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At least 73 records · Page 4

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NASA NTRS1988

Primitive material surviving in chondrites - Mineral grains

Besides chondrules and various kinds of polymineralic inclusion, carbonaceous chondrites commonly contain, embedded in their matrices, isolated grains of mafic silicates and metallic iron. Most of the silicate grains probably originated in chondrules, but some appear to predate chondrule formation and may have formed as individual grains in the solar nebula. If that was the case, their compositions suggest some departure from equilibrium condensation from a gas of solar composition. Metal-grain compositions are broadly suggestive of nebular formation but the exact nature of the conditions in which they were formed remains problematical.

Steele, Ian M.↗

NASA NTRS1988

Interplanetary dust particles

The ways of establishing the extraterrestrial nature of different subsets of interplanetary dust collected in the stratosphere by high-altitude aircraft are discussed. Consideration is given to microanalytic techniques which make it possible to obtain detailed experimental information on the mineralogical and petrographic characteristics, the mid-IR absorption spectra, the Raman spectra, and the isotopic properties of individual particles. The implications of data obtained by these techniques for the origin of interplanetary dust are examined, showing that the particles are less altered than those solar-system material samples found in meteorites. It is suggested that many of the particles come from comets, although an unknown fraction originate from asteroids. Small regions of isotopically distinct material suggest that part of the dust consists of interstellar-cloud material that predates the solar system.

Bradley, John P.↗

NASA NTRS1989

Asteroid volatiles inventory

Asteroids appear in light of telescopic and meteority studies to be the most accessible repositories of early solar system history available. In the cooler regions of the outer asteroid belt, apparently unaffected by severe heating, the C, P, and D populations appear to harbor significant inventories of volatiles; the larger primordial belt population may have had an even greater percentage of volatile-rich, low-albedo asteroids, constituting a potent asteroid for veneering early terrestrial planet atmospheres. The volatile-rich asteroids contain carbon, structurally bound and adsorbed water, as well as remnants of interstellar material predating the solar system.

Lebofsky, L. A.↗

NASA NTRS1989

Identification of relict phases in a once-molten Allende inclusion

Fassaite rims around spinel grains that are enclosed entirely by normally zoned cores of melilite crystals are interpreted as relict grains which predate the melting of Allende type B1 refractory inclusions. Nonrelict subliquidus fassaite did not begin crystallizing from melts of these compositions until the onset of precipitation of the reversely zoned melilite mantles around the same melilite crystal cores. Relict fassaite is fractionated and highly enriched in REE and other refractory lithophiles compared to subliquidus fassaite in the same inclusion. Mass balance suggests that the relict fassaite represents only 3 percent of the original premelt amount of fassaite, and that melilite and at least one other phase were additional condensate precursors of these inclusions.

Kuehner, Scott M.↗

NASA NTRS1989

Strike-slip faulting of ridged plains near Valles Marineris, Mars

This paper identifies and documents several well-preserved examples of Martian strike-slip faults and examines their relationships to wrinkle-ridges. The strike-slip faulting predates or overlaps periods of wrinkle-ridge growth southeast of Valles Marineris, and some wrinkle ridges may have nucleated and grown as a result of strike-slip displacements along the echelon fault arrays. Lateral displacements of several km inferred along these arrays may be related to tectonism in Tharsis.

Schultz, Richard A.↗

NASA NTRS1989

Origin of the DA and non-DA white dwarf stars

Various proposals for the bifurcation of the white dwarf cooling sequence are reviewed. 'Primordial' theories, in which the basic bifurcation of the white dwarf sequence is rooted in events predating the white dwarf stage of stellar evolution, are discussed, along with the competing 'mixing' theories in which processes occurring during the white dwarf stage are responsible for the existence of DA or non-DA stars. A new proposal is suggested, representing a two-channel scenario. In the DA channel, some process reduces the hydrogen layer mass to the value of less than 10 to the -7th. The non-DA channel is similar to that in the primordial scenario. These considerations suggest that some mechanism operates in both channels to reduce the thickness of the outermost layer of the white dwarf. It is also noted that accretion from the interstellar medium has little to do with whether a particular white dwarf becomes a DA or a non-DA star.

