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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 199 records · Page 11
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NASA NTRS1987

Dynamical constraints on the origin of the moon

Six different categories of models for the formation of the moon within the context of the general theory of terrestial planet formation by the accumulation of protoplanets are discussed. These catagories are: (1) rotational fission; (2) precipitation fission; (3) intact capture; (4) disintegrative capture; (5) binary accretion; and (6) giant impact accretion. It appears that the only plausable mechanism proposed thus far involves the formation of the Moon following a giant impact that ejects portions of the differentiated Earth's mantle and parts of the impacting body into circumterrestrial orbit.

Boss, A. P.↗
NASA NTRS1986

Dynamical constraints on the origin of the moon

The paper considers six different categories of models for the formation of the moon within the context of the general theory of terrestrial planet formation by accumulation of planetesimals. These categories are: (1) rotational fission, (2) precipitation fission, (3) intact capture, (4) disintegrative capture, (5) binary accretion, and (6) giant impact accretion. It appears that the only plausible mechanism proposed thus far involves the formation of the moon following a giant impact that ejects portions of the differentiated earth's mantle and parts of the impacting body into circumterrestrial orbit.

Boss, A. P.↗
NASA NTRS1986

The binary fission origin of the moon

The major arguments for and against the binary fission model of lunar origin are reviewed. Unresolved problems include: (1) how the protoearth acquired sufficient angular velocity to fission, and (2) how the earth-moon system lost its excess angular momentum after fission. Despite these uncertainties, the compositional similarities between the earth's mantle and the bulk moon suggest that the fission model is worth considering. The proposed sequence of events in the formation of the moon by binary fission is given.

Binder, Alan B.↗
NASA NTRS1986

When worlds collide - Jetted vapor plumes and the moon's origin

It is proposed that a collision between a Mars-sized planetesimal and the protoearth explains both the anomalously large angular momentum of the earth-moon system and the similarity between the moon's bulk composition and the earth's mantle. An approximate calculation was performed to obtain information on the highest speed, most highly shocked ejecta and the ejection pattern, speed, angles of launch, etc. as a function of the impact parameter and the ratio of the sizes of the colliding protoplanets. Computation results are in agreement with the megaimpact hypothesis.

Melosh, H. J.↗
NASA NTRS1988

On the aspect ratio of Rayleigh-Benard convection cells

Numerical model calculations of isoviscous bottom-heated convection in a two-dimensional box with aspect ratio twelve are reported. The aim was to determine the preferred aspect ratio of convection cells. Time-dependent runs were started from two widely different initial conditions. At Rayleigh numbers in the range 10 to the 5th to 10 to the 6th about eight cells with a mean aspect ratio of 1.5 formed eventually in each case. Thus neither the preference for approximately square cells nor the stability of very elongated cells could be confirmed. The existence of large-scale convective structures in the earth's mantle must be due to deviations from the simple Rayleigh-Benard model of convection.

Christensen, Ulrich↗
NASA NTRS1989

The route to chaos in thermal convection at infinite Prandtl number. I - Some trajectories and bifurcations

The question of whether or not thermal convection in the earth's mantle is chaotic is addressed. It is suggested that the high Prandtl number/high Raleigh number thermal convection associated with mantle convection is chaotic. To test this hypothesis, the route to chaos in a fluid with infinite Prandtl number is examined, using the Saltzman (1962) equations and the Lorenz (1963) equations. It is concluded that thermal convection at infinite Prandtl number becomes chaotic by means of symmetry-breaking pitchfork bifurcations and the appearance of Hopf bifurcations which produce unstable periodic orbits.

Stewart, Cheryl A.↗
NASA NTRS1991

Tomography, the geoid and plate motions

The dynamics of the earth's mantle and its relationship to mantle structure as revealed by seismic tomography, the geoid, and plate motions are discussed in a critical review of U.S. research from the period 1987-1990. Sections are devoted to plates and mantle convection; seismic anomalies, flow, and the geoid; subducted slabs and the 670-km discontinuity; the physical properties of the mantle; plumes; and computational and experimental modeling efforts. Also included is a comprehensive bibliography for the period.

Gable, Carl W.↗
NASA NTRS1992

On the isotopic composition of magmatic carbon in SNC meteorites

SNC meteorites are thought, from many lines of evidence, to come from Mars. A line of investigation which has been pursued in our laboratory over the years involves measurement of the stable isotopic composition of carbon, in its various forms, in SNC meteorites. In order to establish a firm basis for studying the isotopic systematics of carbon in the martian surface environment, it is first necessary to try and constrain the delta C-13 of bulk Mars. Taking all of the available information, it would seem that the delta C-13 of the Earth's mantle lies somewhere in the range of -5 to -7 percent. Preliminary assessment of magnetic carbon in SNC meteorites, would tend to suggest a delta C-13 of 20 to 30 percent, which is conspicuously different from that of the terrestrial mantle. It is not obvious why there should be such a difference between the two planets, although many explanations are possible. One of these possibilities, that previous delta C-13 measurements for magnetic carbon in SNC meteorites are in error to some degree, is being actively investigated. The most recent results seem to constrain the theta C-13 of the magnetic carbon in SNC meteorites to about -20 percent, which is not at odds with previous estimates. As such, it is considered that a detailed investigation of the carbon isotopic systematics of martian surface materials does have the necessary information with which to proceed.

