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At least 1,243 records · Page 69
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Experimental determination of the partitioning of gallium between solid iron metal and synthetic basaltic melt Electron and ion microprobe study

The distribution of Ga between solid Fe metal and synthetic basaltic melt is investigated experimentally at temperatures of 1190 and 1330 C, and over a narrow range of oxygen fugacities. Metal-silicate reversal experiments were conducted, indicating a close approach to equilibrium. The analysis of the partitioned products was performed using electron and ion microprobes. At one bar total pressure, the solid metal/silicate melt partition coefficient D(Ga) is used to evaluate metal-silicate fractionation processes in the earth, moon, and Eucrite Parent Body (EPB). It is found that the depletion of Ga abundances in the EPB is due to the extraction of Ga into a metallic core. Likewise, the depletion of Ga in the lunar mantle is consistent with the extraction of Ga into a smaller lunar core if Ga was originally present in a subchondritic concentration. The relatively high Ga abundances in the earth's mantle are discussed, with reference to several theoretical models.

Drake, M. J.↗
NASA NTRS1984

Impact cratering: The process and its effects on planetary evolution

The potential for silicate-carbon dioxide reactions as a geochemical weathering agent on Venus was studied. A tholetitic basalt close to the composition determined by the XRF experiment at the Venera 14 sites was subjected to high temperature and pressure (with pure CO2 as the pressure medium) for varying time durations. The starting basalt material and the run products were examined optically and by X-ray diffraction and electron microscopy. The kinetics of the silicate-carbonate reactions is discussed. A study to elucidate details of impact processes and to assess the effects of impact cratering on planetary evolution is mentioned.

Grieve, R. A. F.↗
NASA NTRS1984

The rheology of icy satellites

High-temperature creep in orthoenstatite under conditions of controlled oxygen fugacity was studied. It was found that creep was conttrolled by the extremely thin layer of SiO2 which wetted the grain boundaries. Slight reduction of the (Mg, Fe)SiO3 enstatite during hot pressing produced microscopic particles of Fe and the thin film of intergranular SiO2. This result highlights another complication in determining the flow properties of iron bearing silicates which constitute the bulk of terrestrial planets and moons. The Phenomenon may be important in the ductile formation of any extraterrestrial body which is formed in a reducing environment. The rheology of dirty ice was studied. This involves micromechanical modeling of hardening phenomena due to contamination by a cosmic distribution of silicate particles. The larger particles are modeled by suspension theory. In order to handle the distribution of particles sizes, the hardening is readed as a critical phenomenon, and real space renormalization group techniques are used. Smaller particles interact directly with the dislocations. The particulate hardening effect was studied in metals. The magnitude of such hardening in ice and the defect chemistry of ice are studied to assess the effects of chemical contamination by methane, ammonia, or other likely contaminants.

Sammis, C. G.↗
NASA NTRS1984

The problem of iron partition between Earth and Moon during simultaneous formation as a double planet system

A planetary model is described which requires fractional vapor/liquid condensation, planet accumulation during condensation, a late start for accumulation of the Moon, and volatile accretion to the surfaces of each planet only near the end of the accumulation process. In the model, initial accumulation of small objects is helped if the agglomerating particles are somewhat sticky. Assuming that growth proceeds through this range, agglomeration continues. If the reservoir of vapor is being preferentially depleted in iron by fractional condensation, an iron-rich planetary core forms. As the temperature decreases, condensing material becomes progressively richer in silicates and poorer in iron, forming the silicate-rich mantle of an already differentiated Earth. A second center of agglomeration successfully forms near the growing Earth after most of the iron in the reservoir has been used up. The bulk composition of the Moon then is similar to the outer mantle of the accumulating Earth.

Cassidy, W. A.↗
NASA NTRS1984

Origin of the Moon

The similarities in siderophile abundances strongly suggest that the Moon was derived from the Earth's mantle after the Earth's core had formed. The energy required to remove material from the Earth's mantle and place it into geocentric orbit can be supplied most readily by impact processes during accretion of the Earth. Impacts of late-accreting, high-velocity planetismals would evaporate many times their masses of mantle material. These gases would be accompanied by a massive spray of shock-melted silicate droplets. It is suggested that the gases produced from such near-equatorial impacts were rapidly spun out into equatorial geocentric orbit. The evaporated material was selectively recondensed, and, accompanied by the shock melted, devolatilized silicate droplets, accreted to form a sediment ring of Earth-orbiting planetismals. This sediment ring also captured a significant proportion of Earth-bound planetismals. The Moon was formed by accretion from planetismals comprising the sediment ring.

