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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 217 records · Page 12
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NASA NTRS1985

Geophysical interpretation of Venus gravity data

The investigation of the subsurface mass distribution of Venus through the analysis of the data from Pioneer Venus Orbiter (PVO) is presented. The Doppler tracking data was used to map the gravitational potential, which was compared to the topographic data from the PVO radar (ORAD). In order to obtain an unbiased comparison, the topography obtained from the PVO-ORAD was filtered to introduce distortions which are the same as those of our gravity models. The last major software package that was required in order to determine the spectral admittance Z (lambda) was used. This package solves the forward problem: given the topography and its density, and assuming no compensation, find the resulting spacecraft acceleration along a given nominal trajectory. The filtered topography is obtained by processing these accelerations in the same way (i.e., with the same geophysical inverter) as the Doppler-rate data that we use to estimate the gravity maps.

Reasenberg, R. D.↗
NASA NTRS1985

Was core formation violent enough to homogenize the early mantle?

The dynamics of iron, its thermal state and its phase in the accreting Earth probably played a major role in the Earth's early thermal evolution. Plausible impact thermal histories make it possible that pure iron was molten in the accreting Earth after it was about 10% grown. Hence, iron eutectic alloys (FeS, FeO) certainly were. Additionally, the initial temperature of the core is an important constraint on the secular cooling of the early Earth and on the strength of the early geodynamo. Whether iron is solid or molten would influence geochemical equilibria in the upper and lower mantle; the mode of core formation, by spherical or near-spherical blobs, stalk-like instabilities, or something more catastrophic would influence the partitioning of siderophiles between silicate and iron phases. Early descent of iron (during accretion) favors partitioning according to low-pressure phase equilibria, whereas late descent favors higher pressure. The later core formation occurs, the greater the heat pulse, due to the strong dependence of gravitational potential energy on planetary radius. The heat may homogenize the mantle if core formation is global; otherwise, heterogeneity of iron differentiation may leave some of the pre-archean mantle unaffected. The larger the chunks of proto-core (and hence smaller surface/volume ratios) the greater the heterogeneity.

Cooperman, S. A.↗
NASA NTRS1985

Hot coronae around early-type galaxies

The analysis of the X-ray emission from a sample of 55 bright early-type galaxies shows that hot gaseous coronae are a common and perhaps ubiquitous feature of such systems. The X-ray emission can be explained most naturally as thermal bremsstrahlung from hot gas which may be accumulated from mass loss during normal stellar evolution. The presence of these coronae shows that matter previously thought to be expelled in a galactic wind is instead stored in a hot galactic corona which may be heated and powered by supernova explosions. Perhaps the single most important feature of these coronae is that they provide a unique tracer of the gravitational potential in the outer regions of bright early-type galaxies. In this paper the X-ray properties of these coronae and their implications for the presence of massive dark halos around individual early-type galaxies are discussed. Total masses of early-type galaxies up to 5 trillion solar masses are found.

Forman, W.↗
NASA NTRS1986

On transient rheology and glacial isostasy

The effect of transient creep on the inference of long-term mantle viscosity is investigated using theoretical predictions from self-gravitating, layered earth models with Maxwell, Burgers' body, and standard linear solid rheologies. The interaction between transient and steady-state rheologies is studied. The responses of the standard linear solid and Burgers' body models to transient creep in the entire mantle, and of the Burgers' body and Maxwell models to creep in the lower mantle are described. The models' responses are examined in terms of the surface displacement, free air gravity anomaly, wander of the rotation pole, and the secular variation of the degree 2 zonal coefficient of the earth's gravitational potential field. The data reveal that transient creep cannot operate throughout the entire mantle.

