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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Usefulness of multifrequency MST radar measurements, part 2.6B

Scattering of radio waves from atmospheric refractive-index irregularities induced by turbulence was invoked almost four decades ago to explain the characteristics of signals received on VHF/UHF ionospheric and tropospheric forward-scatter links. Due to the bistatic geometry of these links a slender, horizontally extended, common volume or cell is formed in space. The principal contribution to scattering arises from refractive-index fluctuations in this volume at the Bragg wave number K approx. sub B = K approx. sub i -k approx. sub s vectors. It has been surmised that the use of more than one frequency in probing the middle-atmosphere regions should help resolve several issues pertaining to the scattering mechanism. These issues are briefly re-examined in this note. The implications of the radar equation are discussed. The problems arising due to layered structure of turbulence and the choice of frequencies most suitable for multifrequency measurements are considered.

Rastogi, P. K.↗
NASA NTRS1985

Hydrothermal alteration in the 3.5 b.y. old Onverwacht Group of South Africa

K-Ar studies of authigenic and alteration phases from the 3.5 b.y. old Onverwacht group of South Africa delineate hydrothermal metamorphism that ended 3.3 b.y. years ago. A whole rock K-Ar errochron from analysis of barite, dolomite, chert, and serpentinite (komatiite) gives an age of 3.3 b.y. with an intercept of 1,678 + or - 103. The 3.3. b.y. age for the metamorphism in the Onverwacht was confirmed by the Argon isotopes stepwise heating experiments of komatiites and basaltic komatiites from the Onverwacht Group. In addition, the errochron suggests all the phase studied equilibrated with a reservoir of hydrothermal argon with relatively uniform isotopic composition. The concept of hydrothermal activity in the Onverwacht Group is discussed and illustrated with photographs.

Hart, R.↗
NASA NTRS1985

Constraints on the development of the early continental crust from isotopic data

Aspects of the origin and development of the early (AE) continential crust are addressed by radiogenic isotope and trace element studies. The most important ones are: (1) at what time did the earliest continental crust form; (2) what was its composition; (3) by what processes did it grow and by what processes was it destroyed; (4) what were the rates of production and destruction as a function of time during this time period? Nd is isotopic data on the oldest terrestrial rocks indicate that the mantle at this time had already suffered substantial depletion in incompatible elements due to earlier continent forming events. Isotopic data on young volcanic rocks derived from the depleted mantle show no evidence of this early history. The observed isotopic patterns of Nd, Sr, Hf and Pb through time together with the presently observed age spectrum of crustal rocks are considered. These patterns can be modelled by a transport model in which the continental growth and destruction rates are allowed to vary as a function of time. It is suggest that the mass of the continents at 3.8 AE ago was about 25% of the current continental mass. However, due to the very high recycling rates obtained in the early Archean only a few percent of this crust has been preserved up to the present.

Jacobson, S. B.↗
NASA NTRS1985

A review of tectonic aspects of the Limpopo belt and other Archean high-grade gneissic terranes

Published information on the Archean high grade terrains varies a great deal in the detail available. Such information as exists indicates marked differences in the lithic types and proportions present in the central Limpopo belt compared with the better studies of the other Archean high grade terrains. These differences may be important because they are expressed by the presence in the Limpopo belt of subordinate, but significant quantities (about 5% each) of two rock suites likely to have formed on a shallow marine platform of significant size (Eriksson and Kidd, in prep.). These suites consist of thick sections dominantly consisting of either carbonate and calc-silicate, or of pure metaquartzites, often fuchsite bearing, whose lithic characters are unlike those expected for metacherts but are very like those expected for platform arenites. Isotopic ages suggest these sediments are probably older than 3.3 Ga and younger than 3.5 Ga. Studies lead to the conclusions that (1) continental fragments large enough to provide a substrate for significant platform arenite and carbonate sedimentation existed by 3.3 to 3.5 Ga ago; (2) Wilson cycle tectonics seems to adequately explain most major features of the Archean gneissic terranes; and (3) Tibetan-Himalayan style collisional tectonics 2.6 Ga and older accounts for the large scale relationships between the Limpopo belt and the adjacent Archean greenstone granitoid terrane cratons. By inference, other more fragmentary Archean gneissic terranes may have once been part of such collisional zones.

