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At least 1,801 records · Page 100
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Origin and Evolution of the Unusual Object 1996 PW: Asteroids from the Oort Cloud?

The unusual object 1996 PW was discovered on 1996 August 9 by the Near-Earth Asteroid Tracking automated search camera operating from Haleakala in Hawaii. Although asteroidal in appearance, it was soon determined that the object is in a near-parabolic orbit similar to that of a long-period comet. No object that was not an active comet has ever been discovered on such an eccentric orbit. The discovery of 1996 PW prompted us to examine and evaluate its possible origins, including the intriguing possibility that it is an asteroid from the Oort cloud. Current models for the formation of the Oort cloud argue that most of the material there should be from the Uranus-Neptune region and thus cometary, not asteroidal, in composition. We better quantify these models and show that approximately 1% of the Oort Cloud population should be asteroids. We find that 1996 PW has almost certainly been a resident of the Oort cloud. However, we find it equally likely that 1996 PW is an extinct comet or an asteroid. Although not conclusive, our results represent a significant change in our understanding of the Oort cloud, because they suggest that the ejection process sampled material from as close to the Sun as the asteroid belt in the primordial solar nebula and hence much warmer formation temperatures than previously thought. This diverse sample is preserved in the Oort cloud.

Weissman, Paul R.↗
NASA NTRS2001

The Origin of Monsoon Onset: Rotational ITCZ Attractors - Part 2

Through various specially designed numerical experiments with an aqua-planet general circulation model and theoretical arguments. Chao showed the existence of multiple quasi-equilibria of the intertropical convergence zone (ITCZ). He also showed that monsoon onset could be interpreted as an abrupt transition between the quasi-equilibria of the ITCZ. He further showed that the origin of these quasi-equilibria is related to two different types of attraction pulling the ITCZ in opposite directions. One type of attraction on the ITCZ is due to earth's rotation, which pulls the ITCZ toward the equator or two equatorial latitudes symmetric with respect to the equator depending on the choice of convection scheme, and the other due to the peak of the sea surface temperature (SST, which is given in the experiments a Gaussian profile in latitude and is uniform in longitude), which pulls the ITCZ toward a latitude just poleward of the SST peak. The strength of the attraction due to the earth's rotation has a highly nonlinear dependence on the latitude and that due to the SST peak has a linear (at least in a relative sense) dependence on the latitude.

Chao, Winston C.↗
NASA NTRS2000

From Interstellar PAHs and Ices to the Origin of Life

Tremendous strides have been made in our understanding of interstellar material over the past twenty years thanks to significant, parallel developments in observational astronomy and laboratory astrophysics. Twenty years ago the composition of interstellar dust was largely guessed at, the concept of ices in dense molecular clouds ignored, and the notion of large, abundant, gas phase, carbon rich molecules widespread throughout the interstellar medium (ISM) considered impossible. Today the composition of dust in the diffuse ISM is reasonably well constrained to micron-sized cold refractory materials comprised of amorphous and crystalline silicates mixed with an amorphous carbonaceous material containing aromatic structural units and short, branched aliphatic chains. In dense molecular clouds, the birthplace of stars and planets, these cold dust particles are coated with mixed molecular ices whose composition is very well constrained. Lastly, the signature of carbon-rich polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by earlier interstellar chemistry standards, is widespread throughout the Universe. The first part of this lecture will describe how infrared studies of interstellar space, combined with laboratory simulations, have revealed the composition of interstellar ices (the building blocks of comets) and the high abundance and nature of interstellar PAHs. The laboratory database has now enabled us to gain insight into the identities, concentrations, and physical state of many interstellar materials. Within a dense molecular cloud, and especially in the solar nebula during the star and planet formation stage, the materials frozen into interstellar/precometary ices are photoprocessed by ultraviolet light, producing more complex molecules. The remainder of the presentation will focus on the photochemical evolution of these materials and the possible role of these compounds on the early Earth. As these materials are thought to be the building blocks of comets and related to the carbonaceous components of micrometeorites, they are likely to have been important sources of complex organic materials on the early Earth and their composition may be related to the origin of life.

Allamandola, Louis J.↗
NASA NTRS2001

Origin of X-ray Emission from Planetary Nebulae

We proposed to use the entire ROSAT archive to search for observations of Galactic planetary nebulae, and analyze these observations, in order to determine the origin of X-ray emission from planetary nebulae. We have cross-correlated the ROSAT archive with a catalog of Galactic planetary nebulae and found useful ROSAT observations for approx. 80 planetary nebulae. These observations have been analyzed. X-ray emission is detected in 13 nebulae. The X-ray spectra and luminosities of these positive detections have been analyzed, and the 3-sigma upper limits are determined for the non-detections. The X-ray properties of planetary nebulae are correlated with the distance and extinction of the nebula, and the temperature and surface gravity of the central star, in order to explain the observed X-ray emission. The results are reported in a conference and a paper that has been published in the Astrophysical Journal Supplement. We conclude that this program has been successfully completed.