Shipman, Harry L.↗

NASA NTRS1991

Asymptotic integration algorithms for nonhomogeneous, nonlinear, first order, ordinary differential equations

New methods for integrating systems of stiff, nonlinear, first order, ordinary differential equations are developed by casting the differential equations into integral form. Nonlinear recursive relations are obtained that allow the solution to a system of equations at time t plus delta t to be obtained in terms of the solution at time t in explicit and implicit forms. Examples of accuracy obtained with the new technique are given by considering systems of nonlinear, first order equations which arise in the study of unified models of viscoplastic behaviors, the spread of the AIDS virus, and predator-prey populations. In general, the new implicit algorithm is unconditionally stable, and has a Jacobian of smaller dimension than that which is acquired by current implicit methods, such as the Euler backward difference algorithm; yet, it gives superior accuracy. The asymptotic explicit and implicit algorithms are suitable for solutions that are of the growing and decaying exponential kinds, respectively, whilst the implicit Euler-Maclaurin algorithm is superior when the solution oscillates, i.e., when there are regions in which both growing and decaying exponential solutions exist.

Walker, K. P.↗

NASA NTRS1990

Presolar isotopic and chemical signatures in hibonite-bearing refractory inclusions from the Murchison carbonaceous chondrite

The mineralogy and the major-element composition of phases within individual hibonite-bearing inclusions were determined, using SEM with EDAX analysis, for 37 hibonite-bearing inclusions which comprised 26 spinel-hibonite-perovskite inclusions, ten platy hibonite crystal fragments, and one blue hibonite aggregate. Results from all inclusions studied indicated the presence of correlations in hibonite isotopic systematics and chemistry. Namely, no inclusion had both a large Ti-50 anomaly and radiogenic Mg-26, and inclusions that were depleted in the ultrarefractory elements had radiogenic Mg-26 with the exception of those which a had a large Ti-50 anomaly. These features establish a relationship between chemical and isotopic characteristics of the inclusions which must predate their formation. It is suggested that depletions in ultrarefractory elements observed in these inclusions may be a memory of stellar condensation and are not necessarily related to a condensation event in the solar nebula.

Ireland, Trevor R.↗

NASA NTRS1991

New observations on the quartz monzodiorite-granite suite

Five new fragments of quartz monzodiorite (QMD) were identified in particles from soil 15403, which was collected from the boulder sampled as rock 15405, an impact-melt breccia containing clasts of KREEP basalt, QMD, granite, and a more primitive alkali norite. Petrographic and geochemical studies of the fragments show considerable variation in modal proportions and bulk composition. This heterogeneity is due to unrepresentative sampling in small fragments of coarse-grained rocks. Variations in the proportions of accessory minerals have marked effects on incompatible-trace-element concentrations and ratios. Semiquantitative calculations support the derivation of QMD from 60-percent fractional crystallization of a KREEP basalt magma as suggested by Hess (1989). Apollo 15 KREEP basalt cannot be the actual parent magma because the evolved rocks predate volcanic KREEP basalts. It is suggested that ancient KREEP basalt magmas have crystallized as plutons, with alkali norite clasts offering the only direct evidence of this precursor.

Marvin, U. B.↗

NASA NTRS1990

Discretization chaos - Feedback control and transition to chaos

Problems in the design of feedback controllers for chaotic dynamical systems are considered theoretically, focusing on two cases where chaos arises only when a nonchaotic continuous-time system is discretized into a simpler discrete-time systems (exponential discretization and pseudo-Euler integration applied to Lotka-Volterra competition and prey-predator systems). Numerical simulation results are presented in extensive graphs and discussed in detail. It is concluded that care must be taken in applying standard dynamical-systems methods to control systems that may be discontinuous or nondifferentiable.

