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At least 1,423 records · Page 79

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NASA NTRS1978

Ames interactive molecular model building system - A 3-D computer modelling system applied to the study of the origin of life

The investigation of specific interactions among biological molecules must take into consideration the stereochemistry of the structures. Thus, models of the molecules are essential for describing the spatial organization of potentially interacting groups, and estimations of conformation are required for a description of spatial organization. Both the function of visualizing molecules, and that of estimating conformation through calculations of energy, are part of the molecular modeling system described in the present paper. The potential uses of the system in investigating some aspects of the origin of life rest on the assumption that translation of conformation from genetic elements to catalytic elements would have been required for the development of the first replicating systems subject to the process of biological evolution.

Coeckelenbergh, Y.↗

NASA NTRS1978

Clast-laden nature and the origin of Luna 24 olivine-vitrophyres

The study shows that Luna 24 olivine-vitrophyric particles are actually fragments of clast-laden melt and are not from a very low titanium (VLT) mare basalt flow. Diversity of composition and clast population, and morphological similarity to ropy glasses suggest an impact melt origin. Compositionally, the olivine-vitrophyres resemble the high magnesian VLT basalts, which may have been the target and source of the impact melt.

Basu, A.↗

NASA NTRS1978

Petrology, chemistry, and chronology of 14078 - Chemical constraints on the origin of KREEP

Petrographic, chemical and isotopic similarities between 14078 and other Apollo-14 KREEP basalts suggest that these samples were derived from the same parent liquid and possibly from the same cooling unit. The liquid was probably generated via meteorite impact. Subtle differences are noted in the shapes of REE patterns of KREEP-rich samples from different landing sites; the origin of these differences is not well understood. Calculated Ti/Sm values in liquids parental to primitive cumulate samples are similar to values proposed for the whole moon.

Mckay, G. A.↗

NASA NTRS1978

Origin and history of the adcumulate eucrite, Moama as inferred from REE abundances, Sm-Nd and U-Pb systematics

The abundances of Sm-Nd and U-Pb isotopes and of rare earth elements (REE) in Moama cumulate eucrite were investigated. The Sm-Nd data plotted on a Nd-143/Nd-144 vs Sm-147/Nd-144 evolution diagram define a line with a slope corresponding to an age of 4.58 plus or minus 0.05 billion years and an initial Nd-143/Nd-144 ratio of about 0.50684. Moama was found to contain the lowest overall trivalent REE abundances, the most light REE-depleted abundance pattern, and largest positive Eu anomaly of any eucrite yet studied. The REE data indicate that Moama could have been derived by about 1 to 5% fractional crystallization from a liquid with REE contents similar to Juvinas, or about 20 to 30% fractional crystallization from a Sioux County-like liquid. The data are consistent with the hypothesis that the Moama, Serra de Mage, and Moore county cumulate meteorites could have been derived from the same parent liquid. Results of the U-Th-Pb study indicate that the majority of the Pb in Moama is of terrestrial origin, and suggest that the meteorite has undergone a minor thermal event causing re-equilibration of the U-Pb system.

Hamet, J.↗

NASA NTRS1979

Implications of the gas compositional measurements of Pioneer Venus for the origin of planetary atmospheres

Comparisons are made between the volatile inventories of the terrestrial planets, including Pioneer Venus data, and the predictions of three classes of theories for the origin of planetary atmospheres. Serious difficulties arise for the primary atmosphere and external source hypotheses. The grain accretion hypothesis can account for the trends in the volatile inventory from Venus to earth to Mars, if volatiles were incorporated into planet-forming grains at nearly the same temperature for all of these planets, but at systematically lower pressures in the regions of planet formation farther from the center of the solar nebula.

Pollack, J. B.↗

NASA NTRS1979

Magnetite in CI carbonaceous meteorites - Origin by aqueous activity on a planetesimal surface

The composition and morphology of magnetite in CI carbonaceous meteorites appear incompatible with a nebular origin. Mineralization on the meteorite parent body is a more plausible mode of formation. The iodine-xenon age of this material therefore dates an episode of secondary mineralization on a planetesimal rather than the epoch of condensation in the primitive solar nebula.

