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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Rb-Sr age of the Shergotty achondrite and implications for metamorphic resetting of isochron ages

The age of the Shergotty achondrite is determined by Rb-Sr isotope analysis and the metamorphic resetting of isochron ages, which is presumed to have occurred during a shock event in the history of the meteorite, is discussed. The isochron best fitting the Rb-Sr evolution diagram is found to correspond to an age of 165 million years, with an initial Sr-87/Sr-86 value of 0.72260. Different apparent ages obtained by the K-Ar and Sm-Nd methods are interpreted in terms of a model which quantifies the degree of resetting of internal isochron ages by low temperature solid state diffusion. On the basis of these considerations, it is concluded that Shergotty crystallized from a melt 650 million years ago, was shock heated to 300 to 400 C after its parent body was involved in a collision 165 million years ago, and was first exposed to cosmic rays two million years ago.

Nyquist, L. E.↗
NASA NTRS2006

Silica and Pyroxene in IVA Irons; Possible Formation of the IVA Magma by Impact Melting and Reduction of L-LL-Chondrite Materials Followed by Crystallization and Cooling

Group IVA is a large magmatic group of iron meteorites. The mean DELTA O-17 (= delta O-17 - 0.52(raised dot) delta O-18) of the silicates is approx. plus or minus 1.2%o, similar to the highest values in L chondrites and the lowest values in LL chondrites; delta O-18 values are also in the L/LL range. This strongly suggests that IVA irons formed by melting L-LL parental material, but the mean Ni content of IVA irons (83 mg/g) is much lower than that of a presumed L-LL parent (approx. 170 mg/g) and the low-Ca pyroxene present in two IVA meteorites is Fs13, much lower than the Fs20-29 values in L and LL chondrites. Thus, formation from L-LL precursors requires extensive addition of metallic Fe, probably produced by reduction of FeS and FeO. Group IVA also has S/Ni, Ga/Ni, and Ge/Ni ratios that are much lower than those in L-LL chondrites or any chondrite group that preserves nebular compositions, implying loss of these volatile elements during asteroidal processing. We suggest that these reduction and loss processes occurred near the surface of the asteroid during impact heating, and resulted partly from reduction by C, and partly from the thermal dissociation of FeS and FeO with loss of O and S. The hot (approx. 1770 K) low-viscosity melt quickly moved through channels in the porous asteroid to form a core. Two members of the IVA group, Sao Joao Nepomuceno (hereafter, SJN) and Steinbach, contain moderate amounts of orthopyroxene and silica, and minor amounts of low-Ca clinopyroxene. Even though SJN formed after approx. 26% crystallization and Steinbach formed after approx. 77% Crystallization of the IVA core, both could have originated within several tens of meters of the core-mantle interface if 99% of the crystallization occurred from the center outwards. Two other members of the group (Gibeon and Bishop Canyon) contain tabular tridymite, which we infer to have initially formed as veins deposited from a cooling SiO-rich vapor. The silicates were clearly introduced into IVA irons after the initial magma crystallized. Because the y-iron crystals in SJN are typically about 5 cm across, an order of magnitude smaller than in IVA irons that do not contain massive silicates, we infer that the metal was in the gamma-iron field when the silicates were injected. The SJN and Steinbach silicate compositions are near the low-Ca-pyroxene/silica eutectic compositions. We suggest that a tectonic event produced a eutectic-like liquid and injected it together with unmelted pyroxene grains into fissures in the solid metal core. Published estimates of IVA metallographic cooling rates range from 20 to 3000 K/Ma, leading to a hypothesized breakup of the core during a major impact followed by scrambling of the core and mantle debris [Haack, H., Scott, E.R.D., Love, S.G., Brearley, A. 1996. Thermal histories of IVA stony-iron and iron meteorites: evidence for asteroid fragmentation and reaccretion. Geochim. Cosmochim. Acta 60, 3103-3113]. This scrambling model is physically implausible and cannot explain the strong correlation of estimated cooling rates with metal composition. Previous workers concluded that the low-Ca clinopyroxene in SJN and Steinbach formed from protopyroxene by quenching at a cooling rate of 10(sup 12) K/Ma, and suggested that this also supported an impact-scrambling model. This implausible spike in cooling rate by a factor of 10(sup 10) can be avoided if the low-Ca clinopyroxene were formed by a late shock event that converted orthopyroxene to clinopyroxene followed by minimal growth in the clinopyroxene field, probably because melt was also produced. We suggest that metallographic cooling-rate estimates (e.g., based on island taenite) giving similar values throughout the metal compositional range are more plausible, and that the IVA parent asteroid can be modeled by monotonic cooling followed by a high-temperature impact event that introduced silicates into the metal and a low-temperature impact event that partially converted thopyroxene into low-Ca clinopyroxene.

