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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

A comparative Rb-Sr, Sm-Nd, and K-Ar study of shocked norite 78236 - Evidence of slow cooling in the lunar crust

The sample 78236 was chipped from the top of a norite boulder at Station 8 by the Apollo 17 landing team. Jackson et al. (1975) concluded that this rock formed at a depth of 8-30 km in the lunar crust and suggested that it was excavated by a large basin-forming impact event. A petrographic description of the boulder is provided, and isotopic analyses are discussed. Attention is given to a chronology for 78236 which seems to be most consistent with radiometric and other evidence. It is proposed that cumulate norite 78236 formed deep in the lunar crust approximately 4.4 AE ago. The rock cooled slowly in the crust until it was excavated by a major basin-forming event. Excavation may have occurred about 4.2 AE ago, but the time of this event is not well constrained.

Nyquist, L. E.↗
NASA NTRS1982

The intercrater plains of Mercury and the Moon: Their nature, origin and role in terrestrial planet evolution. Chronology of surface history of the Moon

The sequence of events is described that occurred from the time that the ancient lunar crust solidified (about 4.4. billion years ago) and anorthositic high lands dominated the surface, until the global contraction (cooling) that began around 3.3 billion years ago when late stage basalts were emplaced at basin margins where fractures penetrated to subsurface tensional zones. The lunar intercrater plains may be linked with early KREEP volcanism, the LKFM basalt source region, and the first stages of mare volcanism. Ages of KREEP bracket the possible ages of the pre-Imbrian plains, and overlap the initial stages of mare basalt emplacement. Both plains are extruded under the same tensional tectonic regime.

Leake, M. A.↗
NASA NTRS1983

Planetary Radar

The application of modern planetary radar techniques to a comet passing in close proximity to Earth is discussed. These techniques have the potential to determine the nature of cometary origins, structure, and internal dynamics. Moreover, the understanding gained could very likely negate or corroborate one of the prevailing hypotheses regarding the origin of the solar system: that comets are the remainder of the primordial out of which the planets coalesced approximately 4.5 billion years ago. In 1983, two unique opportunities were presented to observe a comet very near to Earth. The last such encounter was several centuries ago.

Bogan, J. R.↗
NASA NTRS1982

A note against a small-body origin for shergottites, nakhlites, and chassignites

With the possible exception of Brachina, shergottites, nakhlites, and chassignites all crystallized about 1.3 b.y. ago on the same planet, and they were probably ejected from it together about 180 m.y. ago. No breccias from the parent planet have yet been identified, thus a dominantly impact-processed surface or a small body internal melt scenario is virtually untenable. These meteorites must have formed in a magmatic complex, and on a planet from which it was difficult to remove material: as far as we know, it only happened once. Everything known about shergottites, nakhlites, and Chassigny is consistent with an igneous origin on Mars.

Ryder, G.↗
NASA NTRS1982

Absence of silicic volcanism on Mars - Implications for crustal composition and volatile abundance

It is believed that explosive basic to ultrabasic eruptions may be responsible for small cinder conelike features on Mars and perhaps for the ancient flank-scoured paterae. Eruptions of this type may have been driven by near-surface water/ice-magma interactions or volatile rich magmas. Noting that the paterae represent a unique style of volcanism that stopped approximately 2 b.y. ago, it is suggested that the volatiles associated with these features were derived from mantle sources and that the cessation of patera formation may coincide with the termination of the period of maximum planetary degassing. Since approximately 2 b.y. ago volcanism has been dominantly effusive, except for such minor explosive ash events as those that blanketed part of the summit of Hecates Tholus. It is proposed that such eruptions were driven by volatiles generated by differentiation of a magma chamber, similar to Icelandic explosive eruptions.

