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At least 955 records · Page 53
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Size distributions in two porous chondritic micrometeorites

Quantitative size measurements of granular units (GUs), and nm-sized minerals in these units, in two porous chondritic micrometeorites are investigated. The matrix of these micrometeorites consist of loosely packed, 0.1 micron-sized, GUs. These objects were a major component of the solar nebula dust that accreted into protoplanets. The matrix in micrometeorite W7010*A2 has a fractal dimension with a small coefficient that supports efficient sticking of carbon-rich GUs during accretion. The fractal nature of the matrix provides a way to calculate the density using the aggregate size. The resulting very low density for porous chondritic micrometeorites is 0.08-0.14 g/cu cm, which supports the view that they are the solid debris from unconsolidated solar system bodies. Chondritic GUs contain ultrafine olivines, pyroxenes, and sulfides, embedded in hydrocarbons and amorphous carbons. Nanocrystals in the micrometeorites W7010*A2 and U2015*B show log normal size distributions. The high incidence of disk-shaped grains, a changeover from disk-shaped to euhedral grains, the unevolved nature of the size distributions, and multiple populations for grains less than 127 nm in size, are consistent with continuous postaccretion nucleation and growth in amorphous GUs, including coarsening via Ostwald ripening.

Rietmeijer, Frans J. M.↗
NASA NTRS1985

Theoretical studies of the marine sulfur cycle

Several reduced sulfur compounds are produced by marine organisms and then enter the atmosphere, where they are oxidized and ultimately returned to the ocean or the land. The oceanic dimethyl sulfide (DMS) flux, in particular, represents a significant fraction of the annual global sulfur input to the atmosphere. In the atmosphere, this gas is converted to sulfur dioxide (SO2), methane sulfonic acid, and other organic acids which are relatively stable and about which little is known. SO2 is a short lived gas which, in turn, is converted to sulfuric acid and other sulfate compounds which contribute significantly to acid rain. Because of the complexity of the sulfur system, it is not well understood even in the unperturbed atmosphere. However, a number of new observations and experiments have led to a significant increase in the understanding of this system. A number of one dimensional model experiments were conducted on the gas phase part of the marine sulfur cycle. The results indicate the measured concentration of DMS and the amplitude of its diurnal cycle are in agreement with estimates of its global flux. It was also found that DMS can make a large contribution to the background SO2 concentration in the free troposphere. Estimates of CS2 concentrations in the atmosphere are inconsistent with estimated fluxes; however, measured reaction rates are consistent with the observed steep tropospheric gradient in CS2. Observations of CS2 are extremely sparse. Further study is planned.

Toon, Owen B.↗
NASA NTRS1985

A field gas chromatograph using technology developed for solar system exploration

Trace gas analysis is an integral part of biospheric studies. Analytical instruments, primarily gas chromatographs (GC), are capable of measuring gases and volatiles to the ppb-level in real time. Trace gases significant in the study of biocycles include nitrous oxide, hydrogen sulfide, other nitrogen and sulfur species, as well as methane and ethylene. The concept of a field gas chromatograph is derived from technology being pursued in the design of ultra-compact instruments for solar system exploration. The instrument breadboard incorporates the specialized porous column packings and the highly sensitive metastable ionization detector developed by the Solar System Exploration Office. These parts ensure a broad capability for which the analysis of ambient N2O is one example. A commercial, portable gas chromatograph is currently being extensively modified to incorporate analytical concepts and components derived from flight GC technology. Data storage devices suitable for field use are presently being studied.

Woeller, F. H.↗
NASA NTRS1953

Study of secondary-flow patterns in an annular cascade of turbine nozzle blades with vortex design

In order to increase understanding of the origin of losses in a turbine, the secondary-flow components in the boundary layers and the blade wakes of an annular cascade of turbine nozzle blades (vortex design) was investigated. A detailed study was made of the total-pressure contours and, particularly, of the inner-wall loss cores downstream of the blades. The inner-wall loss core associated with a blade of the turbine-nozzle cascade is largely the accumulation of low-momentum fluids originating elsewhere in the cascade. This accumulation is effected by a secondary-flow mechanism which acts to transport the low-momentum fluids across the channels on the walls and radially in the blade wakes and boundary layers. The patterns of secondary flow were determined by use of hydrogen sulfide traces, paint, flow fences, and total pressure surveys. At one flow condition investigated, the radial transport of low-momentum fluid in the blade wake and on the suction surface near the trailing edge accounted for 65 percent of the loss core; 30 percent resulted from flow in the thickened boundary layer on the suction surface and 35 percent from flow in the blade wake.