Ringwood, A. E.↗
NASA NTRS1984

The origin of the Moon

Bulk density alone shows that the Moon is depleted in metallic FeNi relative to the Earth or to chondritic meteorites. This depletion implies that the Moon formed not from chondrites but from differentiated material. Origin of the Moon by fission from the Earth offers a simple explanation for its depletion in FeNi, but this mechanism seems unlikely because of associated dynamical difficulties. Lunar volatile element depletions were invoked in support of fission, but volatile contents of eucritic meteorites are similarly low and the eucrites did not form by Earth fission. A more plausible origin of the Moon is accretion from the circumterrestrial swarm. The low FeNi content of the Moon is understood if the mean size of interplanetary silicate particles was much smaller than that for metal particles, since this would have led to preferential capture of silicates into Earth orbit, but the question arises whether the mean particle size of the metallic particles was great enough to prevent their capture into the swarm.

Wasson, J. T.↗
NASA NTRS1984

Chondrule rims and interchondrule matrix in UOC

Opaque rims around chondrules and clasts were distinguished from opaque, interchondrule matrix apparently unrelated spatially to chondrules and clasts. Microprobe and electron microscope techniques were used. The mean chemical composition for dark rim and matrix in Bishunpur and Tieschitz are similar to the opaque matrix of Huss, et al. However, the mean dark rim compositions in Bishunpur have significantly higher Fe, and lower Na, K, Al and Si than opaque interchondrule matrix. The opaque matrix of Huss, et al, essentially lies between these compositions. In Tieschitz only rim material was observed. In Tieschitz the rims are Si-poor and dominated by normative olivine (Fo50). Again there is an Na, K, Al component but is often nepheline normative rather than albitic. It too is probably present as glass, Ashworth (pers.comm.). In Bishunpur rims as well as the silicate-FeS, FeNi layering described by Allen, et al, discontinuous layering was observed within the silicate portion. This is apparently due to variations in the proportions of the components, particularly in the glassy phase. In Bishunpur there is a strong genetic link between matrix and rims, although rims seem to have formed under different, possibly more oxidizing, conditions. Also the presence of the same component in rims, matrix chondrules, and clasts suggests a common source.

Alexander, C.↗
NASA NTRS1985

Geographic variation in the relationships of temperature, salinity or sigma sub t versus plant nutrient concentrations in the world ocean

A NODC data set representing all regions of the world ocean was analyzed for temperature and sigma-t relationships with nitrate, phosphate or silicic acid. Six cubic regressions were for each ten degree square of latitude and longitude containing adequate data. World maps display the locations that allow the prediction of plant nutrient concentrations from temperature or sigma-t. Geographic coverage improves along the sequence: nitrate, phosphate, and silicic acid and is better for sigma-t than for temperature. Contour maps of the approximate temperature of sigma-t at which these nitrients are no longer measurable in a parcel of water are generated, based on a percentile analysis of the temperature or sigma-t at which less than a selected amount of plant nutrient occurs. Results are stored on magnetic tape in tabular form. The global potential to predict plant nutrient concentrations from remotely sensed temperature of sigma-t and to emphasize the latitudinally and longitudinally changing phytoplankton growth environment in present and past oceans is demonstrated.

Kamykowski, D.↗
NASA NTRS1985

Permafrost and Subsurface Ice in the Solar System

The properties and behavior of planetary permafrost are discussed with reference to the ability of such surfaces to sustain loads characteristics of spacecraft landing and planetary bases. In most occurrences, water ice is in close proximity to, or in contact with, finely divided silicate mineral matter. When ice contacts silicate mineral surfaces, a liquid-like, transition zone is created. Its thickness ranges from several hundred Angstron units at temperatures near 0 degrees C to about three Angstrom units at -150 degrees C. When soluble substances are present, the resulting brine enlarges the interfacial zone. When clays are involved, although the interfacial zone may be small, its extent is large. The unfrozen, interfacial water may amount to 100% or more weight at a temperature of -5 degrees C. The presence of this interfacial unfrozen water acts to confer plasticity to permafrost, enabling it to exhibit creep at all imposed levels of stress. Nucleation processes and load-bearing capacity are examined.