Kerridge, J. F.↗

NASA NTRS1979

Petrology and origin of the shergottite meteorites

The results of petrographic studies and 1 atmosphere melting experiments of Shergotty and Zagami shergottites, the origin and evolution of the shergottites, and the relationship of the shergottite parent body to the parent bodies of the other achondrites and to the earth are examined. Shergotty and Zagami differ in the proportions of cumulus pyroxenes and crystallized intercumulus liquid, but their compositions are similar. Shergottites can be distinguished from other achondrite groups by their mineralogies, crystallization sequences, and inferred source region compositions, but their source regions can be related to other achondrite types by addition or loss of volatile components. Shergottites and terrestrial basalts show similar oxidation states and minor element concentrations which may be accounted for by accretion of similar materials to produce the terrestrial upper mantle and the shergottite parent body, or by accretion of the earth's upper mantle from planetisimals similar to the shergottite parent body.

Stolper, E.↗

NASA NTRS1979

Origin of longitudinal grooving in Tiu Vallis, Mars - Isolation of responsible fluid-types

Fluid instability analysis developed for the growth or decay of large, longitudinal roller vortex perturbations imposed at the base of fluids of exponentially stratified viscosities is presented. The origin of longitudinal grooving and fluting in a region of converging flow in Tiu Vallis, Mars, is analyzed, and the propensity of fluids, such as water, mudflow/debris flow, and lava, to develop longitudinal roll patterns as part of their flow field is tested. It is found that the grooving pattern in Tiu Vallis was probably formed by vortex motion in a mudflow or debris-laden slurry, but exact representation of sediment/debris concentration or sediment transport machanism is not yet possible. It is concluded that longitudinal vortex motion modelled in this analysis can develop in water, lava, or ice, but that the specific flow characteristics of these models which include the vortex motion are unreasonable for flow in Tiu Vallis.

Thompson, D. E.↗

NASA NTRS1979

The composition and origin of metal in howardites

The metal compositions of howardite breccias are examined and the significance of variations in metal content to meteorite origins is discussed. It is shown that metals in howardites are found primarily in Ni-poor components, associated with primary clasts and similar in composition to pristine lunar anorthosite metal, and in Ni-rich grains, associated with glassy melt rocks and possibly derived from the projectiles which caused the melting. Variations in primary clast metal compositions among the howardite and the identities of matrix and clast metal contents within many howardites suggest that each howardite is derived from a distinct source region. Evidence of thermal metamorphism in Petersburg is interpreted as the results of deeper burial than in other howardites.

Hewins, R. H.↗

NASA NTRS1980

The origin and evolution of sulfur in an Archean volcano-sedimentary basin, Deer Lake area, Minnesota

Rocks of the Deer Lake area, northcentral Minnesota, consist of Archean (age greater than 2.6 billion years) metasediments and metavolcanics intruded by mafic layered sills. Geologic and sulfur isotopic data suggest that sulfides in the sediments are bacteriogenic, having formed in response to the activity of sulfate reducing bacteria during diagenesis. Deposition of the sediments appears to have occurred in a deep marine basin with restricted circulation of sea water. The bulk of the sulfur in the igneous rocks is of deep seated origin, but basal contacts of the sills show evidence of assimilation of biogenic sulfur from the intruded sediments. This assimilation of biogenic sulfur is the primary geochemical control of local Cu-Ni sulfide mineralization.

Nicol, D. L.↗

NASA NTRS1979

A possible origin for the grooves of Phobos

A hypothesis that the age and distribution of grooves on the surface of Phobos may be explained by a hybrid origin due to varying tidal stresses induced after a large impact altered the satellite's rotation rate is presented. The theory that these grooves are surface manifestations of internal fractures induced by the decay of Phobos orbit is ruled out because this mechanism would require the grooves to be extremely young features, but their implied age is greater than 10 to the 9th power yr. The location and relationship of grooves to the largest crater, Stickney, indicate that they are related to the Stickney impact, but were formed shortly after that event with orientations determined by the shape of Phobos. If the impact destroyed the condition of synchronous rotation, the changing orientation of Phobos in the gravitational field of Mars would subject its interior to stresses, producing the systems of grooves.

Weidenschilling, S. J.↗

NASA NTRS1979

Notes on the origin of the Trojan asteroids

The dynamic plausibility of various ideas on the origin of the Trojans is briefly discussed. We take the point of view that the present, tightly bound population has secularly evolved through some mechanism from less to more tightly bound orbit configurations. The mechanisms considered are changes in the Jovian mass or semimajor axis during planetary formation, collisional interactions with external, asteroidal material, and cometary outgassing.

