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At least 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Oxygen and iron production by electrolytic smelting of lunar soil

Oxygen, present in abundance in nearly all lunar materials, can theoretically be extracted by molten silicate electrolysis from any known lunar rock. Derivation of oxygen by this method has been amply demonstrated experimentally in silicate melts of a variety of compositions. This work can be divided into three categories: (1) measurement of solubilities of metals (atomic) in silicate melts; (2) electrolysis experiments under various conditions of temperature, container material, electrode configuration, current density, melt composition, and sample mass (100 to 2000 mg) measuring energy required and character of resulting products; and (3) theoretical assessment of compositional requirements for steady state operations of an electrolysis cell.

Colson, R. O.↗
NASA NTRS1990

Compositions of large metal nodules in mesosiderites - Links to iron meteorite group IIIAB and the origin of mesosiderite subgroups

The compositions of large metal nodules in 12 mesosiderites, ranging from 3 mm to 4 cm in minimum dimensions and from 10 to 200 mg/g in silicate content, were analyzed by neutron activation; also analyzed were two additional samples (Pennyweight Point and Murchison Downs) which were originally designated iron meteorites but were later classified as Mes nodules. It was found that mesosiderites subgroups classified on the basis of metal composition are virtually the same as those based on silicate composition. The general covariation of the silicate and the metal compositional characteristics suggests that these features were established in the same process or in linked processes. A mesosiderite formation model is proposed.

Hassanzadeh, Jamshid↗
NASA NTRS1991

Aqueous alteration in five chondritic porous interplanetary dust particles

Results are presented on AEM observations carried out on chondritic porous (CP) interplanetary dust particles (IDPs), which include data on alkali-rich layer silicates and new observations of nonstoichiometric plagioclase and alkali feldspars in individual CP IDPs. The compositional similarities found between the feldspar minerals and the layer silicates suggest that the latter have formed from these feldspars during low-temperature aqueous alterations at a stage of diagenesis in the CP IDP parent bodies. Small, but persistent, amounts of layer silicates, carbonates, and barite found in several nominally anhydrous CP IDPs support the suggestion of incipient aqueous alterations in their parent bodies, which may include short-period comet nuclei and outer-belt asteroids.

Rietmeijer, Frans J. M.↗
NASA NTRS1991

Earth's core-mantle boundary - Results of experiments at high pressures and temperatures

Laboratory experiments document that liquid iron reacts chemically with silicates at high pressures (above 2.4 x 10 to the 10th Pa) and temperatures. In particular, (Mg,Fe)SiO3 perovskite, the most abundant mineral of earth's lower mantle, is expected to react with liquid iron to produce metallic alloys (FeO and FeSi) and nonmetallic silicates (SiO2 stishovite and MgSiO3 perovskite) at the pressures of the core-mantle boundary, 14 x 10 to the 10th Pa. The experimental observations, in conjunction with seismological data, suggest that the lowermost 200 to 300 km of earth's mantle, the D-double-prime layer, may be an extremely heterogeneous region as a result of chemical reactions between the silicate mantle and the liquid iron alloy of earth's core. The combined thermal-chemical-electrical boundary layer resulting from such reactions offers a plausible explanation for the complex behavior of seismic waves near the core-mantle boundary and could influence earth's magnetic field observed at the surface.

Knittle, Elise↗
NASA NTRS1991

An infrared reflectance study of low albedo surface constituents

Observational testing of the idea that the distribution of water is the key to understanding the volatile content of the asteroid belt relies on the exploration of the 3 micron absorption feature in hydrated silicates - the only diagnostic spectral band evident in the dark, volatile rich CI and CM meteorites. The existence of the band has demonstrated the presence of hydrated silicates on asteroids. An example of this feature is shown in the spectrum of the CI meteorite Orgueil. The feature is characterized by a sharp reflectance drop at 2.7 microns, due to structural OH, and by an absorption due to H2O that decreases slowly out to about 3.5 microns. In their present observational program, researchers are expanding their observational program to include other low-albedo classes of asteroids - asteroids that range primarily from the middle and outer asteroid belt (greater than 2.5 AU) to the Trojan region at 5.2 AU. Preliminary results indicate that the outer belt and Trojan asteroids do not show feature diagnostic of hydrated silicates. Researchers conclude that these asteroids have not undergone the alteration processes that we see in C-class asteroids.