Wright, I. P.↗
NASA NTRS1992

Models of convection-driven tectonic plates - A comparison of methods and results

Recent numerical studies of convection in the earth's mantle have included various features of plate tectonics. This paper describes three methods of modeling plates: through material properties, through force balance, and through a thin power-law sheet approximation. The results obtained are compared using each method on a series of simple calculations. From these results, scaling relations between the different parameterizations are developed. While each method produces different degrees of deformation within the surface plate, the surface heat flux and average plate velocity agree to within a few percent. The main results are not dependent upon the plate modeling method and herefore are representative of the physical system modeled.

King, Scott D.↗
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

Three-dimensional convection of an infinite-Prandtl-number compressible fluid in a basally heated spherical shell

A numerical investigation is conducted of the effect of compressibility on 3D thermal convection in a basally heated highly viscous fluid spherical shell (representing the earth's mantle). It was found that compressibility has little effect on the spatial structure of steady convection when the superadiabatic temperature drop across the shell, Delta T(sa), is comparable to a characteristic adiabatic temperature. When the Delta T(sa) is about an order of magnitude smaller than the adiabatic temperature, compressibility effect is significant. It is shown that compressibility affects the pattern evolution of the irregular solutions; fewer upwelling plumes are produced with increasing compressibility.

Bercovici, David↗
NASA NTRS1992

Siderophile elements and the earth's formation

Two comments on a work by Murthy (1991) concerning the abundances of siderophile elements in the earth's mantle are presented. In the first comment it is asserted that the basis of Murthy's extrapolation is the assumption that the Gibbs free energy change for the partitioning reaction is independent of temperature, and as this is generally not a valid assumption thermodynamically, and as this is contradicted by most experimental data, the issue of mantle siderophile elements remains unresolved. In the second comment it is asserted that the extrapolation method used by Murthy does not appear to be valid thermodynamically, and that an extrapolation based on generally accepted thermodynamic assumptions yields different results. In a reply, Murthy takes issue with the comments.

Jones, J. H.↗
NASA NTRS1992

Implications for volcanogenic volatile release on the weathering of Mars

Volcanism on Mars has been widespread in both space and time. Notwithstanding important specific differences between the mantles of Earth and Mars, the similarities are such that the suite of gases emitted from martian volcanic activity surely includes gases such as H2O, CO2, S-containing gases (H2S, SO3, or SO2), and Cl-containing gases (e.g. Cl or HCl). Both H2O and CO2 are present in the atmosphere of Mars; both are also present as surface condensates. Spectroscopic observations of the martian atmosphere clearly show that the S- and Cl-containing gases are severely depleted. Likewise, there is no evidence of surface condensates of compounds of these elements. Within the soil, there is direct evidence of incorporation of H2O and some compounds of sulfur and chlorine. None of the resultant weathering products have been directly identified, but both clays and salts have been indirectly implicated. Other aspects of the implications for volcanogenic volatile release on the weathering of Mars are discussed.

Clark, Benton C.↗
NASA NTRS1993

Post-eruptive alteration of silicic ignimbrites and lavas, Gran Canaria, Canary Islands - Strontium, neodymium, lead, and oxygen isotopic evidence

The isotopic composition of lavas from oceanic islands provides important information about the composition and evolution of the earth's mantle. Isotopic analyses of Miocene comenditic, pantelleritic, and trachyphonolitic ignimbrites and lavas from the Canary islands were performed. Results provide evidence for posteruptive mobility of Rb and Sr during low temperature postemplacement interaction with circulating ground water. Calculated Sr isotope ratios define a magmatic trend in the stratigraph section. 87Sr/86Sr ratios in hydrated vitrophyte and devitrified matrix separates indicate significant posteruptive interaction with meteoric water starting soon after deposition. This process extends patchily through the entire pyroclastic flow and may be ongoing. 87Sr/86Sr ratios determined by whole rock analysis of silicic rocks from oceanic islands are suspect and should not be incorporated into mantle tracer studies. Anorthoclase phenocrysts are resistant to these processes and may produce useful data.