Yoder, C. F.↗

NASA NTRS1979

Origin of cosmic magnetic fields

Reasons for describing cosmic hydromagnetic phenomena by introducing electric currents are summarized demonstrating that change from the traditional magnetic field description to a current description provides new aspects of cosmic electrodynamics. It is shown that the kink-instability of electric currents is the basic physical phenomenon responsible for magnetic flux generation in cosmic physics. A model which starts with a poloidal magnetic field is discussed; differential motions change the field configuration so that kinetic energy is transferred into magnetic energy of induced toroidal fields; the currents associated with these become unstable when the toroidal energy exceeds the poloidal energy resulting in an amplification of the original poloidal field.

Alfven, H.↗

NASA NTRS1979

Constraints on the origins of lunar magnetism from electron reflection measurements of surface magnetic fields

The paper describes a new method of detecting lunar surface magnetic fields, summarizes electron reflection measurements and correlations of surface field anomalies to moon geologic features, and discusses the constraints on the origin of lunar magnetism. Apollo 15 and 16 measurements of lunar surface magnetic fields by the electron reflection method show patches of strong surface fields distributed over the lunar surface, and a positive statistical correlation is found in lunar mare regions between the surface field strength and the geologic age of the surface. However, there is a lack of correlation of surface field with impact craters indicating that the mare does not have a strong large-scale uniform magnetization as may be expected from an ancient lunar dynamo. Fields were found in lunar highlands which imply that the rille has a strong magnetization associated with it as intrusive, magnetized rock or as a gap in a uniformly magnetic layer of rock.

Lin, R. P.↗

NASA NTRS1979

Armalcolite/ilmenite - Mineral chemistry, paragenesis, and origin of textures

In order to understand the origin of differing ilmenite/armalcolite textural relationships, bulk rock composition, oxygen fugacity and cooling rate have been experimentally investigated. Results show that armalcolite and ilmenite compositions are dependent upon the liquid composition and temperature at the time of crystallization. Furthermore, the different armalcolite/ilmenite textures rely to a greater extent on f sub O2 than on cooling rate or bulk rock composition, and the Ti(3+) content of armalcolite can be used as an indicator of the approximate f sub O2 prevailing at the time of crystallization.

Stanin, F. T.↗

NASA NTRS1979

Aubrites - Their origin and relationship to enstatite chondrites

A study of aubrites and their origin and relationship to enstatite chondrites is presented. Aubrites contain enstatite, plagioclase, diopside, and olivine; based on similarities in bulk chemistry of the silicate portions and inferred oxygen fugacity it is shown that E6 chondrites are the most likely parent material for chondrites. It is concluded that the addition of 10% plagioclase of An15 composition to the bulk composition of the silicate portion of the aubrites results in good agreement with that of E6 chondrites.

Watters, T. R.↗

NASA NTRS1980

Thermal history, chemical composition and relationship of comets to the origin of life

The role of thermal processes in determining the chemical composition of comets is considered, and implications of possible cometary constituents for the origin and evolution of life on earth are discussed. It is shown that the inclusion of short-lived Al-26 from a nearby supernova explosion into cometary nuclei could lead to comets with surfaces cool enough to retain H2O and interiors warm enough for thermal processing to occur, with the production of complex organic molecules such as amino acids and nucleic acid bases. It is thus suggested that comets may have played a part in seeding the primitive earth with biological polymers capable of self-replication or of evolving towards that capability, and may even be responsible for the subsequent introduction of organic material capable of infecting already existing cells.

Irvine, W. M.↗

NASA NTRS1979

Grooves on Phobos - Their distribution, morphology and possible origin

The global distribution, morphology, age, possible origin and significance of the long, linear depressions termed grooves on the surface of Phobos are discussed, based on Viking Orbiter data. The grooves, which consist of linear strings of coalesced and separate depressions in a loose regolith up to 100-200 m in depth, are observed to define the intersection of several sets of parallel planes with the surface of Phobos, with the widest and deepest grooves occurring just outside the rim of the 10-km crater, Stickney. The superposition of the grooves on older craters, crater density within the grooves and the intersections of groove sets suggest that the grooves are all of the same age, with their formation closely following that of Stickney. The evidence of groove morphology and distribution is used to attribute their formation to the enlargement of preexisting fractures or the formation of new fractures by the Stickney impact, causing the mobilization of the regolith along the fractures. The lack of observable grooves on Deimos is explained by the absence of a crater large enough to have severly fractured its surface.

Thomas, P.↗