FLOW, SUBSONIC↗
NASA NTRS1994

Cubic GaS: A Surface Passivator For GaAs

Thin films of cubic form of gallium sulfide (GaS) formed on surfaces of gallium arsenide (GaAs) substrates via metal/organic chemical vapor deposition (MOCVD). Deposited cubic GaS, crystalline lattice matched to substrate GaAs, neutralizes electrically active defects on surfaces of both n-doped and p-doped GaAs. Enabling important GaAs-based semiconducting materials to serve as substrates for metal/insulator/semiconductor (MIS) capacitors. Cubic GaS enables fabrication of ZnSe-based blue lasers and light-emitting diodes. Because GaS is optically transparent, deposited to form window layers for such optoelectronic devices as light-emitting diodes, solar optical cells, and semiconductor lasers. Its transparency makes it useful as interconnection material in optoelectronic integrated circuits. Also useful in peeled-film technology because selectively etched from GaAs.

Hepp, Aloysius F.↗
NASA NTRS1994

Program For Joule-Thomson Analysis Of Mixed Cryogens

JTMIX computer program predicts ideal and realistic properties of mixed gases at temperatures between 65 and 80 K. Performs Joule-Thomson analysis of any gaseous mixture of neon, nitrogen, various hydrocarbons, argon, oxygen, carbon monoxide, carbon dioxide, and hydrogen sulfide. When used in conjunction with DDMIX computer program of National Institute of Standards and Technology (NIST), JTMIX accurately predicts order-of-magnitude increases in Joule-Thomson cooling capacities occuring when various hydrocarbons added to nitrogen. Also predicts boiling temperature of nitrogen depressed from normal value to as low as 60 K upon addition of neon. Written in Turbo C.

Jones, Jack A.↗
NASA NTRS1993

Mineralogy of chondritic interplanetary dust particle impact residues from LDEF

A detailed structural and compositional analysis of several impactor residues was performed utilizing transmission electron microscopy, energy dispersive spectroscopy, and electron diffraction. Residues from the interior of several craters in gold surfaces were removed with a tungsten needle, mounted in EMBED-812 epoxy, and ultramicrotomed. The presence in these residues of equilibrated ferromagnesian minerals, recrystallization textures, glass, and melted metal and sulfide bodies decorating grain boundaries is indicative of varying degrees of shock metamorphism in all impact residues we have characterized.

Barrett, R. A.↗
NASA NTRS1993

Carbonaceous chondrite clasts in the Kapoeta howardite

A petrographic and mineralogical study of a number of carbonaceous chondrite clasts in the Kapoeta howardite has been carried out. Most of the clasts have mineralogical and chemical properties which link them to the CM carbonaceous chondrites. Some clasts contain chondrules which often have well-developed fine-grained rims, but many have been extensively brecciated. PCP-rich objects are common and pentlandite and pyrrhotite also occur. Calcite has also been found. The remainder of the clasts are extremely fine-grained and appear to be closely related to CI carbonaceous chondrites. In these clasts magnetite framboids are common and finegrained sulfides and magnetite occur disseminated throughout the matrix.

Brearley, Adrian J.↗
NASA NTRS1993

Chemical weathering on Mars: Rate of oxidation of iron dissolved in brines

Salts believed to occur in Martian regolith imply that brines occur on Mars, which may have facilitated the oxidation of dissolved Fe(2+) ions after they were released during chemical weathering of basaltic ferromagnesian silicate and iron sulfide minerals. Calculations show that the rate of oxidation of Fe(2+) ions at -35 C in a 6M chloride-sulfate brine that might exist on Mars is about 10(exp 6) times slower that the oxidation rate of iron in ice-cold terrestrial seawater.

