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At least 289 records · Page 16

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NASA NTRS1992

Bohemian circular structure, Czechoslovakia: Search for the impact evidence

Test of the impact hypothesis for the origin of the circular, 260-km-diameter structure of the Bohemian Massif led to the discovery of glasses and breccias in the Upper Proterozoic sequence that can be compared to autogeneous breccias of larger craters. The black recrystallized glass contains small exsolution crystals of albite-oligoclase and biotite, regularly dispersed in the matrix recrystallized to quartz. The occurrence of these rocks is limited to a 1-sq-km area. It is directly underlain by the breccia of the pelitic and silty rocks cemented by the melted matrix, found on several tens of square kilometers. The melt has the same chemistry as rock fragments in major and in trace elements. It is slightly impoverished in water. The proportion of melted rocks to fragments varies from 1:5 to 10:1. The mineralogy of melt viens is the function of later, mostly contact metamorphism. On the contact of granitic plutons it abounds on sillimanite, cordierite, and small bullets of ilmenite. Immediately on the contact with syenodiorites it contains garnets. The metamorphism of the impact rock melt seems the most probable explanation of the mineralogy and the dry total fusion of rocks accompanied by the strong fragmentation. Other aspects of this investigation are discussed.

Rajlich, Petr↗

NASA NTRS1992

Geochemical aspect of impact cratering: Studies in Vernadsky Institute

Studies of the geochemical effects of impact cratering at the Vernadsky Institute in collaboration with the Institute of Dynamics of Geospheres, Moscow State University, Leningrad State University, and some other institutions were fulfilled by several approaches. At the initial stage, three approaches were used: (1) experimental studies of high-temperature vaporization of geological materials (basalts, granites, and so on) in vacuum that was considered as a model of behavior of impact melt and vapor; (2) search of impact-induced geochemical effects in the rock from terrestrial impact craters; and (3) studies of samples of lunar regolith. The next stage of the studies included experiments on quasi-equilibrium vaporization of geological material in Knudsen cells. The results of this investigation are discussed.

Yakovlev, O. I.↗

NASA NTRS1993

Final report

In eastern Nevada and western Utah is an extensive terrane that has experienced a complex tectonic history of Mesozoic deformation and superposed Tertiary extension. The Mesozoic tectonic history of this area has been the subject of controversy for the past twenty or more years. The debate has centered on whether major Mesozoic geologic structures were due to compressional or extensional tectonic regimes. The goal of our research was to decipher the deformational history of the area by combining detailed geologic mapping, remote sensing data analysis, and U-Pb and K-Ar geochronology. This study area includes the Dolly Varden Mountains and adjacent Currie Hills, located in the semi-arid environment of the northeastern Great Basin in Nevada. Vegetation cover in the Dolly Varden Mountains typically ranges from about 10 percent to 50 percent, with some places along drainages and on high, north-facing slopes where vegetation cover approaches 100 percent. Sagebrush is found at less vegetated lower elevations, whereas pinon pine and juniper are prevalent above 2,000 meters. A variety of geologic materials is exposed in the study area. A sequence of Late Paleozoic and Triassic sedimentary rocks includes limestone, dolomite, chert, sandstone, siltstone and shale. A two-phase granitic stock, called the Melrose, intruded these rocks, resulting in metamorphism along the intrusive contact. Tertiary volcanic rocks cover most of the eastern part of the Dolly Varden Mountains and low-lying areas in the Currie Hills.

Zamudio, Joe↗

NASA NTRS1992

An analysis of differential impact melt-crater scaling and implications for the terrestrial impact record

The volume of impact melt relative to that of the transient cavity is known to increase with the magnitude of the impact event. This paper investigates the influence of that phenomenon on the nature of terrestrial impact craters. A model of impact melting was used to estimate the volume of melt produced during the impact of chondritic projectiles into granite targets at velocities of 15, 25, and 50 km per sec. The results were compared with observed melt volumes at terrestrial craters, which were collated from the literature and paired with the transient-cavity diameters of their respective craters. Results show that the record on terrestrial craters is not inconsistent with the predictions of the model.

