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At least 1,729 records · Page 96
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NASA NTRS1993

The origins of planar magnetic structures in the solar wind

It is shown that the solar-wind settings of planar magnetic structures (PMSs) are generally inconsistent with their interpretation by Nakagawa et al. (1989) as magnetic loops or tongues resulting from newly emerging magnetic flux in the photosphere. It is suggested that PMS events have two origins: (1) compression and/or draping of preexisting magnetic structures on the leading edge of high-speed streams, especially ahead of coronal mass ejections; and (2) discontinuities near the heliospheric current sheet.

Neugebauer, M.↗
NASA NTRS1992

Origin of noble gases in the terrestrial planets

Current models of the origin of noble gases in the terrestrial planets are reviewed. Primary solar system volatile sources and processes are examined along with the current data base on noble gases and its applications to evolutionary processing. Models of atmospheric evolution by hydrodynamic escape are addressed.

Pepin, Robert O.↗
NASA NTRS1991

The nature, origin and evolution of embedded star clusters

The recent development of imaging infrared array cameras has enabled the first systematic studies of embedded protoclusters in the galaxy. Initial investigations suggest that rich embedded clusters are quite numerous and that a significant fraction of all stars formed in the galaxy may begin their lives in such stellar systems. These clusters contain extremely young stellar objects and are important laboratories for star formation research. However, observational and theoretical considerations suggest that most embedded clusters do not survive emergence from molecular clouds as bound clusters. Understanding the origin, nature, and evolution of embedded clusters requires understanding the intimate physical relation between embedded clusters and the dense molecular cloud cores from which they form.

Lada, Charles J.↗
NASA NTRS1993

Quantum mechanical effects of topological origin

Following a brief review of the original Casimir and Aharonov-Bohm effects, some other effects of similar natures are mentioned. A Casimir interaction between AB fluxes is presented. Possible realizations of the Casimir effects for massive charged fields in solid state structures and a new AB effect for photons are suggested.

Duru, I. H.↗
NASA NTRS1993

Using solar flare track densities to determine the origin of interplanetary dust particles

Sandford theoretically explored the use of solar flare track densities in interplanetary dust particles (IDP's) to distinguish whether they were of cometary or asteroidal origin. He determined that there were differences in the characteristic distributions of track densities that would occur from these two possible sources. Flynn examined the heating of IDP's on atmospheric entry and concluded that IDP's must be predominantly from asteroidal sources because these asteroidal particles would have sufficiently low velocities to survive atmospheric heating with little or no modification whereas cometary particles would ordinarily have velocities that are much higher. This paper looks at what we can learn by combining the approaches of Sandford and Flynn.

Blanford, George E.↗
NASA NTRS1993

Arroyo el Mimbral, Mexico, K/T unit: Origin as debris flow/turbidite, not a tsunami deposit

Coarse, spherule-bearing, elastic units have been discovered at 10 marine sites that span the K/T boundary in northeastern Mexico. We examined one of the best exposed sites in Arroyo el Mimbral, northwest of Tampico. The Mimbral outcrop displays a layered elastic unit up to 3 m thick enclosed by marly limestones of the Mendez (Latest Maastrichian) and Velasco (Earliest Danian) Formations. At its thickest point, this channelized elastic unit is comprised of 3 subunits: (1) a basal, poorly-sorted, ungraded calcareous spherule bed 1 m thick containing relict impact glass and shocked mineral grains, (2) a massive set of laminated calcite-cemented sandstones up to 2 m thick with plant debris at its base, (3) capped by a thin (up to 20 cm) set of rippled sandstone layers separated by silty mudstone drapes containing a small (921 pg/g) iridium anomaly. This tripartite elastic unit is conformably overlain by marls of the Velasco Formation. We also visited the La Lajilla site east of Ciudad Victoria; its stratigraphy is similar to Mimbral's, but its elastic beds are thinner and less extensive laterally. The Mimbral elastic unit has been interpreted previously as being deposited by a megawave or tsunami produced by an asteroid impact on nearby Yucatan (Chicxulub crater). However, a presumed 400-m paleodepth of water at the Mimbral site, channeling of the spherule subunit into the underlying Mendez Formation marls, and the overtopping of the basal, spherule-bearing subunit by the laminated sandstone subunit, all suggest a combined debris flow/turbidite origin for this elastic unit similar to that proposed for Upper Pleistocene sand/silt beds occurring elsewhere in the Gulf of Mexico. In this latter model, the sediment source region for the elastic unit is the lower continental shelf and slope escarpment. For the K/T unit at Mimbral, we propose that thick ejecta blanket deposits composed mostly of spherules were rapidly loaded onto the lower shelf and slope from an impact-generated ejecta curtain.

