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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Bunte Breccia of the Ries - Continuous deposits of large impact craters

The 26-km-diameter Ries impact crater in south Germany and the mechanism of ejection and emplacement associated with its formation about 15 Myr ago are discussed in detail, and the implications of the findings for models of crater formation on earth, moon, and planets are considered. Field observations and laboratory tests on 560-m core materials from nine locations are reported. The continuous deposits (Bunte Breccia) are found to be a chaotic mixture resulting from deposition at ambient temperatures in a highly turbulent environment, probably in the ballistic scenario proposed by Oberbeck et al. (1975), with an emplacement time of only about 5 min. Further impact parameters are estimated using the 'Z model' of Maxwell (1977): initial radius = 6.5 km, excavation depth = 1650 m, excavation volume = 136 cu km, and transient cavity volume = 230 cu km. The interpretation of lunar and planetary remote-sensing and in situ evidence from impact craters is reviewed in the light of the Ries findings. Numerous photographs, maps, diagrams, and tables illustrate the investigation.

Horz, F.↗
NASA NTRS1983

Geochemical constraints on the origin of the moon

Hypothesis for the origin of the moon involve variants on capture, double-planet, and fission processes. Double-planet and fission hypotheses are examined in the light of siderophile trace elements. The siderophile trace elements chosen (W, Re, Mo, P, Ga, Ge) have well understood geochemical behavior such that appropriate metal/silicate partition coefficients are available and their abundances in the lunar and terrestrial mantles 4.4-4.5 Gyr ago may be reasonably inferred. The fission hypothesis of Ringwood (1979) is not consistent with the behavior of Re, Mo, and P. The hybrid fission hypothesis of Wanke et al. (1983) overcomes many of the deficiencies of Ringwood's hypothesis, but is not readily reconcilable with the behavior of Re and Ir. The double-planet hypothesis as most recently advanced by Newsom and Drake (1982, 1983) appears to be consistent with siderophile-element behavior in the moon.

Drake, M. J.↗
NASA NTRS1983

The star-forming history of the young cluster NGC 2264

UBVRI H-alpha photographic photometry was obtained for a sample of low-mass stars in the young open cluster NGC 2264 in order to investigate the star-forming history of this region. A theoretical H-R diagram was constructed for the sample of probable cluster members. Isochrones and evolutionary tracks were adopted from Cohen and Kuhi (1979). Evidence for a significant age spread in the cluster was found amounting to over ten million yr. In addition, the derived star formation rate as a function of stellar mass suggests that the principal star-forming mass range in NGC 2264 has proceeded sequentially in time from the lowest to the highest masses. The low-mass cluster stars were the first cluster members to form in significant numbers, although their present birth rate is much lower now than it was about ten million yr ago. The star-formation rate has risen to a peak at successively higher masses and then declined.

Adams, M. T.↗
NASA NTRS1983

Mn-53 profiles in four Apollo surface cores

Mn-53 data are presented for four Apollo cores with an emphasis on the past 10-to-the-7th-year history of these samples: (1) Core 15011 has excess Mn-53 down to a depth of at least 9 cm, which is probably due to the continuous addition of near-surface irradiated material. (2) The Mn-53 profile for 64002 above 12 cm is relatively flat and in excess of that calculated for a static core, which suggests that the upper part of this core was deposited rapidly about 2 m.y. ago. (3) Results for the Apollo 17 drill stem show a large excess of Mn-53 from at least 3 cm downwards. Some magnetic separates obtained from this core have very high Mn-53 activities, probably due to the presence of agglutinates that were recently irradiated very near the surface. (4) The Mn-53 content of the upper part of 74002 is similar to that of trench samples collected nearby and indicates that the gray, orange, and black soils at this site have been exposed together near the surface for at least the last 10 m.y.

Nishiizumi, K.↗
NASA NTRS1983

Isotopic evidence from the eastern Canadian shield for geochemical discontinuity in the Proterozoic mantle

The Nd and Sr isotopic compositions presently reported for anorthosites and related rocks from the Grenville and Nain Provinces of the eastern Canadian shield indicate that the massifs were delivered from at least two distinct mantle source regions which were established before 1650 Myr ago. These regions were episodically involved in magmatism over about 500 Myr. One reservoir was isotopically similar to the depleted, modern midocean ridge basalt source. The other reservoir was chondritic-to-moderately-enriched, and is most easily identified in the Nain Province, but may have occurred scattered throughout Superior Province, as well.