Wasson, John T.↗
NASA NTRS1972

Gosses Bluff impact structure, Australia.

A comprehensive study has been carried out of the Gosses Bluff structure in Central Australia, which is a typical cryptoexplosion structure. The study included detailed geologic mapping, and seismic reflection and refraction, gravity, aeromagnetic, and ground magnetic surveys. It is concluded that the structure is an eroded crater formed by a single nearly instantaneous shock event, and that the event can be explained only by impact.

Milton, D. J.↗
NASA NTRS1974

Gas-rich meteorites - Possible evidence for origin on a regolith

The asymmetry of irradiation features of grains in the Kapoeta and Fayetteville meteorites suggests irradiation on a regolith before meteorite formation. Chondrules and broken grains require approximately 10,000 years of irradiation time between formation or fracturing and compaction into the meteorite. Shock erasure of tracks from irradiated Kapoeta feldspars requires a severe shock event during or after meteorite formation.

Macdougal, D.↗
NASA NTRS1973

Particle track record of Apollo 15 shocked crystalline rocks

Cosmic ray track densities in two mare basalts 15058 and 15555 are multivalued at each depth from the surface, and numerous indications of shock are present, suggesting that shock has lowered track densities in some crystals but not in others. From the minimum track densities, the maximum time since the last shock event is derived for each rock. In separate observations 15017, a black glass, has a surface age of about 14,000 years derived from impact pits, but because of low track retentivity a solar flare record of only 1 year.

Fleischer, R. L.↗
NASA NTRS1981

A semiempirical model for heavy element depletion in the interstellar medium

An empirical relationship is derived which reproduces the observed depletion factors in interstellar gas by relating them to some preshock distribution factor and the binding energy of the elements in appropriate compounds. By fitting this equation to the data, estimates are obtained for both the distribution factor and the critical binding energy, which is related to the shock strength at which grains are completely destroyed. The critical binding energy value therefore provides an estimate of the energy involved in the most recent shock event, while the preshock distribution factor is related to the preshock elemental distribution between gas and dust. It is noted that the minerals most likely to survive are the refractory ones with the largest binding energies.

Trivedi, B. M. P.↗
NASA NTRS1982

The acceleration of charged particles in interplanetary shock waves

Consideration of the theoretical and observational literature on energetic ion acceleration in interplanetary shock waves is the basis for the present discussion of the shock acceleration of the solar wind plasma and particle transport effects. It is suggested that ISEE data be used to construct data sets for shock events that extend continuously from solar wind to galactic cosmic ray energies, including data for electrons, protons, alphas and ions with Z values greater than 2.0, and that the temporal and spatial evolution of two- and three-dimensional particle distribution functions be studied by means of two or more spacecraft.

Pesses, M. E.↗
NASA NTRS1985

Fast and optimal solution to the Rankine-Hugoniot problem

A new, definitive, reliable and fast iterative method is described for determining the geometrical properties of a shock (i.e., theta sub Bn, yields N, V sub s and M sub A), the conservation constants and the self-consistent asymptotic magnetofluid variables, that uses the three dimensional magnetic field and plasma observations. The method is well conditioned and reliable at all theta sub Bn angles regardless of the shock strength or geometry. Explicit proof of uniqueness of the shock geometry solution by either analytical or graphical methods is given. The method is applied to synthetic and real shocks, including a bow shock event and the results are then compared with those determined by preaveraging methods and other iterative schemes. A complete analysis of the confidence region and error bounds of the solution is also presented.

Vinas, A. F.↗
NASA NTRS1986

Fast and optimal solution to the 'Rankine-Hugoniot problem'

A new, definitive, reliable and fast iterative method is described for determining the geometrical properties of a shock (i.e., theta sub Bn, yields N, V sub s and M sub A), the conservation constants and the self-consistent asymptotic magnetofluid variables, that uses the three dimensional magnetic field and plasma observations. The method is well conditioned and reliable at all theta sub Bn angles regardless of the shock strength or geometry. Explicit proof of uniqueness of the shock geometry solution by either analytical or graphical methods is given. The method is applied to synthetic and real shocks, including a bow shock event and the results are then compared with those determined by preaveraging methods and other iterative schemes. A complete analysis of the confidence region and error bounds of the solution is also presented.