Yuen, David A.↗
NASA NTRS1988

Comparisons of global topographic/isostatic models to the Earth's observed gravity field

The Earth's gravitational potential, as described by a spherical harmonic expansion to degree 180, was compared to the potential implied by the topography and its isostatic compensation using five different hypothesis. Initially, series expressions for the Airy/Heiskanen topographic isostatic model were developed to the third order in terms of (h/R), where h is equivalent rock topography and R is a mean Earth radius. Using actual topographic developments for the Earth, it was found that the second and third terms of the expansion contributed 30 and 3 percents, of the first of the expansion. With these new equations it is possible to compute depths (D) of compensation, by degree, using 3 different criteria. The results show that the average depth implied by criterion I is 60 km while it is about 33 km for criteria 2 and 3 with smaller compensation depths at the higher degrees. Another model examined was related to the Vening-Meinesz regional hypothesis implemented in the spectral domain. Finally, oceanic and continental response functions were derived for the global data sets and comparisons made to locally determined values.

Rummel, Reiner↗
NASA NTRS1988

Mantle rheology and satellite signatures from present-day glacial forcings

Changes in the long-wavelength region of the earth's gravity field resulting from both present-day glacial discharges and the possible growth of the Antarctic ice sheet are considered. Significant differences in the responses between the Maxell and Burger body rheologies are found for time spans of less than 100 years. The quantitative model for predicting the secular variations of the gravitational potential, and means for incorporating glacial forcings, are described. Results are given for the excitation of the degree two harmonics. It is suggested that detailed satellite monitoring of present-day ice movements in conjunction with geodetic satellite missions may provide a reasonable alternative for the esimation of deep mantle viscosity.

Sabadini, Roberto↗
NASA NTRS1988

The evolution of cooling flows. II - Galaxies and galaxy formation

Self-similar solutions are presented for the idealized case of a spherical galaxy with a singular isothermal gravitational potential where the ISM is built up from mass loss by stars or by accretion of external gas. For the former case, where a galaxy initially devoid of gas obtains gas from stellar evolution, steady flow only is obtained inside the cooling radius. Several possibilities occur and are discussed for flow outside the cooling radius. For the case involving infall of external gas, the nature of the flow depends on the initial density of the infalling gas. At high density, only a cold, unshocked flow is possible. At low density, the flow is accelerated inward, is heated in a shock front, approaches a hydrostatic state, passes through the cooling radius and enters a steady subsonic cooling flow, and finally passes through a sonic point. The effects of radiative cooling are discussed.

Chevalier, Roger A.↗
NASA NTRS1988

Molecular tori in Seyfert galaxies - Feeding the monster and hiding it

The principal properties of the tori of gas which surround Seyfert nuclei are discussed. The internal state of the clouds and their size distribution function are examined, and it is shown that the Jeans mass scale results in clouds which are individually sufficiently opaque to block out the nucleus, and that the balance of processes which controls their size distribution function also forces the covering factor to be of the order of or greater than unity. Where the gravitational potential is dominated by stars, cloud-cloud collisions keep the molecular clouds close to the equatorial plane. Stirring by stellar processes is never strong enough to compete with collisional losses. The position of the inner edge of the torus is determined by a balance between the inward flow of clouds and the rate at which the nuclear continuum can evaporate them.

Krolik, Julian H.↗
NASA NTRS1988

An accurate and efficient satellite long-term orbit predictor employing 'fictitious' mean orbital elements

By using Von Zeipel's generating function procedure the perturbing earth gravitational potential is averaged with respect to the fast variable (mean anomaly) and a set of 'fictitous' mean orbital elements which can be used as a long-term satellite orbit predictor is obtained. The set of elements is shown to be a function of the nonlinear square of the second zonal harmonic coefficient. It is found that the long-term orbit prediction using the 'fictitous' mean elements is as accurate as that using the osculating elements, but has a computing speed about two orders of magnitude faster. For short-term orbit predictions, the osculating elements approach must be used.