Lebofsky, Larry A.↗
NASA NTRS1991

Intact capture of cosmic dust

The focus of this development effort is to capture dust particles at hypervelocities intact and unmelted in order to preserve volatile organics. At the same time, the capture process must minimize any organic elemental or compound contamination to prevent any compromise of exobiological analyses. Inorganic silicate aerogel has been developed as a successful capture medium to satisfy both requirements of intact capture and minimal organic contamination. Up to 6 km/s, silicate projectiles from a few microns up to 100 microns have been captured intact without any melting and with minimal loss of mass. Carbon in silicate aerogel can be reduced to less than 1 part in 1000 and hydrogen 3 parts in 1000 when baked in air. Under controlled inert gas environments, additional hydrocarbon reduction can be achieved.

Tsou, P.↗
NASA NTRS1992

Material transport in laser-heated diamond anvil cell melting experiments

A previously undocumented effect in the laser-heated diamond anvil cell, namely, the transport of molten species through the sample chamber, over distances large compared to the laser beam diameter, is presented. This effect is exploited to determine the melting behavior of high-pressure silicate assemblages of olivine composition. At pressures where beta-spinel is the phase melted, relative strengths of partitioning can be estimated for the incompatible elements studied. Iron was found to partition into the melt from beta-spinel less strongly than calcium, and slightly more strongly than manganese. At higher pressures, where a silicate perovskite/magnesiowuestite assemblage is melted, it is determined that silicate perovskite is the liquidus phase, with iron-rich magnesiowuestite accumulating at the end of the laser-melted stripe.

Campbell, Andrew J.↗
NASA NTRS1992

Pressure-induced coordination changes in alkali-germanate melts - An in situ spectroscopic investigation

The structure of liquid Na2Ge2O5-H2O, a silicate melt analog, has been studied with Raman spectroscopy to pressures of 2.2 gigapascals. Upon compression, a peak near more than 240 wavenumbers associated with octahedral GeO6 groups grows relative to a peak near 500 wavenumbers associated with tetrahedral GeO4 groups. This change corresponds to an increase in octahedral germanium in the liquid from near 0 percent at ambient pressures to more than 50 percent at a pressure of 2.2 gigapascals. Silicate liquids pausibly undergo similar coordination changes at depth in the earth. Such structural changes may generate decreases in the fusion slopes of silicates at high pressures as well as neutrally buoyant magmas within the transition zone of the earth's mantle.

Farber, Danial L.↗
NASA NTRS1992

Electrolytic smelting of lunar rock for oxygen, iron, and silicon

Preliminary studies of the electrochemical properties of silicate melts such as those available from heating of lunar mare soils indicate that conductivities are high enough for design of a practical electrolytic cell. The nature and kinetics of the electrode reactions, which involve reduction of Fe(++) and Si(IV) and oxidation of silicate anions as the primary, product-forming reactions, are also satisfactory. A survey of the efficiencies for production (amount of product for a given current) of O2, Fe(sup 0), and Si(sup 0) as functions of potential and of electrolyte composition indicate that conditions can be chosen to yield high production efficiencies. We also conclude that electronic conductivity does not occur to a significant extent. Based on these data, a cell with electrodes of 30 sq m in area operating between 1 and 5V with a current between 1.6 and 3.5(10)(exp 5) A for a mean power requirement of 0.54 MW and total energy use of approximately 13 MWhr per 24-hr day would produce 1 ton of O2, 0.81 ton of Fe(sup 0), 0.65 ton of Si(sup 0) (as Fe(sup 0)-Si(sup 0) alloy), and about 3.5 tons of silicate melt of altered composition per 24 hr. Adjustable distance between electrodes could offer flexibility with respect to feedstock and power source.

