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At least 703 records · Page 39
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NASA NTRS1993

The history of mare volcanism in the Orientale Basin: Mare deposit ages, compositions and morphologies

The eruptive history of mare basalts in the Orientale Basin has been studied, using Lunar Orbiter 4 high-resolution photographs, Zond 8 photographs, and recently acquired Galileo EM-1 multispectral images. This work represents a refined set of compositional data incorporating the use of a linear mixing model for mare compositions, crater count data, and a comprehensive morphologic analysis of Orientale Basin mare deposits. Evidence for multiple eruptive episodes has been found, with compositions ranging from medium- to high-Ti basalt (less than 4 to greater than 6 wt. percent TiO2). Eruptive styles included flood, rille-forming, and shield-forming eruptions. Impact crater densities of mare units in the Orientale Basin enable determination of the ages of these deposits, using the method of Neukum et al. Earliest eruptions of mare basalt in the basin occurred at greater than or equal to 3.80 Ga and the latest eruptions occurred at about 2.3-2.5 Ga. Hence, mare volcanism occurred over a period of nearly 1.5 Ga.

Kadel, S. D.↗
NASA NTRS1993

Complex radiation-thermal history of Kaidun meteorite on data of track study of silicate minerals

The results of track study of approximately 80 individual silicate mineral crystals (ol, px, plag) picked out from Kaidun meteorite are presented. A wide range of observed rho(sub VH) value distributions indicate the complex irradiation history of Kaidun minerals. In one anortite crystal having two track groups with different parameters the pre-accretion irradiation traces were observed in all probability.

Kashkarov, L. L.↗
NASA NTRS1993

More on the cooling history of angrite LEW 86010

Antarctic angrite LEW 86010 has many chemical and mineralogical characteristics which suggest it is closely related to Angra dos Reis (ADOR). However, these meteorites have had very different thermal histories. Olivines and pyroxenes in ADOR are nearly homogeneous, suggesting very slow cooling or extensive subsolidus equilibration. In contrast, LEW 86010 pyroxenes are extensively zoned in both major and trace elements, while olivines are nearly homogeneous, suggesting that cooling of this sample was slow enough to homogenize olivines, but too fast to homogenize pyroxenes. In an earlier study, we used Ca diffusion gradients associated with exsolution lamellae of kirsh-steinite in LEW 86010 olivines to estimate the cooling rate of this sample. We obtained cooling rates of a few x.01 deg C/yr, corresponding to a burial depth of approximately 10(exp 2) m under solid rock (thermal diffusivity = 0.004 sq cm/s), or approximately m under regolith material (10(exp -5) sq cm/s). Unfortunately, this calculation was very sensitive to the assumed temperature of initial exsolution, which we approximated from the estimated initial olivine bulk Ca content, and also to the extrapolated values for the Ca diffusion coefficient (D sub Ca). This abstract reports new cooling rate estimates obtained using a refined approach that includes the initial temperature of exsolution as one of the fitted parameters. The purpose of our new calculations is (1) to reduce uncertainties in cooling rate due to uncertainties in bulk olivine Ca content and initial exsolution temperature, (2) to investigate the effects of using different values for D sub Ca, and (3) to determine how well constrained the cooling rate is for any particular Ca diffusion coefficient.

Mckay, G.↗
NASA NTRS1993

Zaoyang chondrite cooling history from pyroxene Fe(2+)-Mg intracrystalline ordering and exolutions