Tang, Charles C. H.↗
NASA NTRS1988

Peculiar velocities of cD galaxies - MX spectroscopy of Abell 1795

Spectroscopic observations of galaxies in the Abell 1795 field have been obtained using the MX multiple-object spectrograph on the Steward Observatory 2.3 m telescope. Redshifts are presented for 46 galaxies, including 41 cluster members. It is found that the A1795 cD galaxy is not at rest in the cluster gravitational potential well; it has a peculiar radial velocity, cz, of 365 km/s, and the hypothesis that the mean cluster velocity is as large as the cD's velocity can be rejected at the 99.5 percent confidence level. This conclusion is supported by spectroscopic data for the 'cooling flow' gas found in the central region of the cluster; this gas, except for the portion coincident with the cD nucleus, lies at the velocity derived for the cluster mean. It is suggested that current models of the formation of cD galaxies are unlikely to account for the large peculiar velocities of the cD galaxies in A1795 and A2670 unless substantial subclustering is still present. However, the available data show no evidence for velocity subclustering in either A1795 or A2670.

Hill, John M.↗
NASA NTRS1988

Driving forces: Slab subduction and mantle convection

Mantle convection is the mechanism ultimately responsible for most geological activity at Earth's surface. To zeroth order, the lithosphere is the cold outer thermal boundary layer of the convecting mantle. Subduction of cold dense lithosphere provides tha major source of negative buoyancy driving mantle convection and, hence, surface tectonics. There are, however, importnat differences between plate tectonics and the more familiar convecting systems observed in the laboratory. Most important, the temperature dependence of the effective viscosity of mantle rocks makes the thermal boundary layer mechanically strong, leading to nearly rigid plates. This strength stabilizes the cold boundary layer against small amplitude perturbations and allows it to store substantial gravitational potential energy. Paradoxically, through going faults at subduction zones make the lithosphere there locally weak, allowing rapid convergence, unlike what is observed in laboratory experiments using fluids with temperature dependent viscosities. This bimodal strength distribution of the lithosphere distinguishes plate tectonics from simple convection experiments. In addition, Earth has a buoyant, relatively weak layer (the crust) occupying the upper part of the thermal boundary layer. Phase changes lead to extra sources of heat and bouyancy. These phenomena lead to observed richness of behavior of the plate tectonic style of mantle convection.

Hager, Bradford H.↗
NASA NTRS1989

Direct numerical simulations of turbulent convection with a variable gravity and Keplerian rotation

Thermal convection was proposed as a possible mechanism for generation and maintenance of turbulence in the inner accretion disk regime of the primordial solar nebula. It is of fundamental interest to design experiments with the basic physical features of the solar nebula conditions cannot be produced in the laboratory, numerical simulations of hydrodynamic flows, which have been very successful in describing aerodynamic flows, can be suitable modified to provide experimental data for solar nebula modelling. The goals are to modify an extant, proven hydrodynamics code with the most important features of the solar nebula and other thin accretion disks: bouyancy terms to generate convection, internal heating representing the release of gravitational potential energy, a variable gravity linearly proportional the the distance from the vertical midplane due to centrifugal balance, rapid rotation with axis aligned with gravity, and Keplerian rotational shear; to determine the effect that these features have on the turbulent convection by introducing them individually and to determine the cumulative nature of the turbulent convection for accretion disk conditions; and to model the convection and the turbulence. In this manner, prior solar nebula models can be tested and their deficiencies rectified.

Cabot, William H.↗
NASA NTRS1989

Light propagation and the distance-redshift relation in a realistic inhomogeneous universe

The propagation of light rays in a clumpy universe constructed by cosmological version of the post-Newtonian approximation was investigated. It is shown that linear approximation to the propagation equations is valid in the region where zeta is approximately less than 1 even if the density contrast is much larger than unity. Based on a gerneral order-of-magnitude statistical consideration, it is argued that the linear approximation is still valid where zeta is approximately greater than 1. A general formula for the distance-redshift relation in a clumpy universe is given. An explicit expression is derived for a simplified situation in which the effect of the gravitational potential of inhomogeneities dominates. In the light of the derived relation, the validity of the Dyer-Roeder distance is discussed. Also, statistical properties of light rays are investigated for a simple model of an inhomogeneous universe. The result of this example supports the validity of the linear approximation.