Cousens, Brian L.↗
NASA NTRS1992

First results from analysis of coordinated AVIRIS, TIMS, and ISM (French) data for the Ronda (Spain) and Beni Bousera (Morocco) peridotites

Ultramafic rocks are relatively rare at the Earth's surface but constitute the vast majority of the Earth by volume. Exposures of ultramafic bodies are therefore crucial for deducing many important processes that occur in the Earth's mantle. An important science question regarding the spatial distribution, abundance, and composition of mafic minerals in ultramafic bodies that can be examined with advanced sensor data is the melting process. When a lherzolite melts, clinopyroxene (cpx) melts first and therefore variations in the modal amount of cpx remaining in the mantle are a reflection of the amount of fractional melting that has occurred. Fe goes preferentially into the melt during melting but a 20 percent batch melting (i.e. closed system) acquires less Fe relative to 20 percent fractional melting (i.e. open system). Since the strength and wavelength of diagnostic absorptions is a strong function of Fe content, it is possible to make maps of the variation in Fe:Mg ratios which can be related to the general melting process. Accurate ground-truth information about local mineralogy provides internal calibration and consistency checks. Investigations using imaging spectrometer are very complementary to field studies because advanced sensor data can provide a synoptic view of modal mineralogy and chemical composition whereas field studies focus on detailed characterization of local areas. Two excellent exposures of ultramafic lithologies are being investigated with visible to mid-infrared imaging spectrometer data: the Ronda peridotite near Ronda, Spain and the Beni Bousera ophiolitic fragment in northern Morocco. Although separated by the Alboran Sea, these bodies are thought to be related and represent fertile sub-continental mantle. The Ronda peridotite is predominantly spinel lherzolite but grades into harzburgite and shows considerable variation in major and trace element compositions. Mafic layering and dykes (i.e. olivine gabbro) are also observed. This indicates some sections of the peridotite have experienced greater degrees of partial melting. The Beni Bousera peridotite also contains mafic layers and dykes and grades into harzburgite representing similar fundamental shifts in the bulk chemistry of this ultramafic body probably related to an episode of partial melting. The specific mode of emplacement of these bodies is controversial and important for understanding the tectonic evolution of this region. Our investigations are not necessarily designed to help resolve this controversy. Rather, these exposures provide excellent and unusual examples of fertile mantle which have undergone variable degrees of partial melting.

Mustard, J. F.↗
NASA NTRS1993

Magmatic volatiles and the weathering of Mars

The sources for volatiles on Mars have been the subject of many hypotheses for exogenous influences including late accretion of volatile-enriched material, impact devolatilization to create massive early atmospheres, and even major bombardment by comets. However, the inventory of chemically active volatiles observable at the contemporary surface of Mars is consistent with domination by endogenous, subsequent planetary processes, viz., persistent magmatic outgassing. Volcanism on Mars has been widespread in both space and time. Notwithstanding important specific differences between the mantles of Earth and Mars, the geochemical similarities are such that the suite of gases emitted from Martian volcanic activity should include H2O, CO2, S-containing gases (e.g. H2S and/or SO2), and Cl-containing gases (e.g., Cl2 and/or HCl). H2O and CO2 exist in the atmosphere of Mars. Both are also present as surface condensates. However, spectroscopic observations of the Martian atmosphere clearly show that the S- and Cl-containing gases are severely depleted, with upper limits of less than or equal to 10(exp -7) the abundance of CO2. Likewise, there is no evidence of polar condensates of compounds of these elements as there is for CO2 and H2O. Within the soil, on the other hand, there has been direct measurement of incorporated H2O and abundant compounds containing S and Cl. Barring some as yet implausible geochemical sequestering process, the S/Cl ratio of about 6:1 in Martian soils implies a limit of 5% on the contribution of matter of solarlike composition (e.g., carbonaceous chondrite or cometary material) to these volatiles. Hence, exogenous sources are minor or not yet observed. From analysis of elemental trends in Martian soils, it has been recently shown that a simple two-component model can satisfy the Viking in situ measurements. Component A includes Si and most or all the Al, Ca, Ti, and Fe. Component B, taken as 16 +/- 3% by weight of the total, contains S and most or all the Cl and Mg. These results constrain several models of Martian soil mineralogy but are consistent with a mixture of silicates (such as Fe-rich clays and accessory minerals and soluble salts). The overall element profile is notably like shergottites, with significant incorporation of chemically reactive atmospheric gases from magmatic degassing.

Clark, B. C.↗
NASA NTRS1994

Measurement and interpretation of crustal deformation rates associated with postglacial rebound

This project involves obtaining GPS measurements in Scandinavia and using the measurements to estimate the viscosity profile of the earth's mantle and to correct tide-gauge measurements for the rebound effect. We report on several aspects of this project. The DSGS was not scheduled to be reoccupied with DOSE receivers during the report period. The permanent network set up by Onsala Space Observatory continues to operate, and the data are being evaluated. An important technical advance we intend for this project is to use the full three dimensional site velocity information for inferring geophysical parameters. During the report period, two papers have been been accepted for publication in the Journal of Geophysical Research and will be published in April. Reprints of these papers are contained in the Appendix.

Davis, James L.↗