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At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Constraints on crustal rheology and age of deformation from models of gravitational spreading in Ishtar Terra, Venus

Gravitational spreading is expected to lead to rapid relaxation of high relief due to the high surface temperature and associated weak crust on Venus. In this study, we use new Magellan radar and altimetry data to determine the extent of gravitational relaxation in Ishtar Terra, which contains the highest relief on Venus as well as areas of extremely high topographic slope. Within Ishtar Terra the only mountain belts found on Venus, Akna, Danu, Freyja, and Maxwell Montes, nearly encircle the smooth, high (3-4 km) plateau of Lakshmi Planum. Finite-element models of this process give expected timescales for relaxation of relief and failure at the surface. From these modeling results we attempt to constrain the strength of the crust and timescales of deformation in Ishtar Terra. Below we discuss observational evidence for gravitational spreading in Ishtar Terra, results from the finite-element modeling, independent age constraints, and implications for the rheology and timing of deformation.

Smrekar, Suzanne E.↗
NASA NTRS1992

A first-order global model of Late Cenozoic climatic change: Orbital forcing as a pacemaker of the ice ages

The development of a theory of the evolution of the climate of the earth over millions of years can be subdivided into three fundamental, nested, problems: (1) to establish by equilibrium climate models (e.g., general circulation models) the diagnostic relations, valid at any time, between the fast-response climate variables (i.e., the 'weather statistics') and both the prescribed external radiative forcing and the prescribed distribution of the slow response variables (e.g., the ice sheets and shelves, the deep ocean state, and the atmospheric CO2 concentration); (2) to construct, by an essentially inductive process, a model of the time-dependent evolution of the slow-response climatic variables over time scales longer than the damping times of these variables but shorter than the time scale of tectonic changes in the boundary conditions (e.g., altered geography and elevation of the continents, slow outgassing, and weathering) and ultra-slow astronomical changes such as in the solar radiative output; and (3) to determine the nature of these ultra-slow processes and their effects on the evolution of the equilibrium state of the climatic system about which the above time-dependent variations occur. All three problems are discussed in the context of the theory of the Quaternary climate, which will be incomplete unless it is embedded in a more general theory for the fuller Cenozoic that can accommodate the onset of the ice-age fluctuations. We construct a simple mathematical model for the Late Cenozoic climatic changes based on the hypothesis that forced and free variations of the concentration of atmospheric greenhouse gases (notably CO2), coupled with changes in the deep ocean state and ice mass, under the additional 'pacemaking' influence of earth-orbital forcing, are primary determinants of the climate state over this period. Our goal is to illustrate how a single model governing both very long term variations and higher frequency oscillatory variations in the Pleistocene can be formulated with relatively few adjustable parameters.

Saltzman, Barry↗
NASA NTRS1993

An artificial intelligence-based structural health monitoring system for aging aircraft

To reduce operating expenses, airlines are now using the existing fleets of commercial aircraft well beyond their originally anticipated service lives. The repair and maintenance of these 'aging aircraft' has therefore become a critical safety issue, both to the airlines and the Federal Aviation Administration. This paper presents the results of an innovative research program to develop a structural monitoring system that will be used to evaluate the integrity of in-service aerospace structural components. Currently in the final phase of its development, this monitoring system will indicate when repair or maintenance of a damaged structural component is necessary.

Grady, Joseph E.↗
NASA NTRS1993

Starburst ages in HII galaxies

The age of the starbursts of a few H2 galaxies is derived from optical photometry and compared with previous results from radio continuum spectra. H2 galaxies are gas rich and characterized by the dominance of giant H2 regions, blue stellar colors, high surface brightness, and narrow emission lines. The presently observed high abundances of bright young stars must be the result of a recent -or even ongoing- starburst. The question, whether the observed starburst is the first one or if it is a recurrent phenomenon, is still open. If the starburst is the first one, H2 galaxies can be interpreted as being among the most unevolved galaxies known - their star formation characteristics possibly paralleling that of normal galaxies early in their evolution. A sample of five H2 galaxies was observed with B, R, and I broadband filters. They have been observed previously at several radio continuum frequencies. The photometric data are given. The results of the photometry and a comparison with radio continuum spectra are given.

Deeg, Hans-Joerg↗
NASA NTRS1993

Ages, chemistry, and type 1A supernovae: Clues to the formation of the galactic stellar halo

We endeavor to resolve two conflicting constraints on the duration of the formation of the Galactic stellar halo - 2-3 Gyr age differences in halo stars, and the time scale inferred from the observed constant values of chemical element abundance ratios characteristic of enrichment by Type II supernovae - by investigating the time scale for the onset of Type Ia supernovae (SNIa) in the currently favored progenitor model - mergers of carbon and oxygen white dwarfs (CO WDs).

