in amplitude of a model oscillatory velocity is noticeably reflected in the center-to-limb behavior of the simulated velocity measure, and a sample model of solar granulation is found to have a strong influence on the formation of the Fourier phase.

Jones, Harrison P.↗
NASA NTRS1992

Biodiversity: molecular biological domains, symbiosis and kingdom origins

The number of extant species of organisms is estimated to be from fewer than 3 to more than 30 x 10(6) (May, 1992). Molecular biology, comparative genetics and ultrastructural analyses provide new insights into evolutionary relationships between these species, including increasingly precise ideas of how species and higher taxa have evolved from common ancestors. Accumulation of random mutations and large macromolecular sequence change in all organisms since the Proterozoic Eon has been importantly supplemented by acquisition of inherited genomes ('symbiogenesis'). Karyotypic alterations (polyploidization and karyotypic fissioning) have been added to these other mechanisms of species origin in plants and animals during the Phanerozoic Eon. The new evolution concepts (coupled with current rapid rates of species extinction and ignorance of the extent of biodiversity) prompted this analysis of the field of systematic biology and its role in the reorganization of extant species into higher taxa. Two superkingdoms (= Domains: Prokaryotae and Eukaryotae) and five kingdoms (Monera = Procaryotae or Bacteria; Protoctista: algae, amoebae, ciliates, foraminifera, oomycetes, slime molds, etc.; Mychota: 'true' fungi; Plantae: one phylum (division) of bryophytes and nine phyla of tracheophytes; and Animalia) are recognized. Two subkingdoms comprise the monera: the great diverse lineages are Archaebacteria and Eubacteria. The criteria for classification using molecular, ultrastructural and genetic data for this scheme are mentioned. For the first time since the nineteenth century, logical, technical definitions for each group are given with their time of appearance as inferred from the fossil record in the primary scientific literature. This classification scheme, which most closely reflects the evolutionary history, molecular biology, genetics and ultrastructure of extant life, requires changes in social organization of biologists, many of whom as botanists and zoologists, still behave as if there were only two important kingdoms (plants and animals).

Non-NASA Center↗
NASA NTRS2000

Photochemistry of Saturn's Atmosphere: Hydrocarbon Chemistry and Comparisons with ISO Observations - 1

To investigate the details of hydrocarbon photochemistry on Saturn, we have developed a one-dimensional diurnally averaged model that couples hydrocarbon and oxygen photochemistry, molecular and eddy diffusion, radiative transfer, and condensation. The model results are compared with observations from the Infrared Space Observatory (ISO) to place tighter constraints on molecular abundances, to better define Saturn's eddy diffusion coefficient profile, and to identify important chemical schemes that control the abundances of the observable hydrocarbons in Saturn's upper atmosphere. From the ISO observations, we determine that the column 12 densities of CH3, CH3C2H, and C4H2 above 10 mbar are 4 (sup +2) (sub -1.5) x 10 (exp 13) cm (sup -2), (1.1 plus or minus 0.3) x 10 (exp 15) cm (exp -2), and (1.2 plus or minus 0.3) x 10 (exp 14) cm (sup -2), respectively. The observed ISO emission features also indicate C2H2 mixing ratios of 1.2 (sup +0.9) (sub -0.6) x 10 (exp -6) at 0.3 mbar and (2.7 plus or minus 0.8) x 10 (exp -7) at 1.4 mbar, and a C2H6 mixing ratio of (9 plus or minus 2.5) x 10 (exp -6) at 0.5 mbar. Upper limits are provided for C2H4, CH2CCH2, C3H8, and C6H2 sensitivity of the model results to variations in the eddy diffusion coefficient profile, the solar flux, the CH4 photolysis branching ratios, the atomic hydrogen influx, and key reaction rates are discussed in detail. We find that C4H2 and CH3C2H are particularly good tracers of important chemical processes and physical conditions in Saturn's upper atmosphere, and C2H6 is a good tracer of the eddy diffusion coefficient in Saturn's lower stratosphere. The eddy diffusion coefficient must be smaller than approximately 3 x 10 (exp 4) sq cm s (sup -1) at pressures greater than 1 mbar in order to reproduce the C2H6 abundance inferred from ISO observations. The eddy diffusion coefficients in the upper stratosphere could be constrained by observations of CH3 radicals if the low-temperature chemistry of CH3 were better understood. We also discuss the implications of our modeling for aerosol formation in Saturn's lower stratosphere-diacetylene, butane, and water condense between approximately 1 and 300 mbar in our model and will dominate stratospheric haze formation at nonauroral latitudes. Our photochemical models will be useful for planning observational sequences and for analyzing data from the upcoming Cassini mission.