Kidd, W. S. F.↗
NASA NTRS1984

Experimental studies of glass refining

The basic components of the experimental apparatus were selected and acquired. Techniques were developed for the fabrication of the special crucibles necessary for the experiments. Arrangements were made for the analysis of glass and gas bubble samples for composition information. Donations of major equipment were received for this project from Owens, Illinois where a similar study had been conducted a few year ago. Decisions were made regarding the actual glass composition to be used, the gas to be used in the first experiments, and the temperatures at which the experiments should be conducted. A microcomputer was acquired, and work was begun on interfacing the video analyzer to it.

Subramanian, R. S.↗
NASA NTRS1985

Biological modulation of planetary atmospheres: The early Earth scenario

The establishment and subsequent evolution of life on Earth had a profound impact on the chemical regime at the planet's surface and its atmosphere. A thermodynamic gradient was imposed on near-surface environments that served as the driving force for a number on important geochemical transformations. An example is the redox imbalance between the modern atmosphere and the material of the Earth's crust. Current photochemical models predict extremely low partial pressures of oxygen in the Earth's prebiological atmosphere. There is widespread consensus that any large-scale oxygenation of the primitive atmosphere was contingent on the advent of biological (autotrophic) carbon fixation. It is suggested that photoautotrophy existed both as a biochemical process and as a geochemical agent since at least 3.8 Ga ago. Combining the stoichiometry of the photosynthesis reaction with a carbon isotope mass balance and current concepts for the evolution of the stationary sedimentary mass as a funion of time, it is possible to quantify, the accumulation of oxygen and its photosynthetic oxidation equivalents through Earth history.

Schidlowski, M.↗
NASA NTRS1985

A solid-state framework for terrestrial upper mantles

A framework is proposed for understanding the upper mantle structure of the terrestrial planets which is based on solid state dislocation processes. It is proposed that the base of the lithosphere on any planet with a Mg-Fe silicate rich upper mantle is defined by the threshold temperatures of low energy dislocation glide systems in olivine and the pyroxenes. This threshold temperature is approximately 1100 to 1200 K and is directly tied to the mobility of olivine and pyroxene dislocations. Using this definition, all terrestrial planets of the Inner Solar System are expected to have mantle lithospheres. Second, the anomalous properties of the asthenosphere in the Earth are related to the rheological properties of an olivine rich differentiate approximately 220 km thick. All of these properties can be the result of the abundant low energy glide systems in olivine which are mobile at temperatures above approximately 1200 K. It is proposed that a true planetary asthenosphere must be both olivine rich and at temperatures above approximately 1200 K. This mode for the asthenosphere does not require partial melting and is therefore less sensitive to the constraints of volatile content in the upper mantle. The thickness of planetary lithospheres is estimated as is maximum thickness of the olivine rich differentiate on each planet. The cooling of the olivine rich layer on Mars (and hence disappearance of the asthenosphere) may coincide with the cessation of global tectonic activity approximately 1 Gyr ago.

Estey, L.↗
NASA NTRS1985

Magnetic field generation in the cores of terrestrial bodies

Efforts to find some scaling law for the dipole moments of planets seem illusory for, although dynamo theory is still in a rudimentary state, once the critical magnetic Reynolds Number is exceeded it appears that the field strength is determined by the energy source, it it is permissible to treat the core as a heat engine. For this reason the lunar magnetic field is of special significance as the paleomagnetic evidence strongly suggests that the surface field was about 1 G 3.9 by diminishing exponentially to about .02 G 3.2 by ago and completely disappearing some time later.