Smecker-Hane, Tammy A.↗
NASA NTRS1992

The activities of comets related to their aging and origin

Attention is given to two indices developed for the purpose of comparing the natures of various classes of comets: the Activity Index (AI), which measures the inherent magnitude increase in brightness from great solar distances to maximum near the perihelion, and the Volatility Index (VI), which measures the variation in magnitude near the perihelion. AI determinations are made for 32 long-period (LP) comets and for 14 short-period (SP). The range of values of AI is of the order of 3 to 10 magnitudes with a median of about 6. No fading with aging among the LP comets was found. No systematic change with period occurs for the VI index. Weak correlations are found with the gas/dust ratio of comets. No correlations are discovered between the two indices, nor of either index with near-perihelion magnitudes or orbital inclination.

Whipple, Fred L.↗
NASA NTRS1991

Thermal stability of the microstructure of an aged Nb-Zr-C alloy

The effects of thermal aging with and without an applied stress on the microstructure of a Nb-Zr-C alloy containing 0.9 wt percent Zr and 0.06 percent C were studied. Chemical analysis, metallographic examination, energy dispersive X-ray spectra of the bulk material, and chemical and X-ray analyses of the phase-extracted residue were used to characterize the microstructure. The samples examined were from a creep strength study involving hot and cold working, and various combinations of exposure to temperatures ranging from 1350 to 1755 K with and without applied load times as long as 34,000 plus hours. The results showed that the initial microstructure consisted primarily of orthorombic precipitates of Nb sub C which were partially or completely transformed to face-centered cubic carbides of Nb and Zr, (Zr, Nb)C, upon prolonged exposure to elevated temperatures. Furthermore, it was found that the microstructure of the alloy is extremely stable owing to the very finely distributed precipitates throughout its matrix and along the grain boundaries. The lattice parameters of the cubic carbides were determined to vary from 0.458 to 0.465 nm as the Zr/Nb ratio varied from 38/62 to 75/25.

Uz, Mehmet↗
NASA NTRS1992

Isotopic signatures of black tektites from the K-T boundary on Haiti - Implications for the age and type of source material

An isotopic study was carried out to characterize the type of black tektites from the Cretaceous-Tertiary (K-T) boundary on Haiti (the first reasonably well-preserved impact-derived material recovered from the K-T boundary), in order to help characterize the tektite source material (i.e., the type of rocks that were melted and ejected during the impact event(s) at around 64.5 Ma). Results show that the isotopic data and all of the element concentration data obtained are consistent with an andesitic-dacitic composition for the tektites and their source material. The Nd isotopic data suggest that the source rocks were not older than Silurian (T(chur) = 400 Ma) in age, and were composed largely of young (less than 1080 Ma) crustal material. Of the suspected K-T boundary impact sites, both the Manson (Iowa) and Chicxulub (Yucatan) structures occur in suitable lithologies to yield the Haitian black tektites.

Premo, W. R.↗
NASA NTRS1992

Evidence for an old Galactic bulge from RR Lyrae stars in Baade's window - Implications for the formation of the Galaxy and the age of the universe

Recent abundance measurements of RR Lyrae variables in the Baade's window field of the Galactic nuclear bulge are examined, and the observed metallicity distributions of the RR Lyraes in different Galactic radial zones are compared with those predicted for the HB population models. It is shown that the observed systematic variation with Galactocentric distance is only explained if the age of the stellar population increases, in the mean, with decreasing R(G). Thus, the oldest stellar population, the RR Lyraes in the Galactic nuclear bulge, is indeed older than that in the halo.

Lee, Young-Wook↗
NASA NTRS1992

The metal-rich globular cluster NGC 6553 - Clump morphology and age

Stellar evolutionary tracks for metal-rich HB stars have been used to construct synthetic population models for the peculiar HB clump in NGC 6553. Although the uncertainties are large, it is possible to derive an age for NGC 6553 in the range 8-13 Gyr, which supports the idea that the disk globular cluster system is younger than the oldest stars in the Galactic halo and bulge.

Demarque, P.↗
NASA NTRS1991

Research on sleep, circadian rhythms and aging - Applications to manned spaceflight

Disorders of sleep and circadian rhythmicity are characteristic of both advancing age and manned spaceflight. Sleep fragmentation, reduced nocturnal sleep tendency and sleep efficiency, reduced daytime alertness, and increased daytime napping are common to both of these conditions. Recent research on the pathophysiology and treatment of disrupted sleep in older people has led to a better understanding of how the human circadian pacemaker regulates the timing of the daily sleep-wake cycle and how it responds to the periodic changes in the light-dark cycle to which we are ordinarily exposed. These findings have led to new treatments for some of the sleep disorders common to older individuals, using carefully timed exposure to bright light and darkness to manipulate the phase and/or amplitude of the circadian timing system. These insights and treatment approaches have direct applications in the design of countermeasures allowing astronauts to overcome some of the challenges which manned spaceflight poses for the human circadian timing system. We have conducted an operational feasibility study on the use of scheduled exposure to bright light and darkness prior to launch in order to facilitate adaptation of the circadian system of a NASA Space Shuttle crew to the altered sleep-wake schedule required for their mission. The results of this study illustrate how an understanding of the properties of the human circadian timing system and the consequences of circadian disruption can be applied to manned spaceflight.