Anderson, D. M.↗
NASA NTRS1985

Chemical Weathering of Soils from the Dry Valleys of Antarctica: a Terrestrial Analog of Martian Weathering Processes

Martian soil subjected to chemical weathering processes could contain the following likely constituents: (1) fresh primary silicate material; (2) partially altered primary silicates; (3) secondary minerals, possibly including clay minerals, evaporites, carbonates, sulfates, hydrates, and zeolites; and (4) altered volcanic glass or impact glass. The soil may also include palogonite and other alteration products and secondary minerals. It is unlikely therefore that an equilibrium assemblage of minerals would be present. From the detailed study of the soils from the Dry Valleys of Antarctica, it is obvious that the complex processes in operation produce major changes in the parent materials, depending upon where the constituents reside and the degree to which weathering and diagenesis operates. It is clear that natural near surface environments, even in very cold and dry regions, may produce extremely complex soils. Extreme caution must be taken when interpreting the results and drawing conclusions, especially about possible processes operating in regoliths in cold, arid environments similar to those of the Dry Valleys or Mars.

Gibson, E. K., Jr.↗
NASA NTRS1985

The distribution of compositional classes in the asteroid belt: A cosmochemical fingerprint?

Studies of the physical properties of the asteroids show a nonrandom distribution of types across the belt for asteroid classes E, S, M, F, C, P, and D. The general trend is for asteroids in the inner belt to have higher albedos and stronger mafic silicate absorption features than those asteroids located further out in the belt. One interpretation of this trend is that the asteroids, which occupy the region between the silicate rich terrestrial planets and the volatile rich outer planets, have preserved in their heliocentric compositional distribution a cosmochemical fingerprint of the thermodynamic conditions present in the solar nebula at the time of their formation. This hypothesis predicts that the differences in the spectral properties among the low albedo classes (C, P, F, P, and D) are due to temperature controlled processes which formed carbonaceous opaques. If this is true then the exact composition of the opaque components could, in principle, be used to determine the thermodynamic conditions between the orbits of Mars and Jupiter during the formation of the asteroids.

Gradie, J.↗
NASA NTRS1985

Far-infrared observations of two southern H II regions - RCW 122 and G 351.6-1.3

The luminosities in excess of 1,000,000 solar luminosities obtained in the present 1-arcmin resolution mapping of the two southern H II regions RCW 122 and G 351.6-1.3 requires the presence of OB star clusters. A luminosity/unit mass of the associated cloud of about 6 solar luminosities/solar mass is obtained by combining near-IR, far-IR, and CO-emission properties for RCW 122, and it is determined that the emitting dust grains are amorphous silicate-like. G 351.6-1.3 has very similar properties, although the fraction of 1-25 micron luminosity corrected for silicate absorption is about five times less than for RCW 122.

Mcbreen, B.↗
NASA NTRS1983

Condensation of chondrules

Analysis of current experimental results concerned with the kinetic constraints on chondrule formation showed that the major physical properties of chondrules could have been produced by direct condensation of metastable liquid silicates droplets from a hot gas in the primordial nebula. It is argued that such a condensation process would have to be followed by crystallization, accretion, and partial comminution of the droplets. The chemical mechanisms driving this process are described, including: nucleation constraints on comminution and crystallization; slow transformations and chemical reactions in chain silicates; and the slow diffusion of ions. It is shown that the physical mechanisms for chondrule condensation are applicable to a broad spectrum of chondrule sources.

Blander, M.↗
NASA NTRS1985

Hydrated interplanetary dust particle linked with carbonaceous chondrites?