Runcorn, S. K.↗
NASA NTRS1985

Shock and statistical acceleration of energetic particles in the interplanetary medium

Definite evidence for particle acceleration in the solar wind came around a decade ago. Two likely sources are known to exist: particles may be accelerated by the turbulence resulting from the superposition of Alfven and Magnetosonic waves (Statistical Acceleration) or they may be accelerated directly at shock fronts formed by the interaction of fast and slow solar wind (CIR's) or by traveling shocks due to sporadic coronal mass ejections. Naurally both mechanisms may be operative. In this work the acceleration problem was tackled numerically using Helios 1 and 2 data to create a realistic representation of the Heliospheric plasma. Two 24 hour samples were used: one where there are only wave like fluctuations of the field (Day 90 Helios 1) and another with a shock present in it (Day 92 of Helios 2) both in 1976 during the STIP 2 interval. Transport coefficients in energy space have been calculated for particles injected in each sample and the effect of the shock studied in detail.

Valdes-Galicia, J. F.↗
NASA NTRS1985

Sidereal variations deep underground in Tasmania

Data from the deep underground vertically directed muon telescopes at Poatina, Tasmania, have been used since 1972 for a number of investigations, including the daily intensity variations, atmospheric influences, and checking for possible effects due to the interplanetary magnetic field. These telescopes have a total sensitive area of only 3 square meters, with the result that the counting rate is low (about 1680 events per hour) and the statistical errors on the results are rather large. Consequently, it was decided several years ago to construct larger detectors for this station. The first of these telescopes has been in operation for two complete years, and the results from it are presented. Results from the new, more stable equipment at Poatina appear to confirm the existence of a first harmonic in the daily variations in sidereal time reported earlier, and are consistent with small or non-existent first harmonics in solar and anti-sidereal time. All the second harmonics appear to be small, if not zero at these energies.

Humble, J. E.↗
NASA NTRS1984

Noble gas contents of shergottites and implications for the Martian origin of SNC meteorites

Three meteorites belonging to the rare group of SNC achondrites, which may have originated in the planet Mars, have been subjected to noble gas isotopic concentration measurements. The elemental and isotopic ratios obtained are unlike those for any other noble gas components except those obtained in analyses of the Martian atmosphere by Viking spacecraft. It is hypothesized that the Kr and Xe gases represent a portion of the Martian atmosphere which was shock-implanted in the case of Elephant Moraine A79001, and that they constitute direct evidence of a Martian origin for the shergottite meteorites. If the SNC meteorites were ejected from Mars at the shergottite shock age of about 180 My ago, they must have been objects more than 6 m in diameter which experienced at least three space collisions to initiate cosmic ray exposure.

Bogard, D. D.↗
NASA NTRS1984

X-ray emission from supernova remnants MSH 14 - 63 and RCW 103

Electron-ion collisions, heavy element enrichment of emissions from reverse-shocked ejecta, abundances in the interstellar medium (ISM), the correspondence between the projected and measured age, ISM particle density, and distance, and classification factors for Type I or II supernovae are examined using a nonequilibrium ionization (NEI) model. The NEI model accounts for the absence of collisional ionization equilibrium and possibly thermal equilibrium between electrons and ions behind the shock fronts, and describes the emission spectra from ejecta and the shocked ISM. When applied to data from the SNR MSH 14 - 63 a non-Coulomb, ion-electron interaction is obtained that is consistent with a shocked ISM. For the SNR RCW 103, however, the processes behind the shock front are not discernable, although the elemental abundances will have solar values if the interactions are non-Coulomb. The age of RCW 103 must be approximated from physical data, as Chinese astronomers would not have been able to see the event from their position on earth two millenia ago.

Nugent, J. J.↗
NASA NTRS1984

Thermal evolution of Venus

A modification of the Boussinesq fluid assumption is the basis of the present theory of three-dimensional and finite amplitude convection in a viscous spherical shell with temperature- and pressure-dependent physical parameters. The theory is applied to the definition of thermal evolution models for Venus which emphasize the effects of certain physical parameters on thermal evolution, rather than the specific thermal history of the planet. It is suggested that a significant portion of the present temperature in the mantle and surface heat flux of Venus is due to the decay of a high temperature that was established in the planet at the completion of its core formation, and that Venus has been highly convective over the course of its history, until about 0.5 Ga ago.