Burns, Roger G.↗
NASA NTRS1993

Anomalous REE patterns in unequilibrated enstatite chondrites: Evidence and implications

We present here a study of Rare Earth Element (REE) microdistributions in unequilibrated enstatite chondrites (EOC's). Although the whole rock REE contents are similar in both unequilibrated and equilibrated chondrites, the host minerals of these refractory elements are different. In the least equilibrated ordinary chondrites (UOC's), the REE reside mainly in glass whereas, in their more equilibrated counterparts, the bulk of the REE is in calcium phosphate, a metamorphic mineral that formed by oxidation of phosphorous originally contained in metal. In the smaller group of enstatite (E) chondrites, calcium phosphate is absent and the phase that contains the highest REE concentrations is a minor mineral, CaS (oldhamite), which contains approximately 50 percent of the total Ca present. In E chondrites, elements typically considered to be lithophiles (such as Ca and Mn) occur in sulfides rather than silicates. This indicates formation under extremely reducing conditions, thus in a region of the solar nebula distinct from those that supplied the more abundant ordinary and carbonaceous chondrites. Previously, we observed a variety of REE patterns in the oldhamite of UEC's; they range from almost flat to some with pronounced positive Eu and Yb anomalies. Here, we searched for complementary REE patterns in other minerals from E chondrites and found them in the major mineral, enstatite. Whenever Eu and Yb anomalies are present in this mineral, they are always negative.

Crozaz, Ghislaine↗
NASA NTRS1993

Iron and nickel isotopic mass fractionation in deep-sea spherules

Magnetite-wuestite spherules collected from deep-sea sediments are thought to have originally been Fe-Ni metal particles at the top of the atmosphere that were oxidized and melted during entry into the earth's atmosphere. Some likely sources for the metal particles are Fe-Ni interplanetary dust particles (IDP's) and metal or sulfide from stony IDP's that separated after melting. Davis et al. reported that four of these spherules are enriched in the heavy isotopes of iron, with enrichments of 8-23%/amu. We have developed a technique for analysis of both iron and nickel isotopes on the same ion microprobe spot and have applied this technique to a number of deep-sea spherules in order to better understand the processes leading to isotopic mass fractionation. Eight spherules show iron and nickel isotopic mass fractionation, with iron and nickel enriched in the heavy isotopes by 10-19%/amu and 4-32%/amu, respectively. If the mass fractionations are due to Rayleigh fractionation during evaporation, these spherules lost 76-94% of their original mass. We have analyzed the four magnetite-wuestite spherules for which iron isotopic data were reported by Davis et al. as well as four new spherules.

Davis, Andrew M.↗
NASA NTRS1992

Application of static and dynamic enclosures in determining DMS and OCS fluxes in Sphagnum peatlands

A static enclosure method was applied to determine the exchange of dimethyl sulfide (DMS) and OCS between the surface of Sphagnum peatlands and the atmosphere. Measurements were performed concurrently with dynamic enclosure measurements with S-free air used as sweep gas. DMS emission rates determined by both methods were comparable between 5 and 500 nmol/m(sup -2)h(sup -1). The dynamic method provided positive OCS flux rates (emission) for measurements performed at sites containing Sphagnum. Conversely, data from the static method indicated that OCS was consumed from the atmosphere. Measurements using both techniques at a site devoid of vegetation showed that peat is a source of both DMS and OCS. Results suggested that OCS is produced in surface peat but it is taken up from the atmosphere by Sphagnum mosses. However, the net effect of both processes is that OCS uptake exceeds emission. The dynamic enclosure technique is adequate to measure rates of emissions of S gases which are produced in peatlands but not consumed, as long as attention is paid to the rate of sweep flow.