Vinas, A. F.↗
NASA NTRS1986

Shock metamorphism and petrography of the Shergotty achondrite

The shock history of the Shergotty meteorite has been evaluated by analyzing the observable shock effects. In addition, the results of petrographic and mineralogical analyses of new Shergotty subsamples are presented. The overall modal composition of these subsamples is similar to that of the other Shergotty samples, except for the rather high whitlockite content. The shock effects observed in the mineral constituents include mosaicism, deformation bands, planar fractures, and mechanical twin lamellae in clinopyroxene; isotropization of plagioclase with very rare remnants of birefringence; and planar deformation structures, mosaicism, and strongly reduced birefringence in quartz. Other shock effects include mechanical twinning of ilmenite, and localized in situ melting of neighboring minerals at the contact of low and high density phases. The observed shock effects could be explained by a single shock event.

Stoeffler, D.↗
NASA NTRS1987

Enhanced low energy (1 MeV) ion fluxes in the outer heliosphere

The Low Energy Charged Particle (LECP) experiment on the Voyager 2 spacecraft in the outer heliosphere (greater than 10 AU) has observed several occasions when there was a peak in the interplanetary ion spectra for ions of energies of about 0.5-1.0 MeV. Such enhancements can last for several days, suggesting that at these times particles of these energies dominate the low energy cosmic population in this region of the heliosphere. Two specific cases are discussed. The enhancements seem to be associated with the passage of transient interplanetary shock events, with the ion anisotropies generally showing outflow. The most straight-forward explanation for the observations seems to involve only a propagation effect of ions from the inner to the outer solar system. This conclusion is supported by simple modeling of the propagation of an event observed at 1 AU to the spacecraft at about 12 AU.

Gold, R. E.↗
NASA NTRS1991

Iodine-xenon studies of petrographically and chemically characterized Chainpur chondrules

INAA, noble gas, and petrographic studies conducted on samples of 18 chondrules and matric material from the Chainpur (LL3) indicate that the I-129/I-127 ratio, R(0), varies by a factor of more than 10 among the chondrules. This corresponds to a greater-than-50 Ma span in apparent I-Xe ages. Models which invoke either gas-dust mixing or nebular heterogeneity cannot satisfactorily explain these data, any more than can hypotheses which attribute the variations to differences in formation age, metamorphic rate, or time of aqueous alteration. It is alternatively suggested that the variations represent periods of low-grade shock events.

Swindle, T. D.↗
NASA NTRS1989

A test of magnetic field draping induced Bz perturbations ahead of fast coronal mass ejecta

ICE plasma and magnetic field data are examined to look for observational evidence of IMF draping ahead of fast coronal mass ejections (CMEs). The utility of the draping model for predicting the Bz perturbations and hence geomagnetic activity associated with the sheath regions ahead of such CMEs is also examined. A simple prediction scheme based on the upstream radial field component is developed and a set of interplanetary shock events previously associated with interplanetary type II bursts, and hence solar source locations, is used. Of 17 events the radial component predictor developed here correctly predicts the direction considered of the Bz perturbations for 13 events (76 percent). While this result is certainly not conclusive, it is considered to be supportive of the draping scenario.

Mccomas, D. J.↗
NASA NTRS1995

The solar origins of two high-latitude interplanetary disturbances

Two extremely similar interplanetary forward/reverse shock events, with bidirectional electron streaming were detected by Ulysses in 1994. Ground-based and Yohkoh/SXT observations show two strikingly different solar events that could be associated with them: an LDE flare on 20 Feb. 1994, and a extremely large-scale eruptive event on 14 April 1994. Both events resulted in geomagnetic storms and presumably were associated with coronal mass ejections. The sharply contrasting nature of these solar events argues against an energetic causal relationship between them and the bidirectional streaming events observed by Ulysses during its S polar passage. We suggest instead that for each pair of events. a common solar trigger may have caused independent instabilities leading to the solar and interplanetary phenomena.

Hudson, H. S.↗
NASA NTRS1999

Magnetism of Carbonados

Carbonados are porous polycrystalline (with crystal sizes up to 100 micrometer) diamonds. Carbonado is found only in alluvial deposits in Bahia, Brazil and in the Central African Republic (CAR). Alluvial deposit host is 1.5 Ga while the carbonados are between 2.6 - 3.8 Ga. The process of fusing the carbonado microcrystals together is not fully understood, partly due to fact that the origin of these carbonado, is not known. Several modes of origins are proposed for carbonado. First, a crustal origin. Carbonados have a light carbon and helium isotopic signature. They contain an enrichment of the rare-earth elements (REE). Carbonados have tightly trapped atmospheric noble gases and contain an evidence of high He content despite the carbonado expected depletion of He at mantle temperatures. Carbonados have high porosity incompatible with high pressure mantle conditions. Second, a mantle origin is proposed based on similar REE pattern to kimberlites. The presence of nitrogen platelet (by IR spectra) indicates high temperature origin and syngenetic inclusions of rutile, ilmenite, and magnetite indicates high pressure and high temperature conditions consistent with mantle origin as well. Third, it is proposed that carbonado diamonds are a result of early impacts into crustal rocks. This is indicated by the rare and controversial occurrence of high pressure diamond polymorph, londsdaleite, frequently found in diamonds from meteorite impact sites, and by observation of planar deformation features, possibly indicating shock events. Finally, it is suggested that carbonados have an extraterrestrial origin, as indicated by a long term annealing based on observation of a zero-phonon line, attributed to paired nitrogen atoms in association with a vacancy.