The results of transmission electron microscope observations of a hydrated interplanetary dust particle (IDP) containing Fe-, Mg-rich smectite or mica as a major phase are reported. The sheet silicate appears to have formed by alteration of anhydrous silicates. Fassaite, a Ca, Al clinopyroxene, also occurs in this particle, and one of the crystals exhibits solar-flare tracks, clearly indicating that it is extraterrestrial. Fassaite is a major constituent of the Ca-, Al-rich refractory inclusions found in the carbonaceous chondrites, so its presence in this particle suggests that there may be a link between hydrated IDPs and carbonaceous chondrites in the early history of the solar system.

Tomeoka, K.↗
NASA NTRS1985

Hot corrosion of sintered alpha-SiC at 1000 C

The hot corrosion of sintered alpha-SiC by thin films of Na2SO4 and Na2CO3 was studied at 1000 C in controlled gas atmospheres. Under all conditions, corrosion led to 10 to 20 times the amount of SiO2 formed in pure oxidation after a 48-h exposure. In addition, small amounts of sodium silicate formed. Melts of Na2SO4/SO3 caused uniform pitting of the SiC substrate; Na2CO3/CO2 melts caused localized pitting and grain-boundary attack. In all cases, the protective SiO2 layer dissolved to form silicate, leading to corrosion. In the sulfate case, free carbon in the SiC promotes this process. In all cases the presence of liquid films is responsible for rapid transport rates and the subsequent rapid reaction.

Jacobson, N. S.↗
NASA NTRS1985

Molten salt corrosion of SiC: Pitting mechanism

Thin films of Na2SO4 and Na2CO3 at 1000 C lead to severe pitting of sintered alpha-SiC. These pits are important as they cause a strength reduction in this material. The growth of product layers is related to pit formation for the Na2CO3 case. The early reaction stages involve repeated oxidation and dissolution to form sodium silicate. This results in severe grain boundary attack. After this a porous silica layer forms between the sodium silicate melt and the SiC. The pores in this layer appear to act as paths for the melt to reach the SiC and create larger pits.

Jacobson, N. S.↗
NASA NTRS1986

Oblique impact: A process for providing meteorite samples of other planets

Cratering flow calculations for a series of oblique to normal impacts of silicate projectiles onto a silicate halfspace were carried out to determine whether the gas produced upon shock vaporizing both projectile and planetary material could entrain and accelerate surface rocks and thus provide a mechanism for propelling SNC meteorites from the Martian surface. The difficult constraints that the impact origin hypothesis for SNC meteorites has to satisfy are that these meteorites are lightly to moderately shocked and yet were accelerated to speeds in excess of the Martian escape velocity. Two dimensional finite difference calculations demonstrate that at highly probable impact velocities, vapor plume jets are produced at oblique impact angles of 25 deg to 60 deg and have speeds as great as 20 km/sec. These plumes flow nearly parallel to the planetary surface. It is shown that upon impact of projectiles having radii of 0.1 to 1 km, the resulting vapor jets have densities of 0.1 to 1 g/cu.cm. These jets can entrain Martian surface rocks and accelerate them to velocities 5 km/sec. It is suggested that this mechanism launches SNC meteorites to Earth.

Okeefe, J. D.↗
NASA NTRS1986

A model for the biological precipitation of Precambrian iron-formation

A biological model for the precipitation of Precambrian iron formations is presented. Assuming an oxygen deficient atmosphere and water column to allow sufficient Fe solubility, it is proposed that local oxidizing environments, produced biologically, led to precipitation of iron formations. It is further suggested that spheroidal structures about 30 mm in diameter, which are widespread in low grade cherty rion formations, are relict forms of the organic walled microfossil Eosphaera tylerii. The presence of these structures suggests that the organism may have had a siliceous test, which allowed sufficient rigidity for accumulation and preservation. The model involves precipitation of ferric hydrates by oxidation of iron in the photic zone by a variety of photosynthetic organisms. Silica may have formed in the frustules of silica secreting organisms, including Eosphaera tylerii. Iron formates formed, therefore, by a sediment rain of biologically produced ferric hydrates and silica and other organic material. Siderite and hematite formed diagenetically on basin floors, and subsequent metamorphism produced magnetite and iron silicates.

Laberge, G. L.↗