Bohor, Bruce F.↗
NASA NTRS1993

On the origin of (4)He and (40)Ar in natural gold

In a first report on our investigations of noble gases in native gold we demonstrated that placer gold contains an excess of radiogenic (4)He and (40)Ar relative to the concentrations expected from in situ decay of U, Th, and K, respectively, during the geologic age of about 30 Ma of the samples. We also showed that the U/Th-(4)He age of 36 Ma of vein-type gold from the Southern Alps agrees with its K-Ar formation age derived from associated muscovite and biotite. We now studied the question of the origin of the (4)He and (40)Ar excesses of placer gold. We conclude that gold contains two components of noble gases, a low-temperature component from fluid inclusions or phases which release noble gases at less than 800 C and a high-temperature component released when gold melts (1064 C). In some samples extremely high U and K concentrations or an unreasonably high formation age would be required to explain the observed (4)He abundances. Thus, trapped (4)He and (40)Ar must be present in gold.

Eugster, O.↗
NASA NTRS1993

Origin of the Vredefort structure, South Africa: Impact model

A model is presented for the evolution of the Vredefort structure, based on reasoned constraints on the original size of the Vredefort structure from observational data and comparison with other terrestrial impact craters. The models for complex craters (ring and multi-ring basins) of Croft, Grieve, and co-workers, and Schultz and co-workers, were used to reconstruct the Vredefort impact event, using a final crater diameter of 300 km, as estimated by Therriault. The sequence of events (stages 2-5) is illustrated diagramatically. The stages are: initial penetration, excavation and compression, dynamic rebound and uplift, maximum radial growth and collapse, and final crater form.

Therriault, A. M.↗
NASA NTRS1993

The Martian valley networks: Origin by niveo-fluvial processes

The valley networks may hold the key to unlocking the paleoclimatic history of Mars. These enigmatic landforms may be regarded as the Martian equivalent of the Rosetta Stone. Therefore, a more thorough understanding of their origin and evolution is required. However, there is still no consensus among investigators regarding the formation (runoff vs. sapping) of these features. Recent climatic modeling precludes warm (0 degrees C) globally averaged surface temperatures prior to 2 b.y. when solar luminosity was 25-30 percent less than present levels. This paper advocates snowmelt as the dominant process responsible for the formation of the dendritic valley networks. Evidence for Martian snowfall and subsequent melt has been discussed in previous studies.

Rice, J. W., Jr.↗
NASA NTRS1994

Photospheric origins of chromospheric and coronal activity

This contract is for a two-year research study of the origins of activity in the upper atmosphere of the sun. The approach is to collect high resolution images of the lower atmosphere on observing runs at the Swedish Solar Observatory on La Palma, Canary Islands, Spain. The best observations are analyzed and compared with data from other telescopes and/or theoretical models, to study magnetic flux emergence, coronal heating, and various dynamic phenomena and transients. Software for analysis and visualization of the data is developed as needed. The contract is being performed by the Solar and Astrophysics Laboratory, part of the Lockheed Palo Alto Research Laboratory (LPARL) of the Research and Development Division (RDD) of Lockheed Missiles and Space Co., Inc. (LMSC).