Haskin, Larry A.↗
NASA NTRS1992

Venus steep-sided domes: Relationships between geological associations and possible petrogenetic models

Venus domes are characterized by steep sides, a circular shape, and a relatively flat summit area. In addition, they are orders of magnitude larger in volume and have a lower height/diameter ratio than terrestrial silicic lava domes. The morphology of the domes is consistent with formation by lava with a high apparent viscosity. Twenty percent of the domes are located in or near tessera (highly deformed highlands), while most other (62 percent) are located in and near coronae (circular deformational features thought to represent local mantle upwelling). These geological associations provide evidence for mechanisms of petrogenesis and several of these models are found to be plausible: remelting of basaltic or evolved crust, differentiation of basaltic melts, and volatile enhancement and eruption of basaltic foams. Hess and Head have shown that the full range of magma compositions existing on the Earth is plausible under various environmental conditions on Venus. Most of the Venera and Vego lander compostional data are consistent with tholeiitic basalt; however, evidence for evolved magmas was provided by Venera 8 data consistent with a quartz monzonite composition. Pieters et al. have examined the color of the Venus surface from Venera lander images and interpret the surface there to be oxidized. Preliminary modeling of dome growth has provided some interpretations of lava rheology. Viscosity values obtained from these models range from 10(exp 14) - 10(exp 17) pa*s, and the yield strength has been calculated to be between 10(exp 4) and 10(exp 6) Pa, consistent with terrestrial silicic rocks. The apparent high viscosity of the dome lavas suggests that the domes have a silicic composition or must augment their viscosity with increased visicularity or crystal content. Sixty-two percent of the Venus domes are associated with coronae, circular features that have been proposed as sites of mantle upwelling, and 20 percent of the domes are located near tessera, relatively high areas of complex deformed terrain. We have investigated several models that are consistent with these geologic associations. The first case involves the differentiation of basalt in a magma reservoir in the crust, perhaps produced by partial melting within a mantle plume. The second case is melting at the base of thickened basaltic crust, and the final case is volatile exsolution and enhancement within a basaltic magma reservoir. The association of domes with tessera might be explained by crustal remelting, while the association with coronae may be consistent with chemical differentiation of a magma reservoir or the exsolution and concentration of volatiles in the reservoir before eruption.

Pavri, B.↗
NASA NTRS1992

Interstellar dust from the Milky Way to the Magellanic Clouds

Interstellar dust in the Magellanic Clouds, with a weak or nearly absent 2175 A extinction feature, may be of interest in studies of galaxies in early stages of chemical evolution. To this inquiry, we extend the graphite-silicate grain model, introduced by Mathis, Rumpl, and Nordsieck and developed by Draine and Lee (1984) from the Milky Way to the Magellanic Clouds. We find that the empirical extinction curves in the Large and Small Magellanic Clouds can be reproduced by adjusting only the relative abundances of graphite and silicate grains, while leaving all other model properties fixed to those appropriate for the Galactic extinction curve. Using the graphite-silicate models, we calculate the absorption and scattering optical depths, the mass-density ratio of interstellar dust to neutral hydrogen, and the Kramers-Kronig relation for all three galaxies. We also present a fitting function for the three extinction curves, valid not only over the observed range of wavelengths but also over the full range as predicted by the models. All the quantities we derived here are independent of the dust-to-gas ratios in the Milky Way and the Magellanic Clouds, and can be applied to other galaxies if they contain Galactic or Magellanic-type dust.