Grantham, Walter J.↗

NASA NTRS1991

The history of lunar volcanism

The timing and extent of volcanism must have reflected the internal thermal evolution of the moon and a knowledge of the history of lunar volcanism is important for understanding the global evolution of the moon. Recent studies show that lunar volcanism began prior to formation of at least some of the major lunar basins, although the exact provenance of the earliest volcanism remains somewhat obscure. It was shown that the ferroan anorthosites are the best candidates to be remnants of the postulated lunar magma ocean. The Mg-suite rocks are the second major category of highland rocks, and highly evolved granites and quartz-monzodiorites comprise a third suite of lunar highland rocks. Basalts form the Apollo 12, 15, and 17 landing sites demonstrated that the lunar mantle is greatly depleted in LREE beneath the widely separated landing sites, an observation supportive of a global process of differentiation. Also, KREEP volcanism apparently predated basin formation in the Serenitatis region, but appears to have been contemporaneous with basin formation in the Imbrium region. Recent studies showed that the earliest lunar basaltic volcanism was of mare-like composition.

Nyquist, Laurence E.↗

NASA NTRS1991

Geology and tectonics of Beta Regio, Guinevere Planitia, Sedna Planitia, and Western Eistla Regio, Venus - Results from Arecibo image data

The geology of a portion of the equatorial region of Venus (1 deg S to 45 deg N and from 270 deg eastward to 30 deg) is assessed on the basis of new radar images obtained from the Arecibo Observatory. Nine geologic units are mapped on the basis of their radar characteristics, and their distribution and correspondences with topography are examined. Plains are the most abundant unit, comprising 80 percent of the area imaged, and are divided into bright, dark, and mottled. Mottled plains contain abundant lava flows and domes, suggesting that volcanism forming planes is a significant process in the equatorial region of Venus. Tesserae are found primarily on Beta Regio and its eastern flank and are interpreted to be locally stratigraphically older units, predating episodes of faulting and plains formation. On the basis of spatial and temporal relationships between geologic units, the highlands of Beta Regio and Western Eistla Regio are interpreted to have formed in association with areas of mantle upwelling which uplift plains, cause rifting, and, in the case of Beta Regio, disrupt a large region of tesserae.

Senske, D. A.↗

NASA NTRS1991

Transportation analyses for the lunar-Mars initiative

This paper focuses on certain results of an ongoing NASA-sponsored study by Boeing, including (1) a series of representative space exploration scenarios; (2) the levels of effort required to accomplish each; and (3) a range of candidate transportation system as partial implementations of the scenarios. This effort predated release of the Synthesis report; the three levels of activity described are not responses to the Synthesis architectures. These three levels (minimum, median and ambitious), do envelope the range of scope described in the four Synthesis architecture models. The level of analysis detail was to the current known level of detail of transportation hardware systems and mission scenarios. The study did not include detailed analysis of earth-to-orbit transportation, surface systems, or tracking and communications systems. The influence of earth-to-orbit systems was considered in terms of delivery capacity and cost. Aspects of additional options, such as in situ resource utilization are explored as needed to indicate potential benefits. Results favored cryogenic chemical propulsion for low activity levels and undemanding missions (such as cargo and some lunar missions), nuclear thermal propulsion for median activity levels similar to the Synthesis architectures, and nuclear thermal propulsion with aerobraking or nuclear electric propulsion for high activity levels. Solar electric propulsion was seen as having an important role if the present high unit cost (i.e., dollars per watt) of space photovoltaics could be reduced by a factor of five or more at production rates of megawatts per year.

Woodcock, Gordon R.↗

NASA NTRS1992

The continuous plankton recorder survey: A long-term, basin-scale oceanic time series

In the 1920s, before the advent of echo sounders, fishery biologists were greatly concerned with assisting the fisherman to locate schools of pelagic fish. One of the approaches they developed was to relate the distribution of the planktonic food organisms to the presence of the schools of predators such as herring (Clupea harengus). The British planktologist, Alister Hardy, who had already carried out extensive studies on the feeding preferences of herring (Hardy, 1926a), initiated a program to examine the fishermen's contention that herring schools avoided 'green', i.e., phytoplankton-rich, water but could be correlated with high concentrations of zooplankton. This practical program was centered on the use of a specially developed instrument, the 'Plankton Indicator', designed to be used by the fisherman to assist in the search for suitable waters. It had limited success in its main aim but, as a collecting device, it embodied several profoundly important features. It was a simple instrument which was robust enough to be deployed and recovered by the crew of commercial vessels (in this case fishing vessels) while they were underway. The Indicator however, was no more than a high speed net which integrated the plankton over the area of sampling, but Hardy had also become interested in describing the patchiness of planktonic populations. He thus developed the Continuous Plankton Recorder (CPR) where he substituted the fixed filter screen of the Indicator by a continually moving length of silk mesh. The screen traversed at constant speed across the path of the incoming water and the trapped organisms were retained in place by sandwiching beneath an additional second mesh screen. Thus, knowing the speed of the towing vessel and the shooting and hauling positions, the spatial patterns of the plankton could be determined. Hardy took the first CPR to the Antarctic where he used it in the Southern Atlantic (Hardy, 1926b) and later deployed it in the North Sea to make some of the earliest contiguous records of plankton patchiness.