Chu, You-Hua↗
NASA NTRS2001

The Origin of Hot Subluminous Horizontal-Branch Stars in Omega Centauri and NGC 2808

Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch (EHB) are found in the ultraviolet color-magnitude diagrams of both omega Cen (DCruz et.al. 2000) and NGC 2808 (Brown et al. 2001). In order to investigate the origin of these subluminous stars, we have constructed a detailed set of evolutionary sequences that follow the evolution of low-mass stars continuously from the zero-age main sequence through the helium-core flash to the HB for a wide range in the mass loss along the red-giant branch (RGB). Stars with the largest mass loss evolve off the RGB to high effective temperatures before igniting helium in their cores (Castellani & Castellani 1993). Our results indicate that the location of the subluminous EHB stars, as well as the high temperature gap along the EHB of NGC 2808, can be explained if these stars undergo a late helium-core flash while descending the white-dwarf cooling curve. Under these conditions the convection zone produced by the main helium flash will penetrate into the stellar envelope, thereby mixing most, if not all, of the envelope hydrogen into the hot helium-burning interior, where it is rapidly consumed (Sweigart 1997). This phenomenon is analogous to the "born-again" scenario for producing hydrogen-deficient stars following a very late helium-shell flash. This "flash mixing" of the envelope during a late helium-core flash greatly enhances the envelope helium and carbon abundances and, as a result, leads to a discontinuous increase in the HB effective temperature. We argue that the hot HB gap observed in NGC 2808 is associated with this theoretically predicted dichotomy in the HB properties. Using new helium- and carbon-rich stellar atmospheres, we show that the changes in the envelope abundances due to flash mixing will suppress the ultraviolet flux in the spectra of hot EHB stars. We suggest that such changes in the emergent spectral energy distribution are primarily responsible for explaining the hot subluminous EHB stars in omega Cen and NGC 2808. Moreover, we demonstrate that models without flash mixing lie, at most, only approximately 0.1 mag below the EHB, and hence fail to explain the observations.

Sweigart, Allen V.↗
NASA NTRS2002

Near-Earth Asteroid Origin for the Farmington Meteorite

We report nitrogen and xenon isotopic signatures in separated metal and non-magnetic phases of a catastrophically degassed L5 chondrite and discuss implications for the collisional event and the impactor. Additional information is contained in the original extended abstract.

Marti, K.↗
NASA NTRS2002

Isotopic Constraints on the Origin of Lunar Ferroan Anorthosites

Ferroan anorthosites have whole rock Nd isotopic compositions that are too radiogenic to have crystallized from a chondritic magma ocean. Mechanisms to disturb the whole rock Sm-Nd isotopic compositions are explored. Additional information is contained in the original extended abstract.

Borg, L. E.↗
NASA NTRS2002

Circumstellar Oxide Grains in Ordinary Chondrites and Their Origin

Three new circumstellar hibonite and 14 circumstellar corundum grains have been found based on their highly anomalous oxygen. Al-Mg isotopes have been measured for these circumstellar oxides and for 14 oxides with a normal oxygen composition. Additional information is contained within the original extended abstract.

Krestina, Natalia↗
NASA NTRS2002

Age and Origin of the Crustal Dichotomy in Eastern Mars

The crustal dichotomy in eastern Mars is largely due to the very large impact which produced the Utopia Basin. Buried impact basins on the Utopia Basin constrain the Utopia impact (and therefore the dichotomy) to be very Early Noachian in age. Additional information is contained in the original extended abstract.

Frey, H. V.↗
NASA NTRS2002

Origin of Water on Mars

Dynamical simulations suggest that the Earth's water budget was delivered primarily from the asteroid belt, in the form of large planetary embryos. The same simulations present a very different picture for Mars its water came from a mixture of cometary and small asteroidal bodies. Additional information is contained in the original extended abstract.

Lunine, J. I.↗
NASA NTRS2002

Investigating the Origin of Th in Mare Basalts of the Western Procellarum Region

Clementine spectral reflectance and compositional data and Lunar Prospector gamma-ray data are used to map individual basalt flows in the western Procellarum and to investigate whether Th was inherent to the basalts or the result of surface contamination. Additional information is contained in the original extended abstract.