Futamase, Toshifumi↗
NASA NTRS1988

Tilted-ring models of the prolate spiral galaxies NGC 5033 and 5055

Observations of the kinematics of H I in the disks of spiral galaxies have shown that isovelocity contours often exhibit a twisted pattern. The shape of a galaxy's gravitational potential well (whether due to luminous matter or dark matter) can be determined from the direction of the twist. If this twist is a manifestation of the precession of a nonsteady-state disk, it is shown that the twists of NGC 5033 and 5055 imply an overall prolate shape, with the major axis of the potential well aligned along the rotation axis of the disk. Therefore, the luminous disks of these galaxies must be embedded in dark halos that are prolate spheroids or prolatelike triaxial figures.

Christodoulou, Dimitris M.↗
NASA NTRS1988

Cluster influences on the internal dynamics of a galaxy

As part of a study of cluster influences, an attempt is made to map out damage to a galaxy under several different kinds of buffeting a galaxy suffers as it sweeps along its orbit through a cluster. It is shown that a cluster's observational characteristics are determined by the shape of its gravitational potential. It is noted the model galaxy must have full freedom to do whatever the physical galaxy wants to do.

Miller, R. H.↗
NASA NTRS1989

A combined optical/X-ray study of the Galaxy cluster Abell 2256

The dynamics of Abell 2256 is investigated by combining X-ray observations of the intracluster gas with optical observations of the galaxy distribution and kinematics. Magnitudes and positions are presented for 172 galaxies and new redshifts for 75. Abell 2256 is similar to the Coma Cluster in its X-ray luminosity, mass, and galaxy density. Both the X-ray surface brightness and the galaxy surface density distributions exhibit an elliptical morphology. The radial galaxy distribution is steeper than the density profile of the X-ray-emitting gas, yet the galaxy velocity dispersion is higher than the equivalent value for the gas. Under the simplest assumptions that the galaxy velocity distribution is isotropic and the gas is isothermal, the galaxies and gas cannot be in hydrostatic equilibrium in a common gravitational potential. Models consistent with available data have mass-to-light ratios which increase with radius and galaxy orbits that are anisotropic with a radial bias.

Fabricant, Daniel G.↗
NASA NTRS1989

Discovery of CO emission from NGC 1275

CO radio emission has been discovered in both the J=1-0 and J=2-1 rotational lines from the elliptical galaxy NGC 1275. The CO, which may condense from the cooling flow by means of which mass is thought to be accumulating, exhibits such a small velocity dispersion that it may collapse into stars before it virializes in the gravitational potential of this galaxy. If the star formation rate is as high as presently calculated, the initial mass function will be weighted toward the production of low-mass stars.

Lazareff, B.↗
NASA NTRS1989

Dynamical models of highly flattened oblate elliptical galaxies with De Vaucouleurs' surface-brightness profiles

Two surveys of dynamical models of E6 elliptical galaxies were constructed with de Vaucouleurs'surface-brightness distributions. One set of models has a constant mass-to-light ratio. The other set consists of models embedded in oblate isothermal gravitational potentials. In both sets, the observed axis ratio of the models is 0.55 when viewed edge-on. When viewed edge-on, it is possible to construct models in either potential that have velocity-dispersion profiles that increase or decrease with radius along either or both principal axes. The observational data seem to indicate that dispersion profiles decrease or remain constant as function of radius. The most surprising result of this work is that in order for a model to have velocity-dispersion profiles that decrease with radius out to 1.5 r(e) along both principal axes, it must be dominated by fairly thin-walled tube orbits at large radii. This is very different from the spherical case, where radial orbits dominate at large radii.

Fillmore, James A.↗