Tarbell, Theodore↗
NASA NTRS1993

Temporal changes in the geographic distribution, elevation, and potential origin of the Martian outflow channels

Observational evidence of outflow channel activity on Mars suggests that water was abundant in the planet's early crust. However, with the decline in the planet's internal heat flow, a freezing front developed within the regolith that propagated downward with time and acted as a thermodynamic sink for crustal H2O. One result of this thermal evolution is that, if the initial inventory of water on Mars was small, the cryosphere may have grown to the point where all the available water was taken up as ground ice. Alternatively, if the inventory of H2O exceeds the current pore volume of the cryosphere, then Mars has always possessed extensive bodies of subpermafrost groundwater. We have investigated the relative age, geographic distribution, elevation, and geologic setting of the outflow channels in an effort to accomplish the following: (1) identify possible modes of origin and evolutionary trends in their formation; (2) gain evidence regarding the duration and spatial distribution of groundwater in the crust; and (3) better constraint estimates of the planetary inventory of H2O.

Tribe, S.↗
NASA NTRS1994

Absolute calibration and atmospheric versus mineralogic origin of absorption features in 2.0 to 2.5 micron Mars spectra obtained during 1993

We obtained new high resolution reflectance spectra of Mars during the 1993 opposition from Mauna Kea Observatory using the UKIRT CGS4 spectrometer. Fifty spectra of 1600-2000 km surface regions and a number of standard star spectra were obtained in the 2.04 to 2.44 micron wavelength region on 4 February 1993 UT. Near-simultaneous observations of bright standard stars were used to perform terrestrial atmospheric corrections and an absolute flux calibration. Using the known magnitude of the stars and assuming blackbody continuum behavior, the flux from Mars could be derived. A radiative transfer model and the HITRAN spectral line data base were used to compute atmospheric transmission spectra for Mars and the Earth in order to simulate the contributions of these atmospheres to our observed data. Also, we examined the ATMOS solar spectrum in the near-IR to try to identify absorption features in the spectrum of the Sun that could be misinterpreted as Mars features. Eleven absorption features were detected in our Mars spectra. Our data provide no conclusive identification of the mineralogy responsible for the absorption features we detected. However, examination of terrestrial spectral libraries and previous high spectral resolution mineral studies indicates that the most likely origin of these features is either CO3(sup 2-), HCO3(-), or HSO4(-) anions in framework silicates or possibly (Fe, Mg)-OH bonds in sheet silicates.

Bell, James F., III↗
NASA NTRS1994

Cr, Mn, and Ca distributions for olivine in angritic systems: Constraints on the origins of Cr-rich and Ca-poor core olivine in angrite LEW87051

Angrite meteorites are a type of basaltic achondrites that are noted for their very old cyrstallization ages (4.55 b.y.) and unusual chemical and mineralogical properties. In spite of great interest, only four angrites have been found. LEW87051 is the smallest one which weighs 0.6 g. It is a porphyritic rock with coarse subhedral to euhedral olivines set in a fine-grained groundmass which clearly represents a crystallized melt. The largest uncertainty about the petrogenesis of LEW87051 is the relationship between the large olivine crystals and the groundmass. Prinz et al. suggests that olivines are xenocrysts, while McKay et al. proposed a fractional cyrstallization model based on experimental studies. However, the crystals have Cr-rich and Ca-poor cores which do not match experimental olivines. Although Jurewicz and McKay tried to explaine the zoning of the rim by diffusion, some features are not explained. There also exists a definite composition boundary of Fe(2+) and MnO between the core and the rim. To clarify the origin of these olivines, we have performed experiments using LEW87051 analogs to measure the effects of oxygen fugacity on distribution coefficients of various elements in an angritic system.