The Zaoyang ordinary chondrite fell as a single 14.15-kg mass in Hubey province (China) in October 1984 and was classified as a non-brecciated H5 chondrite, shock facies b. Cooling rate in pyroxenes can be calculated down to about 1000 C by using fine textures and at still lower temperatures (700 to 200 C) by intracrystalline ordering processes. The crystal chemistry of clinopyroxene and orthopyroxene from the matrix of the H5 Zaoyang chondrite has been investigated by X-ray structure refinement and detailed microprobe analysis. By comparison with terrestrial pyroxenes cell and polyhedral volumes in clino- and orthopyroxenes show a low crystallization pressure. Fe(2+) and Mg are rather disordered in M1 and M2 sites of clino- and orthopyroxenes; the closure temperatures of the exchange reaction are 600 and 512 C respectively, which is consistent with a quite fast cooling rate, estimated of the order of one degree per day. The closure temperature for the intercrystalline Ca-Mg exchange reaction for clino- and orthopyroxene showing clinopyroxene lamellae about 10 microns thick. Kinetic evaluations based on the thickness of exolved lamellae give a cooling rate of not more than a few degrees per 10(exp 4) years. The different cooling rates obtained from Fe(2+)-Mg intracrystalline partitioning and exolution lamellae suggest an initial episode of slow cooling at 900 C, followed by faster cooling at temperatures of 600-500 C at low pressure conditions. The most probable scenario of the meteorite history seems that the exolved orthopyroxene entered the parental chondrite body after exolution had taken place at high temperature. Subsequent fast cooling occurred at low temperature after the formation of the body.

Molin, G. M.↗
NASA NTRS1993

Focus history of the Hubble Space Telescope: Launch to May 1993

Since the launch of the Hubble Space Telescope (HST) the secondary mirror of the telescope has been moved several times in order to collimate the telescope and also to define a position of best focus. In addition to these moves the focus position changes over time because of water desorption by the graphite epoxy in the metering truss. The authors report here the focus history of the telescope based on a knowledge of the mirror moves made and an analysis of desorption monitoring data obtained by the Faint Object Camera (FOC) in the F/96 mode and of the routine calibration data obtained by the Wide Field and Planetary Cameras. Focus values are extracted using two different methods. In the first method the distance between the center of the point spread function (PSF) and the shadows of the pads supporting the HST primary mirror are related to the focus error. In the second method an analytical formula for the PSF with variable aberration coefficients is fitted to the data. Focus positions derived from the two methods show good agreement. The data show that a desorption of about 83 microns has taken place since Aug. 16, 1990. The desorption has clearly not leveled off as expected from the trend of the earlier data. Long term variations of the secondary mirror position of approximately 3-15 microns from the 'best' focus position have been observed. Variations of the order of 2-5 microns over an orbital period have also been noted. Focus changes resulting from secondary mirror movements greater than approximately 5 microns changes the point spread function significantly and makes deconvolution and quantitative measurements difficult.

Hasan, H.↗
NASA NTRS1993

Mineralogy and cooling history of magnesian lunar granulite 67415

Apollo granulite 67415 was investigated by mineralogical techniques to gain better understanding of cooling histories of lunar granulities. Cooling rates were estimated from chemical zoning of olivines in magnesian granulitic clasts by computer simulation of diffusion processes. The cooling rate of 10 deg C/yr obtained is compatible with a model of the granulite formation, in which the impact deposit was cooled from high temperature or annealed, at the depth of about 25 m beneath the surface.

Takeda, Hiroshi↗
NASA NTRS1994

An analysis of gamma ray burst time histories

Gamma ray burst time histories, ranging in durations from milliseconds to thousands of seconds, are as varied as the number of bursts. They show a wide array of structures from those that are very smooth to those that contain a seemingly uncountable number of spikes riding on top of other spikes. These profiles have tantalized researchers for years - they obviously hold important information on the nature of GRB's, but to date no one has been successful in analyzing them. For the past year the author has been working on algorithms to analyze these data. Two approaches have been followed in this investigation. The first is an attempt to quantify the amount of structure, or spikiness, in a profile. The second involves applying the latest theorems on chaos and fractals with the aim of extracting useful information from what seems to be a random collection of shot noise.