Grieve, R. A. F.↗

NASA NTRS1993

The effects of atmospheric pressure on infrared reflectance spectra of Martian analogs

The use of terrestrial samples as analogs of Mars soils are complicated by the Martian atmosphere. Spectral features due to the Martian atmosphere can be removed from telescopic spectra of Mars and ISM spectra of Mars, but this does not account for any spectral differences resulting from atmospheric pressure or any interactions between the atmosphere and the surface. We are examining the effects of atmospheric pressure on reflectance spectra of powdered samples in the laboratory. Contrary to a previous experiment with granite, no significant changes in albedo or the Christiansen feature were observed from 1 bar pressure down to a pressure of 8 micrometers Hg. However, reducing the atmospheric pressure does have a pronounced affect on the hydration features, even for samples retained in a dry environment for years.

Bishop, Janice L.↗

NASA NTRS1993

The correlation of alkalis and aluminum during high-temperature volatilization of albite and nepheline

Our previous study of high-temperature evaporation of mafic targets has shown a systematic enrichment of newly-formed condensates in silicon and alkalis and depletion in refractory elements (Al, Ca, Ti). This effect, which is consistent with the volatility of the elements, was also tested for several experiments with acidic targets. In one case it was the impact evaporation of granite targets using a light-gas gun facility. Analyses of the condensate which was collected during the impact show that it was enriched in Al and Na so that the composition of the condensate became close to nepheline. In the work of Nagahara, an incongruent evaporation of alkali feldspars under stationary conditions has been also proposed. These discoveries required a more detailed study of high-temperaure evaporation of monomineralic Al-silicate targets. In these experiments, we have used samples of oligoclase and of nepheline. Vaporization of samples was performed in a hermetic cell filled with helium at normal temperature and pressure using a powerful Nd-laser focused on the sample. Laser pulses had the following parameters: luminous energy approximately 600 J, duration approximately 10(exp -3) s, density of luminosity was approximately 5 x 10(exp 6) W/sq cm, the temperature of vaporization is about 3000-4000 K. About 4 cm from the sample, a nickel foil was placed on which the condensate was collected. Films of condensate were analyzed using X-Ray Photo-Electron spectrometer ES-2401. The measurement of internal chemical composition and of the charging state of elements through the whole depth of the condensed films was made layer by layer with argon-ion etching of 200 A of the film per step. Results of this study are given. The bottom layers of condensed film correspond to the front parts of the evaporated cloud or to condensation at the beginning of the evaporative process and surface layers to condensation at the end of the process.

Dikov, Yu. P.↗

NASA NTRS1993

The Karakul depression in Pamirs: The first impact structure in Central Asia

The Karakul depression was picked out as the possible impact structure by the study of space images of Tadjikistan. Its striking similarity with some complex impact craters such as Dellen and Gosses Bluff is evident. Our investigations of the Karakul depression in 1987 and 1989-1991 years allowed to determine it as the impact crater with the central uplift. The target of the crater is presented by slightly metamorphosed sedimentary rocks of Paleozoic, intensively folded and intruded with granites during the Gercynian orogeny. The modern structure of north-eastern Pamirs was formed by Alpinian orogeny; the raise of the area in Neogene was about 4000-7000 m.

Gurov, E. P.↗

NASA NTRS1993

Manson impact structure, Iowa: First geochemical results for drill core M-1

The Manson Impact Structure is a large complex impact crater centered ca. S km north of the town of Manson, Iowa. It is the largest intact impact structure recognized in the United States (35 km in diameter). Its Ar-40/Ar-39 age is indistinguishable from that of the Cretaceous-Tertiary (K-T) boundary. The Manson structure may be one element of the events at the K-T boundary. The crater is completely covered by Quaternary glacial sedimentary deposits that are normally underlain by Cretaceous clastic sediments and flat-lying carbonate sediments of Phanerozoic age, as well as Proterozoic red clastic, metamorphic, volcanic, and plutonic rock sequences. The study of a reflection seismic profile, provided by Amoco, was critical in interpreting the structure. In the 35 km diameter zone that marks the extension of the crater the normal rock sequence is disturbed due to the impact, and at the center of the structure granitic basement rocks are present that have been uplifted from about 4 km depth. Our studies consist of detailed petrological and geochemical characterization of all cores, with emphasis on a detailed description of all rock types found in the core samples and their relationship to target rocks. Geochemical data on samples from the Manson M-1 core are presented.