Ashwal, L. D.↗
NASA NTRS1983

Silicon material technology status

Silicon has been the basic element for the electronic and photovoltaic industries. The use of silicon as the primary element for terrestrial photovoltaic solar arrays is projected to continue. The reasons for this projection are related to the maturity of silicon technology, the ready availability of extremely pure silicon, the performance of silicon solar cells, and the considerable present investment in technology and manufacturing facilities. The technologies for producing semiconductor grade silicon and, to a lesser extent, refined metallurgical grade silicon are considered. It is pointed out that nearly all of the semiconductor grade silicon is produced by processes based on the Siemens deposition reactor, a technology developed 26 years ago. The state-of-the-art for producing silicon by this process is discussed. It is expected that efforts to reduce polysilicon process costs will continue.

Lutwack, R.↗
NASA NTRS1983

Neogene Caribbean plate rotation and associated Central American tectonic evolution

A theoretical model of the opening of the Cayman Trough is developed on the basis of geological evidence from a wide area. It is proposed that strike slip motion began about 30 Myr ago and proceeded at a rate of 37 + or - 6 mm/yr for a total of 1100 km of relative plate displacement, and that Central America Underwent an anticlockwise rotation with internal plate deformation. Maps of the reconstructed motion are provided.

Wadge, G.↗
NASA NTRS1984

Holocene eruptive activity of El Chichon volcano, Chiapas, Mexico

Geologic and radiometric-age data indicate that El Chichon was frequently and violently active during the Holocene, including eruptive episodes about 600, 1250, and 1700 years ago and several undated, older eruptions. These episodes, involving explosive eruptions of sulfur-rich magma and associated domegrowth processes, were apparently separated by intervals of approximately 350 to 650 years. Some of El Chichon's eruptions may correlate with unusual atmospheric phenomena around A.D. 1300 and possibly A.D. 623.

Tilling, R. I.↗
NASA NTRS1984

Mass ejection from the peculiar emission star R Aquarii

It is pointed out that the composite emission object R Aquarii embodies a number of characteristic properties which distinguish it from other peculiar emission stars. The visual spectrum indicates the presence of a cool Mira variable in close association with a hot unresolved ionizing source of radiation which appears to be responsible for the high excitation nebular emission observed. It has been suggested by Hubble (1943) and Baade (1944) that the distinguishing meniscus-shaped nebula was formed by a nova outburst which occurred about 600 years ago. Wallerstein and Greenstein (1980) reported the appearance of a new feature in R Aquarii which resembles a brilliant 'spike' or 'jet' protruding a distance approximately 7 arcsec from the central compact region. Attention is given to several models which have been considered to explain the emission feature.

Michalitsianos, A. G.↗
NASA NTRS1984

Cosmic ray propagation in the local superbubble

It is suggested that a ring of HI gas lying in the galactic plane is part of a supershell which formed some 3 x to the 7th power years ago. The consequences of a closed magnetic supershell for cosmic ray propagation are examined and it is concluded that there is no evidence which precludes the production and trapping of cosmic rays in such a region. A consequence of superbubble confinement is that the mean age of cosmic rays would be independent of energy. This can be tested by high energy observations of the isotopic composition of Be.

Steitmatter, R. E.↗
NASA NTRS1984

Two lunar global asymmetries

The Moon's center of mass is displaced from its center of figure about 2 km in a roughly earthward direction. Most maria are on the side of the Moon which faces the Earth. It is assumed that the Moon was initially spherically symmetric. The emplacement of mare basalts transfers mass which produces most of the observed center of mass displacement toward the Earth. The cause of the asymmetric distribution of lunar maria was examined. The Moon is in a spin orbit coupled relationship with the Earth and the effect of the Earth's gravity on the Moon is asymmetric. The earth-facing side of the Moon is a gravitational favored location for the extrusion of mare basalt magma in the same way that the topographically lower floor of a large impact basin is a gravitationally favored location. This asymmetric effect increases inversely with the fourth power of the Earth Moon distance. The history of the Earth-Moon system includes: formation of the Moon by accretion processes in a heliocentric orbit ner that of the Earth; a gravitational encounter with the Earth about 4 billion years ago resulting in capture of the Moon into a geocentric orbit and heating of the Moon through dissipation of energy related to tides raised during close approaches to the Earth(5) to produce mare basalt magma; and evolution of the Moon's orbit to its present position, slowly at first to accommodate more than 500 million years during which magmas were extruded.