Lestrade, John Patrick↗
NASA NTRS1993

Constraining proposed combinations of ice history and Earth rheology using VLBI determined baseline length rates in North America

We predict the present-day rates of change of the lengths of 19 North American baselines due to the glacial isostatic adjustment process. Contrary to previously published research, we find that the three dimensional motion of each of the sites defining a baseline, rather than only the radial motions of these sites, needs to be considered to obtain an accurate estimate of the rate of change of the baseline length. Predictions are generated using a suite of Earth models and late Pleistocene ice histories, these include specific combinations of the two which have been proposed in the literature as satisfying a variety of rebound related geophysical observations from the North American region. A number of these published models are shown to predict rates which differ significantly from the VLBI observations.

Mitrovica, J. X.↗
NASA NTRS1994

New Developments Regarding the KT Event and Other Catastrophes in Earth History

Papers presented at the conference on New Developments Regarding the KT Event and Other Catastrophes in Earth History are included. Topics covered include: trajectories of ballistic impact ejecta on a rotating earth; axial focusing of impact energy in the earth's interior: proof-of-principle tests of a new hypothesis; in search of Nemesis; impact, extinctions, volcanism, glaciations, and tectonics: matches and mismatches.

Source record↗
NASA NTRS1994

Ozone control of biological activity during Earth's history, including the KT catastrophe

There have been brief periods since the beginning of the Cambrian some 600 m.y. ago when mass extinctions destroyed a significant fraction of living species. The most widely studied of these events is the catastrophe at the KT boundary that ended the long dominance of the dinosaurs. In addition to mass extinctions, there is another profound discontinuity in the history of Earth's biota, the explosion of life at the end of the Precambrian era which is an episode that is not explained well at all. For some 3 b.y. before the Cambrian, life had been present on Earth, but maintained a low level of activity which is an aspect of the biota that is puzzling, especially during the last two-thirds of that period. During the last 2 b.y. before the Cambrian, conditions at the Earth's surface were suitable for a burgeoning of the biota, according to most criteria: the oceans neither boiled nor were fozen solid during this time, and the atmosphere contained sufficient O for the development of animals. The purpose of this paper is to suggest that mass extinctions and the lackluster behavior of the Precambrian biota share a common cause: an inadequate amount of ozone in the atmosphere.

Sheldon, W. R.↗
NASA NTRS1993

Use of SPOT and ERS-1 SAR data to study the tectonic and climatic history of arid regions

In order to separate the effects of the different tectonic and climatic processes on the shapes of desert piedmonts, a modified conic equation was fitted to digital topographic data for individual alluvial fans in Death Valley (California, U.S.). The topographic data were obtained from a SPOT panchromatic stereo pair and from the airborne interferometric SAR (Synthetic Aperture Radar) (TOPSAR). The conic fit allows parameters for the epex position, slope, and radial curvature to be compared with unit age, uplift rate, and climatic conditions. Preliminary results indicate that slope flattens with age and radial curvature is concave up, but decreases with age. Work is continuing on correlation of fit residuals and apex position with fan unit age. This information will help in the determination of tectonic uplift rates and the climatic history of the western U.S. ERS-1 SAR images were used to study an area of western China where a large strike slip fault crosses a series of alluvial fans and stream valleys. Previous analysis of SPOT panchromatic images of the area shows that offsets fans and streams can be recognized. Measurement of the rate of motion of this fault will help in the overall model of deformation of the Asian tectonic plate in response to the collision of the Indian plate.

Farr, Tom G.↗
NASA NTRS1994

Way Station to Space: The History of Stennis Space Center

The video traces the history of the Stennis Space Center from its origins as a test facility for President Kennedy's initiative to put a man on the moon to its present day tasks as a leading center for propulsion research and its contributions towards the development of Space Station Freedom.