Kletetschka, G.↗
NASA NTRS2000

Evaluation of Thermo-Mechanical Stability of COTS Dual-Axis MEMS Accelerometers for Space Applications

Microelectromechanical systems in MEMS is one of the fastest growing technologies in microelectronics, and is of great interest for military and aerospace applications. Accelerometers are the earliest and most developed representatives of MEMS. First demonstrated in 1979, micromachined accelerometers were used in automobile industry for air bag crash- sensing applications since 1990. In 1999, N4EMS accelerometers were used in NASA-JPL Mars Microprobe. The most developed accelerometers for airbag crash- sensing are rated for a full range of +/- 50 G. The range of sensitivity for accelerometers required for military or aerospace applications is much larger, varying from 20,000 G (to measure acceleration during gun and ballistic munition launches), and to 10(exp -6) G, when used as guidance sensors (to measure attitude and position of a spacecraft). The presence of moving parts on the surface of chip is specific to MEMS, and particularly, to accelerometers. This characteristic brings new reliability issues to micromachined accelerometers, including cyclic fatigue cracking of polysilicon cantilevers and springs, mechanical stresses that are caused by packaging and contamination in the internal cavity of the package. Studies of fatigue cracks initiation and growth in polysilicon showed that the fatigue damage may influence MEMS device performance, and the presence of water vapor significantly enhances crack initiation and growth. Environmentally induced failures, particularly, failures due to thermal cycling and mechanical shock are considered as one of major reliability concerns in MEMS. These environmental conditions are also critical for space applications of the parts. For example, the Mars pathfinder mission had experienced 80 mechanical shock events during the pyrotechnic separation processes.

Sharma, Ashok K.↗
NASA NTRS2000

Mineralogy, Petrology, Chemistry, and Ar-39 - Ar-40 and Ages of the Caddo County IAB Iron: Evidence for Early Partial Melt Segregation of a Gabbro Area Rich in Plagioclase-Diopside

We found coarse-grained gabbroic material rich in plagioclase and diopside in the Caddo County IAB iron meteorite. The polished thin sections studied were made from areas rich in Al and Ca detected by a micro-focus X-ray fluorescence (XRF) mapping technique. The gabbro is not a clast within a breccia, but rather this area is located mainly at silicate-metal boundaries only a few cm away from an area with fine-grained, ultramafic silicate similar to winonaites. Medium-grained orthopyroxene and olivine are found in transitional areas showing no disturbance of their crystalline textures. A vein-like region, starting at the area rich in fine-grained mafic silicate, extends towards the gabbroic area with a gradual increase in abundance of plagioclase and diopside. This texture and our accumulated knowledge of the formation mechanism of IAB/winonaltes meteorites, suggest that the gabbroic materials were formed by inhomogeneous segregation of partial melts of chondritic source materials. Compositional data on two mineralogically distinct samples of the gabbro-rich portion of the inclusion were obtained by INAA. Compared to an average of LAB silicate inclusions or winonaites, the Caddo County gabbro is enriched in the incompatible lithophile elements Na, Ca, Sc, REE and Hf, which is consistent with a melt origin for the gabbro. The cosmogenic space exposure age of Caddo County (511 Ma) is significantly younger than exposure ages of some other IAB meteorites, An 39Ar-40Ar age determination of the gabbroic material indicates a series of upward steps in age from 4.516 Ga to 4.523 Ga, with a few high temperature ages up to 4.54 Ga. The older age could approximate the primary recrystallization age of silicates. The stepped Ar age spectrum may indicate differences in Ar closure temperatures during slow cooling of -2-20'C/Myr in the parent body. Alternatively, the younger Ar-Ar ages may date a shock event which occurred while Caddo County was hot and which also created textures such as a linear metal vein with rounded zigzag walls in a diopside crystal and slightly miss-oriented rounded plagioclase domain.

Takeda, Hiroshi↗