Flor, E. L.↗
NASA NTRS2002

Support for the 2001 Gordon Research Conference on the Origins of Solar Systems

This grant provided partial support for participants at the Gordon Research Conference on Origins of Solar Systems, held at Connecticut College, June 17 through June 22, 2001. This conference was chaired by the PI (David Stevenson) and Pat Cassell (NASA Ames) was Vice-Chair. In addition to the money provided by this grant ($15,000), the Gordon Research Conference organization provided about $22,000 (in large part from registration fees) and BPI provided $10,000 (through Director David Black); the latter targeted primarily for students and postdocs. Accordingly the grant money from NASA was used to cover the costs of registration and travel for about one half of the invited speakers (22 in total) and discussion leaders (about 10 in total).

Stevenson, David J.↗
NASA NTRS2001

From the Origins of Life to Intelligence: The Emergence of Symbolic Constructs

Intelligence cannot be understood - and cannot be 'artificially recreated' - without also understanding how it fits as a phenomenon in the evolution of matter. I say 'matter' instead of 'life' because my thesis is that the distinction between matter, with its associated interactions, and life, is simply in the complexity of these interactions and in the number of 'symbolic levels' that are defined by these interactions. Most of us think of symbols only in the context of language. This is understandable, since it is at this level where it is easiest to draw a distinction between natural objects and their 'names', i.e., the different sets of 'abstract' objects that can be manipulated to produce models of the 'real' world. Of course sets of abstract objects can also acquire names and be manipulated at higher and higher conceptual levels. When we use the words 'philosophy' or 'the Declaration of Independence' we use abstract constructs that will only make sense in other specific abstract contexts. Additional information is contained in the original extended abstract.

Colombano, Silvano P.↗
NASA NTRS2002

Origin, Structure, and Evolution of Grooved Terrain on Ganymede and Comparison to Europa

'Ridge and trough terrain' (RTT) is a generic term for sets of subparallel ridges and troughs observed on several icy outer planet satellites. Ganymede's grooved terrain is the best studied example of ridge and trough terrain. However, the >500 a resolution of Voyager images limited the conclusions that could be drawn regarding its detailed morphology and deformational style, as well as the relationship of bright grooved terrain to dark terrain on Ganymede. The goal of this study was to utilize Galileo high resolution images to examine and categorize the detailed local-scale morphology of ridge and trough terrains on Ganymede, in order to determine the morphological variety and deformational styles of the terrain, and its evolutionary sequence. This included comparison of the ridges and troughs that occur within dark terrain to those within bright terrain. This study addressed implications for the satellite's local-scale stress history, its lithospheric character, and its broader-scale geological history. Conclusions from the Ganymede were applied to gain insight into the origin of ridge and trough terrains on Europa. Attached are the abstracts of the seven papers and list of one popular press article and 13 abstracts which resulted from this study. No subject inventions are associated with this work.

Pappalardo, Robert T.↗
NASA NTRS2001

Chemical Constraints Governing the Origin of Metabolism: The Thermodynamic Landscape of Carbon Group Transformations

The thermodynamics of organic chemistry under mild aqueous conditions was examined in order to begin to understand its influence on the structure and operation of metabolism and its antecedents. Free energies were estimated for four types reactions of biochemical importance carbon-carbon bond cleavage and synthesis, hydrogen transfer between carbon groups, dehydration of alcohol groups, and aldo-keto isomerization. The energies were calculated for mainly aliphatic groups composed of carbon, hydrogen, and oxygen. The energy values showed that (1) when carbon-carbon bond cleavage involves two different types of functional groups, transfer of the shared electron-pair to the more reduced carbon group is energetically favored over transfer to the more oxidized carbon group, and (2) the energy of carbon-carbon bond transformation is strongly dependent on the type of functional group that donates the shared electron-pair during cleavage, and the group that accepts the shared electron-pair during synthesis, and (3) the energetics of C-C bond transformation is determined primarily by the half-reaction energies of the couples: carbonyl/carboxylic acid, carboxylic acid/carbon dioxide, alcohol/carbonyl, and hydrocarbon/alcohol. The energy of hydrogen-transfer between carbon groups was found to depend on the functional group class of both the hydrogen-donor and hydrogen-acceptor. From these and other observations we concluded that the chemistry of the origin of metabolism (and to a lesser degree modem metabolism) is strongly constrained by the (1) limited disproportionation energy of organic substrates that can be dissipated in a few irreversible reactions, (2) the energy-dominance of few half-reaction couples in carbon-carbon bond transformation that establishes whether a chemical reaction is energetically irreversible, reversible or unfeasible, and (3) the dependence of the transformation-energy on the oxidation state of carbon groups (functional group type) which is contingent on prior reactions in the synthetic pathway.

Weber, Arthur L.↗

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