Pei, Yichuan C.↗
NASA NTRS1992

0.7- to 23-micron photometric observations of P/Halley 2986 III and six recent bright comets

Infrared photometry over 0.7-23 micron range is presented for P/Halley 1986 III and six other recent bright comets. It is concluded that comets can be classified by their thermal infrared energy distributions. IR Type I comets have low continuum superheat and the 10-micron silicate emission is muted or absent. IR Type II comets have large continuum superheat and strong silicate emission features. Simultaneous measurements of P/Halley and Bradfield 1980 XV are generally consistent with the steady-state model for nuclear ablation. P/Halley's dust coma had an average albedo of 0.20 at a scattering angle of 130 deg. The correlation between superheat and 20-micron silicate excess, the flattened forward-scattering peak of the albedo curve, and the relatively low backscattering albedo are all consistent with laboratory and theoretical results for nonspherical and fluffy grains. The results appear to be especially consistent with core-mantle grain models such as those proposed by Greenberg and Hage (1989).

Gehrz, R. D.↗
NASA NTRS1993

SO2-rock interaction on Io - Reaction under highly oxidizing conditions

Laboratory simulations are used here to test the possibility that interactions of SO2 and silicates can produce Na-S compounds which can account for the observed surface enrichment of Na relative to Si on Io. It is concluded that there are three viable hypotheses for the nature and origin of the Na compounds on the surface of Io: (1) Na2SO4 from SO2-silicate interactions, (2) Na-sulfides from elemental S, silicate interactions, and (3) directly erupted volcanic Na. The present studies indicate the elemental Si is not required to form Na compounds in crustal igneous interactions. Thus, the presence of Na compounds provides no constraint on elemental S but igneous formation of Na-S compounds is not tied to models of Io which contain elemental S.

Johnson, M. L.↗
NASA NTRS1993

Magnetite-sulfide chondrules and nodules in CK carbonaceous chondrites - Implications for the timing of CK oxidation

CK carbonaceous chondrites contain rare (about 0.1 vol pct) magnetite-sulfide chondrules that range from about 240 to 500 microns in apparent diameter and have ellipsoidal to spheroidal morphologies, granular textures, and concentric layering. They resemble the magnetite-sulfide nodules occurring inside mafic silicate chondrules in CK chondrites. It seems likely that the magnetite-sulfide chondrules constitute the subset of magnetite-sulfide nodules that escaped as immiscible droplets from their molten silicate chondrule hosts during chondrule formation. The intactness of the magnetite-sulfide chondrules and nodules implies that oxidation of CK metal occurred before agglomeration. Hence, the pervasive silicate darkening of CK chondrites was caused by the shock mobilization of magnetite and sulfide, not metallic Fe-Ni and sulfide as in shock-darkened ordinary chondrites.

Rubin, Alan E.↗
NASA NTRS1993

Spectral evidence of size dependent space weathering processes on asteroid surfaces

Most compositional characterizations of the minor planets are derived from analysis of visible and near-infrared reflectance spectra. However, such spectra are derived from light which has only interacted with a very thin surface layer. Although regolith processes are assumed to mix all near-surface lithologic units into this layer, it has been proposed that space weathering processes can alter this surface layer to obscure the spectral signature of the bedrock lithology. It has been proposed that these spectral alteration processes are much less pronounced on asteroid surfaces than on the lunar surface, but the possibility of major spectral alteration of asteroidal optical surfaces has been invoked to reconcile S-asteroids with ordinary chondrites. The reflectance spectra of a large subset of the S-asteroid population have been analyzed in a systematic investigation of the mineralogical diversity within the S-class. In this sample, absorption band depth is a strong function of asteroid diameter. The S-asteroid band depths are relatively constant for objects larger than 100 km and increase linearly by factor of two toward smaller sizes (approximately 40 km). Although the S-asteroid surface materials includes a diverse variety of silicate assemblages, ranging from dunites to basalts, all compositional subtypes of the S-asteroids conform to this trend. The A-, R-, and V-type asteroids which are primarily silicate assemblages (as opposed to the metal-silicate mixtures of most S-asteroids) follow a parallel but displaced trend. Some sort of textural or regolith equilibrium appears to have been attained in the optical surfaces of asteroids larger than about 100 km diameter but not on bodies below this size. The relationships between absorption band depth, spectral slope, surface albedo and body size provide an intriguing insight into the nature of the optical surfaces of the S-asteroids and space weathering on these objects.