Hartung, J. B.↗
NASA NTRS1984

Synthesis of proterozoic data as a prerequisite for tectonic and thermal modeling

Rocks of the Pongola supergroup form an elongate belt in the Archean Kaapvaal Caton of southern Africa. Because these rocks exhibit many features that are characteristic of rocks deposited in continental rifts, it is suggested that the Pongola supergroup was deposited in such a rift. The age of these rocks (approximately 3.0 Ga) makes the Pongola structure the world's oldest well-preserved rift so far recognized, and comparison of the Pongola Rift with other rifts formed more recently in Earth history reveals striking similarities. Rocks of the Ventersdorp Supergroup were deposited in a system of northeast trending grabens on the Kaapvaal Craton approximately 2.64 Ga ago. It is suggested that it was this collision which initiated the Ventersdorp rifting. The Ventersdorp rift province is related to an extension in the Kaapval Craton associated with the collision.

Burke, K. C.↗
NASA NTRS1985

Lineaments on Ganymede: New Evidence for Late Tectonic Activity

Families of lineaments in the Gilomesh and Ninki basins of Ganymede imply post impact tectonic activity. The grooves, previously considered the youngest tectonic features, are estimated to have formed between 3.8 and 3.1 Gyr ago. One rayed crater however, is probably less than 1 Gyr old, implying tectonic activity on Ganymede has extended nearly to the present. Like the grooves, the lineaments appear to be extensional. The parallel trends and nearly contiguous associations of the lineaments with the grooves imply that both are products of the same stress systems. The young inferred age of the lineaments implies that they (and presumably also the grooves) are not associated with stresses in the cooling of fresh deposits of bright terrain, but are probably associated with underlying convective stress patterns, the long implied duration of an extensional stress regime in Ganymede's lithosphere is consistent with the stress models of derived assuming a differentiated interior.

Croft, S. K.↗
NASA NTRS1985

Interstellar Al-26 and excess Mg-26

Evidence of a possible supernova explosion that occured very near the Earth is presented by the presence of radioactive 26 Aluminum (26 Al). The time of the supernova explosion is placed at approximately 100,000 years ago and is partially supported by cosmic gamma ray measurements from both HEAO 3 and the Solar Maximum Mission.

Clayton, D. D.↗
NASA NTRS1985

Periodic cometary showers: Real or imaginary?

Since the initial reports in 1980, a considerable body of chemical and physical evidence has been accumulated to indicate that a major impact event occurred on earth 65 million years ago. The effects of this event were global in extent and have been suggested as the cause of the sudden demise or mass extinction of a large percentage of life, including the dinosaurs, at the end of the geologic time period known as the Cretaceous. Recent statistical analyses of extinctions in the marine faunal record for the last 250 million years have suggested that mass extinctions may occur with a periodicity of every 26 to 30 million years. Following these results, other workers have attempted to demonstrate that these extinction events, like that at the end of the Cretaceous, are temporally correlated with large impact events. A recent scenario suggests that they are the result of periodic showers of comets produced by either the passage of the solar system through the galactic plane or by perturbations of the cometary cloud in the outer solar system by a, as yet unseen, solar companion. This hypothesized solar companion has been given the name Nemesis.

Grieve, R. A. F.↗
NASA NTRS1985

Hertzian contact in two and three dimensions

The basic solution to the problem of mechanical contact between elastically deforming solids was proposed by Hertz over a century ago and has been used by tribologists and others ever since in a steadily increasing number of applications. While the theoretical development is not conceptually difficult and treatments exist to suit all tastes, it is nonetheless interesting to trace the relationships among the solutions in different dimensions. Such an approach is used herein to shed light on the curious and sometimes perplexing behavior of line contacts. A number of the more frequently used contact expressions together as a convenient reference and for comparative purposes.

Tripp, J. H.↗
NASA NTRS1985

Global Geomorphology

Any global view of landforms must include an evaluation of the link between plate tectonics and geomorphology. To explain the broad features of the continents and ocean floors, a basic distinction between the tectogene and cratogene part of the Earth's surface must be made. The tectogene areas are those that are dominated by crustal movements, earthquakes and volcanicity at the present time and are essentially those of the great mountain belts and mid ocean ridges. Cratogene areas comprise the plate interiors, especially the old lands of Gondwanaland and Laurasia. Fundamental as this division between plate margin areas and plate interiors is, it cannot be said to be a simple case of a distinction between tectonically active and stable areas. Indeed, in terms of megageomorphology, former plate margins and tectonic activity up to 600 million years ago have to be considered.

Douglas, I.↗