Koeberl, Christian↗

NASA NTRS1992

AVIRIS as a tool for carbonatite exploration: Comparison of SPAM and Mbandmap data analysis methods

Data acquired with the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) of the Mountain Pass, San Bernadino County, California, area were analyzed to evaluate the use of narrow-band imaging data for carbonatite exploration. Carbonatites are igneous carbonate-rich rocks that are economically important in part because they are the major source for rare-earth minerals. Because the 224 AVIRIS spectral channels have a nominal spectral resolution of 10 nm, narrow absorption features such as those displayed by the rare-earth elements neodymium (Nd) and samarium (Sm) may be detected. The Mountain Pass region encompasses a well-exposed sequence of sedimentary, metamorphic, and igneous rocks, including an alkalic carbonatite intrusion. The carbonatite was emplaced in Precambrian granitic rocks that are fault bounded by a thick suite of Paleozoic, sedimentary rock, predominantly dolomite. The carbonatite stock, the major source of light rare-earth elements (REE) in the United States, affords the opportunity to test AVIRIS capabilities for detecting REE absorption features. Nd-bearing minerals display narrow, sharp and distinctive spectral features at 580, 740, 800, and 870 nm. Minerals bearing Sm display similarly sharp features near 1090, 1250, 1410, and 1550 nm. The more common REE, lanthanum and cerium, do not display absorption features in the visible/near-infrared region in their natural oxidation states.

Kingston, Marguerite J.↗

NASA NTRS1993

Geochronology and petrogenesis of the western highlands alkali suite: Radiogenic isotopic evidence from Apollo 14

Several rocks of alkalic affinity, from the western highlands of the Moon, have been analyzed for their Nd and Sr isotopic compositions. One sample yields a Sm-Nd mineral isochron of 4110 = 41 Ma. This age, in conjunction with U-Pb zircon ages on two other alkalic rocks from the Apollo 14 landing site suggests a distinct western highlands 'event' which was approximately 100 Ma in duration. Since the last dregs of the lunar magma ocean likely crystallized prior to 4.3 Ga, this alkalic 'event' may have included the re-melting of evolved plutons or the remobilization of urKREEP trapped liquid from upper mantle cumulates. Alkalic lithologies such as granites and felsites have been known from the Moon since the earliest days of the Apollo lunar sample returns. However, not until 1977 were alkali-rich rocks recognized from typical highlands suites such as ferroan anorthosites (FAN) and norites and Mg-suite rocks. In the intervening years, several other alkali suite samples have been discovered and characterized, mostly through labor-intesive breccia pull-apart studies of clasts and analyses of coarse-fine fractions of soils. We will speculate on the origins of this suite of lunar highlands rocks.

Snyder, Gregory A.↗

NASA NTRS1993

The age of the Pretoria Saltpan impact crater, South Africa

The Pretoria Saltpan impact crater, situated about 40 km NNW of Pretoria, South Africa, has a diameter of about 1.13 km. The structure was formed in 2.05 Ga Nebo granite of the Bushveld Complex. The impact origin of the crater was recently established by the discovery of characteristic shock-metamorphic features in breccias found in drill cores at depths greater than 90 m. Impact glass fragments were recovered by standard magnetic separation techniques and handpicking from the melt breccias. As no reliable crater age was known so far, several hundred sub-millimeter-sized glass fragments were studied for fission tracks. The results show that the Saltpan impact crater has an age of 220 +/- 52 ka. This is in agreement with field geological observations.

Storzer, Dieter↗

NASA NTRS1993

SPH modelling of energy partitioning during impacts on Venus

Impact cratering of the Venusian planetary surface by meteorites was investigated numerically using the Smoothed Particle Hydrodynamics (SPH) method. Venus presently has a dense atmosphere. Vigorous transfer of energy between impacting meteorites, the planetary surface, and the atmosphere is expected during impact events. The investigation concentrated on the effects of the atmosphere on energy partitioning and the flow of ejecta and gas. The SPH method is particularly suitable for studying complex motion, especially because of its ability to be extended to three dimensions. In our simulations, particles representing impactors and targets are initially set to a uniform density, and those of atmosphere are set to be in hydrostatic equilibrium. Target, impactor, and atmosphere are represented by 9800, 80, and 4200 particles, respectively. A Tillotson equation of state for granite is assumed for the target and impactor, and an ideal gas with constant specific heat ratio is used for the atmosphere. Two dimensional axisymmetric geometry was assumed and normal impacts of 10km diameter projectiles with velocities of 5, 10, 20, and 40 km/s, both with and without an atmosphere present were modeled.