Mikouchi, T.↗
NASA NTRS1994

Origins of energetic ions in the Earth's magnetosheath

This final report describes activities under NASA contract NAS5-31213 to Lockheed Missiles and Space Company. The report covers the entire contract period from 8 May 1991 to 7 Jun. 1994. This is a contract under the NASA Guest Investigator Program for the analysis and interpretation of the combined scientific data from the Hot Plasma Composition Experiment (HPCE) and the Charge Energy Mass (CHEM) spectrometer on the AMPTE/Charge Composition Explorer (CCE) spacecraft. These combined data sets have been used to survey the energetic ion environment in the earth's magnetosheath to determine the origins and relative strengths of the energetic ion populations found there.

Source record↗
NASA NTRS1993

Indications for common origin and gravitational interaction in three binary LMC clusters

Three close pair clusters of the LMC, NGC 2006/SL 538, NGC 2011 a/b, and NGC 2042 a/b, have been studied in order to establish their binarity. The stellar content in the outer region of each cluster has been investigated by means of low resolution objective UK Schmidt prism spectra, the cores of the clusters have been examined using low resolution integrated IUE spectra, whereas their density profiles and their observed dynamical parameters have been derived by means of star counts. The integrated spectra of their cores and the stellar content of their outer cluster regions have shown a common origin and a very young age (less than 2 x 10 exp 7 yr) for each member of the pairs, whereas their dynamical study has shown that they are gravitationally interacting. Comparing the age of their stellar content with their dynamical and relaxation times, it has been found that these clusters are physically associated and had no time to relax by stellar encounters, no time to merge, and no time to be destroyed by dynamical friction.

Kontizas, E.↗
NASA NTRS1994

Differences between recalculated and original Dobson total ozone data from Hradec Kralove, Czechoslovakia, 1962-1990

Backward reevaluation of long-term total ozone measurements from the Solar and Ozone Observatory of Czech Hydrometeorological Institute at Hradec Kralove, Czechoslovakia, was performed for the period 1962-1990. The homogenization was carried out with respect to the calibration level of the World Primary Standard Spectrophotometer No. 83 - WPSS by means of day-by-day recalculations of more than 25,000 individual measurements using the R-N tables reconstructed after international comparisons and regular standard lamp tests of the Dobson spectrophotometer No. 74. The results showed significant differences among the recalculated data and those original ones published in the bulletins Ozone Data for the World. In the period 1962-1979 they reached 10-19 D.U. (3.0-5.5%) for annual averages and even 26 D.U. (7.0%) for monthly averages of total ozone. Such differences exceed several times accuracy of measuring and can significantly influence character of trends of total ozone in Central Europe. Therefore the results from Hradec Kralove support the calls for reevaluation of all historical Dobson total ozone data sets at individual stations of Global Ozone Observing System.

Vanicek, Karel↗
NASA NTRS1994

Crystallization trends of precursor pyroxene in ordinary chondrites: Implications for igneous origin of precursor

Various observations suggest that the precursor of the fine fragments and the relict pyroxene was formed by an igneous process in a grandparent body of the chondrite. Furthermore, the fact that the precursors appear as fragments in the matrix as well as relicts in the chondrule suggests a shock origin for the chondrite by collision of two or more bodies, which had already to some extent undergone igneous differentiation.

Fujita, T.↗
NASA NTRS1994

Experimental constraints on models for origins of chondrules: Peak temperatures

Chondrule peak temperature constraints for mathematical models pertaining to chondrule origin are discussed in this article. The peak temperatures experienced by chondrules provide a parameter that may allow discrimination between alternative heating mechanisms. Successful mechanisms must provide just enough energy to reach such temperatures and not overshoot them. In principle, determination of peak temperatures from chondrule textures and experiments that duplicate them should be very easy. However, this approach is complicated by the fact that there are several different experimental paths that reproduce chondrule properties. The key texture depends not only on the peak temperature but also on the heating time and, in the case of totally molten droplets, on interactions with other particles. Addressed here are different types of peak temperature constraints; (1) near-liquidus melting; (2) flash melting; (3) dust seeding; (4) volatile element constraints; and (5) barred olivine chondrules.

Hewins, R. H.↗