Demello, William Zamboni↗
NASA NTRS1992

Effects of inorganic sulfur addition on fluxes of volatile sulfur compounds in Sphagnum peatlands

Short and long-term impacts of increased S deposition on fluxes of volatile S compounds (VSC's) from Sphagnum peatlands were investigated in an artificially acidified (sulfuric and nitric acids) poor fen (Mire 239) at the Experimental Lakes Area (ELA), Ontario, Canada. Additional experiments were conducted in a poor fen (Sallie's Fen) in Barrington, NH, USA. At Mire 239, emissions of VSC's were monitored, before and after acidification, at control (unacidified) and experimental sections within two major physiographic zones of the mire (oligotrophic and minerotrophic). The experimental segments of the mire received S amendments since 1983, in amounts equivalent to the annual S deposition in the highest polluted areas of Canada and U.S. Dimethyl sulfide (DMS) was the predominant VSC released from the mire and varied largely with time and space (i.e., from 2.5 to 127 nmol/m(sup -2)h(sup -1)). Sulfur addition did not affect DMS emissions in a period of hours to a few days, although it stimulated production of DMS and MSH in the anoxic surficial regions of the peat. DMS emissions in the experimental oligotrophic segment of the mire was approximately 3-fold greater than in the control oligotrophic segment, and approximately 10-fold greater than in the minerotrophic zones. These differences could be due to a combination of differences in types of vegetation, nutritional status, and S input. At Sallie's Fen, DMS fluxes were approximately 8 times higher from a Sphagnum site than from a bare peat site. Fluxes of VSC's were not significantly affected by sulfate amendments at both sites, while DMS and MSH concentrations increases greatly with time in the top 10 cm of the peat column. Our data indicated that although Sphagnum is not the direct source of DMS released from Sphagnum peatlands, it might play a role in regulating DMS emissions to the atmosphere.

Demello, William Zamboni↗
NASA NTRS1993

Crustal structure and igneous processes in a chondritic Io

Liquid sulfur can form when metal-free C1 or C2 chondrites are heated. It may be obtained either by direct melting of native sulfur in disequilibrated C1 or C2 chondrites or by incongruent melting of pyrite and other sulfides in thermodynamically equilibrated rocks of the same composition. Hence, Lewis considered C2 chondrites to be the best meteoritic analog for Io's bulk composition. Metal-bearing C3 and ordinary chondrites are too chemically reduced to yield liquid sulfur and are not thought to represent plausible analogs of Io's bulk composition. An important aspect of Lewis' work is that CaSO4 and MgSO4 are predicted to be important in Io. Real C1 and C2 chondrites contain averages of, respectively, 11 percent and 3 percent by mass of salts (plus water of hydration). The most abundant chondritic salts are magnesium and calcium sulfates, but other important components include sulfates of sodium, potassium, and nickel and carbonates of magnesium, calcium, and iron. It is widely accepted that chondritic salts are formed by low-temperature aqueous alteration. Even if Io originally did not contain salts, it is likely that aqueous alteration would have yielded several percent sulfates and carbonates. In any event, Io probably contains sulfates and carbonates. This report presents the results of a model of differentiation of a simplified C2 chondrite-like composition that includes 1.92 percent MgSO4, 0.56 percent CaSO4, 0.53 percent CaCO3, and 0.094 percent elemental sulfur. The temperature of the model is gradually increased; ensuing fractional melting results in these components extruding or intruding at gravitationally stable levels in Io's crust. Relevant phase equilibria were reviewed. A deficiency of high-pressure phase equilibria renders the present model qualitative.

Kargel, J. S.↗
NASA NTRS1993

Densities of 5-15 micron interplanetary dust particles

We have measured the densities of about 100 5-15 micron stratospheric IDPs. Great care was taken to minimize selection bias in the sample population. Masses were determined using an absolute x-ray analysis technique with a transmission electron microscope, and volumes were found using scanning electron microscope imagery. Unmelted chondritic particles have densities between 0.5 and 6.0 g/cc. Roughly half of the particles have densities below 2 g/cc, indicating appreciable porosity, but porosities greater than about 70 percent are rare. IDPs with densities above 3.5 g/cc usually contain large sulfide grains. We find no evidence of bimodality in the unmelted particle density distribution. Chondritic spherules (melted particles) have densities near 3.5 g/cc, consistent with previous results for deep sea spherules.