Gamble, John C.↗

NASA NTRS1993

The stellar velocity dispersion in the nucleus of M82

We observed the 2-0 (C-12)O absorption bandhead from integrated starlight from the nuclear bulge of the starburst galaxy M82. The velocity dispersion of the stellar component in the central arcsecond is significantly larger than that of the ionized gas in the same region, implying that the stars and gas represent different kinematic systems. The M/L(K) in this nuclear region is essentially identical to that in our own nuclear bulge, but is much higher than in the starburst disk of M82 at larger radii. This implies that the near-infrared continuum from the central r is less than 7.5 pc of M82 is dominated by a nuclear bulge population that predates the enveloping starburst. This work is a first step in separating these two distinct populations in the central region of this well-observed starburst galaxy.

Gaffney, Niall I.↗

NASA NTRS1993

Paleomagnetism and tectonics of the Crescent Formation, northern Olympic Mountains, Washington

Results are presented of a paleomagnetic analysis of the Crescent Formation basalts of the northern Olympic Mountains, carried out with the purpose of constraining the emplacement and deformation history of the rocks of the northern Coast Range. It was found that (1) the stable remanent magnetization measured within the Crescent Formation appears to be early, predating significant deformation, and probably is primary; (2) a correction for bedding rotations about strike within four different structural domains produces a circular distribution of virtual geomagnetic poles; and (3) the Crescent Formation, where sampled in the north, records no significant net rotation or displacement.

Warnock, Andrew C.↗

NASA NTRS1993

Impacts, tillites, and the breakup of Gondwanaland

Mathematical analysis demonstrates that substantial impact crater deposits should have been produced during the last 2 Gy of Earth's history. Textures of impact deposits are shown to resemble textures of tillites and diamictites of Precambrian and younger ages. The calculated thickness distribution for impact crater deposits produced during 2 Gy is similar to that of tillites and diamictites of 2 Ga or younger. We suggest, therefore, that some tillites/diamictites could be of impact origin. Extensive tillite/diamictite deposits predated continental flood basalts on the interior of Gondwanaland. Significantly, other investigators have already associated impact cratering with flood basalt volcanism and continental rifting. Thus, it is proposed that the breakup of Gondwanaland could have been initiated by crustal fracturing from impacts.

Oberbeck, Verne R.↗

NASA NTRS1993

Th-230 - U-238 series disequilibrium of the Olkaria basalts Gregory Rift Valley, Kenya: Petrogenesis

Strong mixing trends on a (Th-230/Th-232) versus Th diagram show that the basalts are mixed magmas which have undergone interaction with the crust. Instantaneous Th/U ratios are less than time integrated ones but these exceed the Th/U ratios in the MORB and OIB sources. This indicates that the mantle may have undergone some metasomatic fluxing, crustal contamination of the basalts will also enhance these ratios. Early activity on the Akira plain is represented by early basalts and hawaiites. The early basalt samples are known to predate the earliest comendites. The most recent phase of activity is represented by another cinder cone 40-50 m high being feldspar and clinopyroxene phyric. Inclusions which occur in the comendites vary in size and distribution. The largest and most porphyritic are the trachytes (up to 40 cm) with alkali feldspar phases up to 6 mm and small pyroxenes in the ground mass. The second set of inclusions are smaller (up to 10 cm) and are largely aphyric. The distribution of the inclusions are not uniform, the Broad Acres (C5) lavas contain 2-5 percent. The size of the inclusions decrease from south to north, as does the abundance of the trachytic inclusions. The major element variations in the Naivasha basalts, hawaiites and magmatic inclusions are discussed.

Black, S.↗