Moses, Julianne I.↗
NASA NTRS2012

Observations of GEO Debris with the Magellan 6.5-m Telescopes

Optical observations of geosynchronous orbit (GEO) debris are important to address two questions: 1. What is the distribution function of objects at GEO as a function of brightness? With some assumptions, this can be used to infer a size distribution. 2. Can we determine what the likely composition of individual GEO debris pieces is from studies of the spectral reflectance of these objects? In this paper we report on optical observations with the 6.5-m Magellan telescopes at Las Campanas Observatory in Chile that attempt to answer both questions. Imaging observations over a 0.5 degree diameter field-of-view have detected a significant population of optically faint debris candidates with R > 19th magnitude, corresponding to a size smaller than 20 cm assuming an albedo of 0.175. Many of these objects show brightness variations larger than a factor of 2, suggesting either irregular shapes or albedo variations or both. The object detection rate (per square degree per hour) shows an increase over the rate measured in the 0.6-m MODEST observations, implying an increase in the population at optically fainter levels. Assuming that the albedo distribution is the same for both samples, this corresponds to an increase in the population of smaller size debris. To study the second issue, calibrated reflectance spectroscopy has been obtained of a sample of GEO and near GEO objects with orbits in the public U.S. Space Surveillance Network catalog. With a 6.5-m telescope, the exposures times are short (30 seconds or less), and provide simultaneous wavelength coverage from 4500 to 8000 Angstroms. If the observed objects are tumbling, then simultaneous coverage and short exposure times are essential for a realistic assessment of the object fs spectral signature. We will compare the calibrated spectra with lab-based measurements of simple spacecraft surfaces composed of a single material.

Seitzer, Patrick↗
NASA NTRS1974

Exospheric temperature and composition from satellite beacon measurements

Routine measurements of the slab thickness of the ionosphere, from 1965 to 1971, are used to infer the changes in neutral temperature and ion composition at a mean latitude of 40 S. Values of neutral temperature at solar maximum are 5 to 10% above Northern Hemisphere backscatter results. The diurnal and seasonal changes agree closely with satellite drag and backscatter measurements, except that the maximum temperature occurs after sunset in winter. Winter night-time values of the O(+)/H(+) transition height were 500 km in 1965-1966, 800 km in 1968-1969, and 700 km in 1971. Changes in the transition height lag about six months behind the changes in solar flux. Diurnal variations have a minimum just before sunrise and a maximum 1 to 3 hr after noon. On winter nights the transition height descends to the level set by chemical equilibrium. On summer nights the transition height is always above this level, giving a continual production of H(+) which serves as an additional source for maintaining the night-time ionosphere in the winter hemisphere.

Titheridge, J. E.↗
NASA NTRS1978

A magnetohydrodynamic theory for the lunar response to time variations in a spatially uniform ambient magnetic field

An analytic theory based on principles of magnetohydrodynamics is derived for interpreting the lunar magnetic response to magnetic field fluctuations in the lobes of the geomagnetic tail. Only the case of magnetic perturbations parallel to a spatially uniform ambient magnetic field is treated. The theoretical frequency-dependent MHD transfer function depends on both the lunar electrical conductivity profile and the Alfven speed of the tail lobe plasma. For circular frequencies much less than the Alfven speed/lunar radius, the MHD transfer function reduces to the vacuum transfer function, while at higher frequencies the finite propagation speed of magnetic disturbances affects the response. These characteristics of the MHD transfer function are in qualitative agreement with observation. A carefully selected set of magnetic field transient events could be used together with this theory to infer lunar electrical conductivity profiles and the tail lobe plasma density (via its dependence on the Alfven speed).

Hood, L. L.↗
NASA NTRS1984

Effect of an oscillating flow direction on leading edge heat transfer

An experimental investigation was conducted to examine the effect of a periodic variation in the angle of attack on heat transfer at the leading edge of a gas turbine blade. A circular cylinder was used as a large-scale model of the leading edge region. The cylinder was placed in a wind tunnel and was oscillated rotationally about its axis. The incident flow Reynolds number and the Strouhal number of oscillation were chosen to model an actual turbine condition. Incident turbulence levels up to 4.9 percent were produced by grids placed upstream of the cylinder. The transfer rate was measured using a mass transfer technique and heat transfer rates inferred from the results. A direct comparison of the unsteady and steady results indicate that the effect is dependent on the Strouhal number, turbulence level, and the turbulence length scale, but that the largest observed effect was only a 10 percent augmentation at the nominal stagnation position.

Marziale, M. L.↗