Love, S. G.↗
NASA NTRS1993

Volatility in the lunar crust: Trace element analyses of lunar minerals by PIXE proton microprobe

In situ determination of mineral compositions using microbeam techniques can characterize magma compositions through mineral-melt partitioning, and be used to investigate fine-grained or rare phases which cannot be extracted for analysis. Abundances of Fe, Mn, Sr, Ga, Zr, Y, Nb, Zn, Cu, Ni, Se, and Sb were determined for various mineral phases in a small number of lunar highlands rocks using the PIXE proton microprobe. Sr/Ga ratios of plagioclase and Mn/Zn ratios of mafic silicates show that the ferroan anorthosites and Mg-suite cumulates are depleted in volatile lithophile elements to about the same degree compared with chondrites and the Earth. This links the entire lunar crust to common processes or source compositions. In contrast, secondary sulfides in Descartes breccia clasts are enriched in chalcophile elements such as Cu, Zn, Ni, Se, and Sb, and represent a potential resource in the lunar highlands.

Norman, M. D.↗
NASA NTRS1993

Origin of the Apollo 15 very low Ti green glass. A perspective from the compositional diversity in the very low Ti glasses

The very low Ti green glasses from the Apollo 15 site have intrigued scientists for over 20 years. Their primitive composition has been used to understand magmatic processes and the structure of the moon. The compositional variability observed in the Apollo 15 glass population has long been a point of debate. Initial studies did not recognize the compositional diversity in the glasses. Stolper et al. documented the major element variability and concluded it could not be produced by magmatic processes and therefore concluded that these glasses must be of impact origin. Subsequent studies confirmed a volcanic origin for the glass population and attempted to elucidate magmatic processes to account for its compositional variability. Models that have been proposed for these glasses include the following: (1) the crystallization of single or multiple phases (olivine, pyroxene, Fe metal, immiscible sulfide); (2) the incompatible behavior of Ni and Co during multiple phase crystallization at extremely low fO2; and (3) magma or source mixing. All of these models have problems. Type (1) models appear not to be consistent with recent trace element studies on the glasses; model (2) is dependent on the debatable incompatible behavior of Ni and Co, and, in models of type (3), the origin and nature of mixing models are somewhat unconstrained. This study compares the Apollo 15 green glasses with the very low Ti picritic glasses from other landing sites.

Shearer, C. K.↗
NASA NTRS1993

The Sm-Nd systematics of silicate inclusions in iron meteorites: Results from Caddo (IAB)

The timing of events leading to the formation of silicate-rich and metal-rich regions in planetesimals remains an important problem in the study of planetary formation and differentiation in the early solar system. The IAB irons are especially important as they are considered to represent a magmatic differentiation series. Iron meteorites present a particular challenge for chronological studies, due to the relative paucity of phases serving as hosts for radioactive parent-daughter nuclides. Recent work using the Re-Os system, following on the pioneering work by Herr et al. and Luck and Allegre, appears promising, but investigators up to now have concentrated on whole rock isochrons. Silicate clasts enclosed within iron meteorites can provide information about the chronology and thermal history of irons. Extensive work on Rb-Sr, K-Ar, and I-Xe has been reported on silicate inclusions in iron meteorites. We report the initial results from our Sm-Nd study of an inclusion with the Caddo IAB iron, the first Sm-Nd isotopic study of a silicate clast embedded within an iron meteorite. Our results include measurements of the standard long-lived Sm-147/Nd-143 (tau = 152 AE) system, as well as the shorter-lived SM-146/Nd-142 (tau = 0.149 AE) system, which has been shown to be very useful in deciphering the history of the early solar system. The Caddo silicate clast was described by Palme et al., who kindly provided us with a major part of the inclusion. The inclusion is coarse-grained consisting predominantly of olivine, clinopyroxene, and plagioclase, with lesser amounts of orthopyroxene, Fe-Ni metal, sulfide, and phosphate. The relatively large grain size (up to 3 mm) and 120 degree grain boundaries suggest extensive metamorphism at high temperatures. Based on study of a thin section, there is evidence for metal invading along grain boundaries in some regions of the inclusion, suggesting that the Fe-Ni metal was molten when the silicate clast was incorporated. Metamorphic recrystallization may have occurred during this event.

Stewart, Brian W.↗