Arkani-Hamed, J.↗
NASA NTRS1985

Integrated Power and Attitude Control Systems for Space Station

Integrated Power and Attitude Control Systems (IPACS) studies performed over a decade ago established the feasibility of simultaneously storing electrical energy in wheels and utilizing the resulting momentum for spacecraft attitude control. It was shown that such a system possessed many advantages over other contemporary energy storage and attitude control systems in many applications. More recent technology advances in composite rotors, magnetic bearings, and power control electronics have triggered new optimism regarding the feasibility and merits of such a system. The paper presents the results of a recent study whose focus was to define an advanced IPACS and to evaluate its merits for the Space Station application. A system and component design concept is developed to establish the system performance capability. A system level trade study, including life-cycle costing, is performed to define the merits of the system relative to two other candidate systems. It is concluded that an advanced IPACS concept is not only feasible, but offers substantial savings in mass, and life-cycle cost.

Oglevie, R. E.↗
NASA NTRS1985

Satellite magnetic anomalies over subduction zones - The Aleutian Arc anomaly

Positive magnetic anomalies seen in MAGSAT average scalar anomaly data overlying some subduction zones can be explained in terms of the magnetization contrast between the cold subducted oceanic slab and the surrounding hotter, nonmagnetic mantle. Three-dimensional modeling studies show that peak anomaly amplitude and location depend on slab length and dip. A model for the Aleutian Arc anomaly matches the general trend of the observed MAGSAT anomaly if a slab thickness of 7 km and a relatively high (induced plus viscous) magnetization contrast of 4 A/m are used. A second source body along the present day continental margin is required to match the observed anomaly in detail, and may be modeled as a relic slab from subduction prior to 60 m.y. ago.

Clark, S. C.↗
NASA NTRS1984

Endpoints of stellar evolution - X-ray surveys of the Local Group

The galaxies of the Local Group offer a variety of settings in which to study galactic source populations unhampered by local interstellar-absorption and distance ambiguities. A review of the subject of Local Group X-ray astronomy from the first discovery of a source in the Large Magellanic Cloud 15 years ago to the most recent analysis of the Einstein Observatory data (in which over 200 X-ray sources were detected in Local Group members) is presented. Following a detailed presentation of the latest results, two examples of the constraints these data impose on stellar and galactic evolution are discussed: study of the distribution of properties for the 30 detected supernova remnants in the Large Magellanic Cloud, and a comparison of the populations of classical X-ray binaries in M31, M33, the Magellanic Clouds, and the Galaxy. A prospectus for future research, including a discussion of the contributions which could be made by the next generation of X-ray observatories, is provided.

Helfand, D. J.↗
NASA NTRS1984

Refractory materials for high-temperature thermoelectric energy conversion

Theoretical work of two decades ago adequately explained the transport behavior and effectively guided the development of thermoelectric materials of high conversion efficiencies of conventional semiconductors (e.g., SiGe alloys). The more significant contributions involved the estimaiation of optimum doping concentrations, the reduction of thermal conductivity by solid solution doping and the development of a variety of materials with ZT approx. 1 in the temperature range 300 K to 1200 K. ZT approx. 1 is not a theoretical limitation although, experimentally, values in excess of one were not achieved. Work has continued with emphasis on higher temperature energy conversion. A number of promising materials have been discovered in which it appears that ZT 1 is realizable. These materials are divided into two classes: (1) the rare-earth chalcogenides which behave as itinerant highly-degenerate n-type semiconductors at room-temperature, and (2) the boron-rich borides, which exhibit p-type small-polaronic hopping conductivity.

Wood, C.↗
NASA NTRS1984

Comments on the BLAG model - The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years

A self-consistent method of determining initial conditions for the model presented by Berner, Lasaga, and Garrels (1983) (henceforth, the BLAG model) is derived, based on the assumption that the CO2 geochemical cycle was in steady state at t = -100 m.y. (million years). This initialization procedure leads to a dissolved magnesium concentration higher than that calculated by Berner, Lasaga, and Garrels and to a low ratio of dissolved calcium to bicarbonate prior to 60 m.y. ago. The latter prediction conflicts with the geologic record of evaporite deposits, which requires that this ratio remain greater than 0.5. The contradiction is probably caused by oversimplifications in the BLAG model, such as the neglect of the cycles of organic carbon and sulfur.

Kasting, J. F.↗