Gaffey, M. J.↗
NASA NTRS1993

Micrometeorite dynamic pyrometamorphism: Observation of a thermal gradient in iron-nickel sulfide

Dynamic pyrometamorphism describes the mineralogical changes in a micrometeorite that occur in response to flash-heating during atmospheric entry. Mineral reactions during this event will be endothermic and act as local sinks for thermal energy which delays a uniform temperature distribution in decelerating micrometeorites. The most common pyrometamorphic reaction is formation of magnetite nanocrystal decorations on Mg,Fe-silicate grains and magnetite or (partial) maghemite rims. Constituent silicates also respond to this thermal event and show formation of olivine-maghemite symplectites and growth of garnet and partial laihunitisation. In continued effort to constrain dynamic pyrometamorphism of unmelted interplanetary dust particles (IDP's), I determined the mineralogical composition, and Fe,Ni-sulfide chemistry, in the sulfide-rich particles L2005C39, L2005E40, and L2006A28 using a JEOL 2000FX analytical electron microscope equipped with a TN5500 energy dispersive spectrometer for in situ microanalysis. Sulfide compositions in these three IDP's are distinctly different from those in silicate-rich particle L2005T13 which might suggest a delicate balance of the fO2/fS2 ratios during dynamic pyrometamorphism.

Rietmeijer, Frans J. M.↗
NASA NTRS1993

A new Antarctic meteorite with chromite, orthopyroxene and metal with reference to a formation model of S asteroids

Mineralogy of a unique meteorite from Antarctica, EET84302 and the related primitive achondrites, was studied to gain better understanding on relationship between S asteroids and this meteorite group. EET84302 consists of metal-rich (M) region with metal filling interstices of silicate grains of orthopyroxene (Opx), olivine Fa8, plagioclase An24, and augite (Aug), chromite-rich (C) region with major chromite and Opx, and silicate-rich (S) region with mainly Opx. This is the first example of chromite-rich achondrite. Distribution of three areas (C, M, S) are in line with a proposed model of primitive achondrites and S asteroids. Since reflectance spectra of some members of primitive achondrites combined with those of iron meteorites, resemble those of the S asteroids, there is an expectation that S asteroids are modified products of chondrites. The trends of the variations in their mineral assemblages are in line with those of the S asteroids. Hiroi proposed a stony-iron model of S asteroids, which assumes that a few domains of various primitive achondrites are embedded in iron matrix. Polished thin sections (PTS) of EET84302,19 and ALH81187,16 were supplied from Meteorite Working Group (MWG). A previously studied PTS of EET84302 was similar in both texture and mineralogy to silicate inclusions in some iron meteorites. PTS EET84302,19 was studied by electron probe microanalyzer (EPMA) and scanning electron microscope (SEM). The reflectance spectra of EET84302 showed a weathering problem and was not used to match with those of S asteroids.

Takeda, Hiroshi↗
NASA NTRS1994

Preliminary results of sulfide melt/silicate wetting experiments in a partially melted ordinary chondrite

Recently, mechanisms for core formation in planetary bodies have received considerable attention. Most current theories emphasize the need for large degrees of silicate partial melting to facilitate the coalescence and sinking of sulfide-metal liquid blebs through a low strength semi-crystalline silicate mush. This scenario is based upon observations that sulfide-metal liquid tends to form circular blebs in partially molten meteorites during laboratory experiments. However, recent experimental work by Herpfer and Larimer indicates that some sulfide-Fe liquids have wetting angles at and slightly below 60 deg in an olivine aggregate, implying an interconnected melt structure at any melt fraction. Such melt interconnectivity provides a means for gravitational compaction and extraction of the majority of a sulfide liquid phase in small planetary bodies without invoking large degrees of silicate partial melting. Because of the important ramifications of these results, we conducted a series of experiments using H-chondrite starting material in order to evaluate sulfide-liquid/silicate wetting behavior in a more complex natural system.

Jurewicz, Stephen R.↗