Takata, T.↗

NASA NTRS1993

Original size of the Vredefort structure, South Africa

The Vredefort structure is located approximately 120 km southwest of Johannesburg, South Africa, and is deeply eroded. Controversies remain on the origin of this structure with the most popular hypotheses being: (1) by impact cratering about 2.0 Ga; (2) as a cryptoexplosion structure about 2.0 Ga; and (3) by purely tectonic processes starting at about 3.0 Ga and ending with the Vredefort event at 2.0 Ga. In view of recent work in which the granophyre dikes are interpreted as the erosional remants of a more extensive impact melt sheet, injected downward into the underlying country rocks, the impact origin hypothesis for Vredefort is adopted. In order to estimate the original dimensions of the Vredefort impact structure, it is assumed that the structure was initially circular, that its predeformation center corresponds to the center of the granitic core, and that the pre-Vredefort geology of the area prior to approximately 2.0 Ga ago is as suggested by Fletcher and Reimold. The spatial relationship between shock metamorphic effects, the shock pressures they record, and the morphological features of the crater were established for a number of large terrestrial craters. The principles of crater formation at large complex impact structures comparable in size to Vredefort were also established, although many details remain unresolved. An important conclusion is that the transient crater, which is formed directly by excavation and displacement by the shock-induced cratering flow-field (i.e., the particle velocity flow field existing in the region of the transient crater but behind the initial outgoing shock front), is highly modified during the late stage processes. The original transient crater diameter lies well within the final rim of the crater, which is established by structural movements during late-stage cavity modification.

Therriault, A. M.↗

NASA NTRS1993

A lunar/Martian anchor emplacement system

On the Moon or Mars, it is necessary to have an anchor, or a stable, fixed point able to support the forces necessary to rescue a stuck vehicle, act as a stake for a tent in a Martian gale, act as a fulcrum in the erection of general construction poles, or support tent-like regolith shields. The anchor emplacement system must be highly autonomous. It must supply the energy and stability for anchor deployment. The goal of the anchor emplacement system project is to design and build a prototype anchor and to design a conceptual anchor emplacement system. Various anchors were tested in a 1.3 cubic meter test bed containing decomposed granite. A simulated lunar soil was created by adjusting the moisture and compaction characteristics of the soil. We conducted tests on emplacement torque, amount of force the anchor could withstand before failure, anchor pull out force at various angles, and soil disturbances caused by placing the anchor. A single helix auger anchor performed best in this test bed based on energy to emplace, and the ultimate holding capacity. The anchor was optimized for ultimate holding capacity, minimum emplacement torque, and minimum soil disturbance in sandy soils yielding the following dimensions: helix diameter (4.45 cm), pitch (1.27 cm), blade thickness (0.15 cm), total length (35.56 cm), shaft diameter (0.78 cm), and a weight of 212.62 g. The experimental results showed that smaller diameter, single-helix augers held more force than larger diameter augers for a given depth. The emplacement system consists of a flywheel and a motor for power, sealed in a protective box supported by four legs. The flywheel system was chosen over a gear system based on its increased reliability in the lunar environment.

Clinton, Dustin↗

NASA NTRS1993

Mineralogic variability of the Kelso Dunes, Mojave Desert, California derived from Thermal Infrared Multispectral Scanner (TIMS) data