Source record↗
NASA NTRS1993

Thermal history of comets during residence in the Oort cloud - Effect of radiogenic heating in combination with the very low thermal conductivity of amorphous ice

The thermal history of long-period comets initially composed of amorphous ice is studied. It is shown that such comets with a small nucleus thermal conductivity (kappa) experience a runaway increase in the internal temperature during residence in the Oort cloud. The temperature increase is a result of rapid release of the latent heat at crystallization triggered by gradual heating due to decay of radioactive nuclides. The time of the runaway temperature increase is about ten to a hundred million years after the formation of the nucleus depending on the fraction of refractory grains which contain radioactive nuclides. Most of the amorphous ice in the nuclides except just beneath the surface transforms into crystalline ice due to the runaway temperature increase. This implies that the ice in short-period comets is crystalline from the initial time when the long-period comet becomes a short-period one. In comets with large kappa the temperature does not rise much compared to the small kappa case and the initial amorphous ice is preserved. A criterion for the crystallization of the nucleus ice is derived.

Haruyama, Jun'ichi↗
NASA NTRS1993

Experimental aeroelasticity in wind tunnels - History, status, and future in brief

The state of the art of experimental aeroelasticity in the United States is assessed. A brief history of the development of ground test facilities, apparatus, and testing methods is presented. Several experimental programs are described that were previously conducted and helped to improve the state of the art. Some specific future directions for improving and enhancing experimental aeroelasticity are suggested.

Ricketts, Rodney H.↗
NASA NTRS1994

Shock and thermal history of iron and chondritic meteorites

This research grant included a study of the shock and thermal history of iron and chondritic meteorites. The important research findings are to be found in the 20 publications that were published as a result of the research support. A complete bibliographic reference to all these papers is given.

Goldstein, Joseph I.↗
NASA NTRS1994

Degassing history and evolution of volcanic activity of terrestrial planets based on radiogenic noble gas degassing models

Current abundances of radiogenic noble gases (4-He, 40-Ar, 129-Xe) in the planetary atmospheres provide valuable information on planetary evolution through their degassing histories. Of these, 40-Ar (from 40-K with half life 1.25 Ga) can be used to envisage long-term degassing from their interior. Present amounts of 36 and 40-Ar should have information on timing and degree of degassing from the interior. On the other hand, 129-Xe (from 129-I with half life 17 Ma) is used in discussing early planetary degassing. Because life time of helium in the atmosphere is short, atmospheric 4-He (from 235-U, 238-U, 232-Th) data might be used only for current degassing. The relative abundance of 40-Ar in the Cytherean atmosphere is 0.26 of that in the Earth's atmosphere. We have executed numerical calculation of a 40-Ar degassing model along with mantle evolution and melt migration. Our result shows that the average magma production rate on Venus is about 5 (km(exp 3)/yr). Duration of plate motion on Venus, if existed, would be less than several hundred million years.

Sasaki, S.↗
NASA NTRS1994

Water abundance and accretion history of terrestrial planets

According to a widespread believe, Earth's water was either added in form of a late volatile-rich veneer or as we have argued repeatedly that of all the water which was added to the Earth only that portion remained which was added towards the end of accretion when the mean oxygen fugacity of the accreting material became so high that metallic iron could not exist any longer. Prior to this moment, all the water in the latter scenario would have been used up for the oxidation of iron. Fe + H2O yields FeO + H2. Huge quantities of hydrogen would continuously be produced in this scenario which escaped. In the same moment the hydrogen on its way to the surface would lead to an efficient degassing of the growing Earth's mantle. The fact that - assuming C1 abundances - the amount of iridium in the Earth's mantle agrees, within a factor of two with the total water inventory of the Earth's mantle and crust is taken as evidence for the validity of such a scenario. In both scenarios, the Earth's mantle would remain dry and devoid of other volatiles. Some species soluble in metallic iron like carbon and hydrogen will probably partly enter the core in some portions. It is generally assumed that today a considerable portion of the earth's total water inventory resides in the mantle. It is also clear that over the history of the Earth the water of the Earth's oceans has been recycled many times through the mantle. This is the consequence of plate subduction. In a similar way mantle convection was probably responsible to being water into the originally dry mantle. As a consequence, today the Earth is wet both inside and outside.

Waenke, H.↗