Czeisler, Charles A.↗
NASA NTRS1993

Transverse flexural tests as a tool for assessing damage to PMR-15 composites from isothermal aging in air at elevated temperatures

To date, the effect of thermo-oxidative aging on unidirectional composite mechanical properties has been monitored by the measurement of interlaminar shear strength (ILSS) and either three or four point longitudinal flexural strength (LFS) of the composites being tested. Both results are affected by the fiber-to-matrix bonding, the former being dependent on the shear resistance of the interface and the latter on the degree of load sharing by the fibers through the fiber/matrix interface. Recently, fiber/matrix interfacial bond strengths have been monitored using a transverse flexural strength (TFS) test method. This test method was used to evaluate the effect of fiber surface treatment on the fiber/matrix.

Bowles, Kenneth J.↗
NASA NTRS1993

The evolution of angular momentum among zero-age main-sequence solar-type stars

We consider a survey of rotation among F, G, and K dwarfs of the Pleiades in the context of other young clusters (Alpha Persei and the Hyades) and pre-main-sequence (PMS) stars (in Taurus-Auriga and Orion) in order to examine how the angular momentum of a star like the sun evolves during its early life on the main sequence. The rotation of PMS stars can be evolved into distributions like those seen in the young clusters if there is only modest, rotation-independent angular momentum loss prior to the ZAMS. Even then, the ultrafast rotators (UFRs, or ZAMS G and K dwarfs with v sin i equal to or greater than 30 km/s) must owe their extra angular momentum to their conditions of formation and to different angular momentum loss rates above a threshold velocity, for it is unlikely that these stars had angular momentum added as they neared the ZAMS, nor can a spread in ages within a cluster account for the range of rotation seen. Only a fraction of solar-type stars are thus capable of becoming UFRs, and it is not a phase that all stars experience. Simple scaling relations (like the Skumanich relation) applied to the observed surface rotation rates of young solar-type stars cannot reproduce the way in which the Pleiades evolve into the Hyades. We argue that invoking internal differential rotation in these ZAMS stars can explain several aspects of the observations and thus can provide a consistent picture of ZAMS angular momentum evolution.

Soderblom, David R.↗
NASA NTRS1993

Possible effects of anthropogenically-increased CO2 on the dynamics of climate - Implications for ice age cycles

A dynamical model, developed to account for the observed major variations of global ice mass and atmospheric CO2 during the late Cenozoic, is used to provide a quantitative demonstration of the possibility that the anthropogenically-forced increase of atmospheric CO2, if maintained over a long period of time (perhaps by tectonic forcing), could displace the climatic system from an unstable regime of oscillating ice ages into a more stable regime representative of the pre-Pleistocene. This stable regime is characterized by orbitally-forced oscillations that are of much weaker amplitude than prevailed during the Pleistocene.

Saltzman, Barry↗
NASA NTRS1993

The natural thermoluminescence of meteorites. VI - Carbon-14, thermoluminescence and the terrestrial ages of meteorites

A relationship is noted between the natural thermoluminescence (TL) levels and the C-14-derived terrestrial ages for meteorite finds from the U.S. Prairie States and Roosevelt County, NM; those in the Sahara are also in accord with calculated TL decay curves, for 'storage' temperatures equal to the approximate average annual temperatures at individual sites. This discussion is limited to the empirical correspondence between the two methodologies, and to theoretical decay curves for a single 'average' ordinary chondrite.

Benoit, P. H.↗
NASA NTRS1941

Effect of aging on mechanical properties of aluminum-alloy rivets

Curves and tabular data present the results of strength tests made during and after 2 1/2 years of aging on rivets and rivet wire of 3/16-inch nominal diameter. The specimens were of aluminum alloy: 24S, 17S, and A17S of the duralumin type and 53S of the magnesium-silicide type.

Roop, Frederick C↗
NASA NTRS1932

Effect of Aging on Taut Rubber Diaphragms

As part of an investigation of special compositions of rubber suitable for use as diaphragms for aircraft instruments, six samples were used as taut diaphragms in instruments and allowed to age for five years. Two of the instruments were in operating condition after this period of time and one had remarkably little change in performance. In making the rubber tetraethyl thorium disulfide was employed as a vulcanizing agent.

Henrickson, H B↗