Mineral identification and mapping of alluvial material using thermal infrared (TIR) remote sensing is extremely useful for tracking sediment transport, assessing the degree of weathering and locating sediment sources. As a result of the linear relation between a mineral's percentage in a given area (image pixel) and the depth of its diagnostic spectral features, TIR spectra can be deconvolved in order to ascertain mineralogic percentages. Typical complications such as vegetation, particle size and thermal shadowing are minimized upon examination of dunes. Actively saltating dunes contain little to no vegetation, are very well sorted and lack the thermal shadows that arise from rocky terrain. The primary focus of this work was to use the Kelso Dunes as a test location for an accuracy analysis of temperature/emissivity separation and linear unmixing algorithms. Accurate determination of ground temperature and component discrimination will become key products of future ASTER data. A decorrelation stretch of the TIMS image showed clear color variations within the active dunes. Samples collected from these color units were analyzed for mineralogy, grain size, and separated into endmembers. This analysis not only revealed that the dunes contained significant mineralogic variation, but were more immature (low quartz percentage) than previously reported. Unmixing of the TIMS data using the primary mineral endmembers produced unique variations within the dunes and may indicate near, rather than far, source locales for the dunes. The Kelso Dunes lie in the eastern Mojave Desert, California, approximately 95 km west of the Colorado River. The primary dune field is contained within a topographic basin bounded by the Providence, Granite Mountains, with the active region marked by three northeast trending linear ridges. Although active, the dunes appear to lie at an opposing regional wind boundary which produces little net movement of the crests. Previous studies have estimated the dunes range from 70% to 90% quartz mainly derived from a source 40 km to the west. The dune field is assumed to have formed in a much more arid climate than present, with the age of the deposit estimated at greater than 100,000 years.

Ramsey, Michael S.↗

NASA NTRS1992

Quest for excellence 5

The highlights of the 1992 Malcolm Baldrige National Quality Award winners - AT and T Network Systems Group, Transmission Systems Business Unit; Texas Instrument Defense Systems and Electronics Group; AT and T Universal Card Services; The Ritz Hotel CO; and The Granite Rock Company are presented, along with brief information about the company and their beliefs and business and production strategies for quality manufacturing and products.

Source record↗

NASA NTRS1994

Quantitative estimation of granitoid composition from thermal infrared multispectral scanner (TIMS) data, Desolation Wilderness, northern Sierra Nevada, California

We have produced images that quantitatively depict modal and chemical parameters of granitoids using an image processing algorithm called MINMAP that fits Gaussian curves to normalized emittance spectra recovered from thermal infrared multispectral scanner (TIMS) radiance data. We applied the algorithm to TIMS data from the Desolation Wilderness, an extensively glaciated area near the northern end of the Sierra Nevada batholith that is underlain by Jurassic and Cretaceous plutons that range from diorite and anorthosite to leucogranite. The wavelength corresponding to the calculated emittance minimum lambda(sub min) varies linearly with quartz content, SiO2, and other modal and chemical parameters. Thematic maps of quartz and silica content derived from lambda(sub min) values distinguish bodies of diorite from surrounding granite, identify outcrops of anorthosite, and separate felsic, intermediate, and mafic rocks.

Sabine, Charles↗

NASA NTRS1994

Apollo 12 ropy glasses revisited

We analyzed ropy glasses from Apollo 12 soils 12032 and 12033 by a variety of techniques including SEM/EDX, electron microprobe analysis, INAA, and Ar-39-Ar-40 age dating. The ropy glasses have potassium rare earth elements phosphorous (KREEP)-like compositions different from those of local Apollo 12 mare soils; it is likely that the ropy glasses are of exotic origin. Mixing calculations indicate that the ropy glasses formed from a liquid enriched in KREEP and that the ropy glass liquid also contained a significant amount of mare material. The presence of solar Ar and a trace of regolith-derived glass within the ropy glasses are evidence that the ropy glasses contain a small regolith component. Anorthosite and crystalline breccia (KREEP) clasts occur in some ropy glasses. We also found within these glasses clasts of felsite (fine-grained granitic fragments) very similar in texture and composition to the larger Apollo 12 felsites, which have a Ar-39-Ar-40 degassing age of 800 +/- 15 Ma. Measurements of 39-Ar-40-Ar in 12032 ropy glass indicate that it was degassed at the same time as the large felsite although the ropy glass was not completely degassed. The ropy glasses and felsites, therefore, probably came from the same source. Most early investigators suggested that the Apollo 12 ropy glasses were part of the ejecta deposited at the Apollo 12 site from the Copernicus impact. Our new data reinforce this model. If these ropy glasses are from Copernicus, they provide new clues to the nature of the target material at the Copernicus site, a part of the Moon that has not been sampled directly.

Wentworth, S. J.↗