Francis, P. W.↗
NASA NTRS1983

Ar-40/Ar-39 and U-Th-Pb dating of separated clasts from the Abee E4 chondrite

Ar-40/Ar-39 and U-Th-Pb are investigated for three clasts from the Abee (E4) enstatite chondrite, yielding Ar-40/Ar-39 plateau ages (and/or maximum ages) of 4.5 Gy, while two of the clasts give average ages of 4.4 Gy. The 4.4-4.5 Gy range does not resolve possible age differences among the clasts. The U-Th-Pb data are consistent with the interpretation that initial clast formation occurred 4.58 Gy ago, and that the clasts have since remained closed systems which have been contaminated with terrestrial Pb. The thermal history of Abee deduced from Ar data seems consistent with that deduced from magnetic data, suggesting that various Abee components experienced separate histories until brecciation no later than 4.4 Gy ago, experiencing no significant subsequent heating.

Bogard, D. D.↗
NASA NTRS1983

Archean crust-mantle geochemical differentiation

Isotope measurements on carbonatite complexes and komatiites can provide information on the geochemical character and geochemical evolution of the mantle, including the sub-continental mantle. Measurements on young samples establish the validity of the method. These are based on Sr, Nd and Pb data from the Tertiary-Mesozoic Gorgona komatiite and Sr and Pb data from the Cretaceous Oka carbonatite complex. In both cases the data describe a LIL element-depleted source similar to that observed presently in MORB. Carbonatite data have been used to study the mantle beneath the Superior Province of the Canadian Shield one billion years (1 AE) ago. The framework for this investigation was established by Bell et al., who showed that large areas of the province appear to be underlain by LIL element-depleted mantle (Sr-85/Sr-86=0.7028) at 1 AE ago. Additionally Bell et al. found four complexes to have higher initial Sr ratios (Sr-87/Sr-86=0.7038), which they correlated with less depleted (bulk earth?) mantle sources, or possibly crustal contamination. Pb isotope relationships in four of the complexes have been studied by Bell et al.

Tilton, G. R.↗
NASA NTRS1983

Pieces of the ancient lunar crust - Ages and composition of clasts in consortium breccia 67915

The composition and chronology of clasts representing three minor lithologies in consortium breccia 67915 are discussed. The lithologies studied are: sodic ferrogabbro, pristine troctolitic anorthosite (67915,26), and granulitic troctolitic anorthosite (67915,67). These lithologies were presumed to represent samples of the ancient lunar crust. The pristine troctolitic anorthosite, a typical member of the pristine anorthosite suite, contains little or no trapped liquid consistent with its two-phase mineralogy. The plagioclase separates have Ar-39-Ar-40 plateau ages of 4.10 + or - 0.06 b.y., and a final rise in the age pattern may represent a 'memory' of the earlier evolution. The granulitic troctolitic anorthosite, which has 13 times larger REE abundances, displays a continuous rise of the Ar-40/Ar-39 ratio. The last 25 percent of the Ar-39 release data establish a minimum age of over 3.5 b.y., but the Pu-244/Nd chronology suggests an age of about 4.3 b.y. The sodic ferrogabbro samples, which appear to be members of the Mg-gabbronorite group of pristine rocks, show large Ar losses but indicate plateaus in the low-temperature data, suggesting a metamorphic event more recently than 260 m.y. ago, a time that is consistent with the North Ray impact event 50 m.y. ago.

Marti, K.↗
NASA NTRS1984

Close encounters and collisions of comets with the earth

A computer search for earth-approaching comets among those listed in Marsden's (1983) updated orbit catalog has identified 36 cases at which minimum separation distance was less than 2500 earth radii. A strong representation of short period comets in the sample is noted, and the constant rate of the close approaching comets in the last 300 years is interpreted to suggest the lack of long-period comets intrinsically fainter than an absolute magnitude of about 11. A comet-earth collision rate derived from the statistics of these close encounters implies an average period of 33-64 million years between any two events. This rate is comparable with the frequency of geologically recent global catastrophes which appear to be associated with extraterrestrial object impacts, such as the Cretaceous-Tertiary extinction 65 million years ago and the late Eocene event 34 million years ago.

Sekanina, Z.↗
NASA NTRS1984

Apollo 16 - Impact melt sheets, contrasting nature of the Cayley plains and Descartes mountains, and geologic history

Apollo 16 stations four and five rake samples have been examined petrographically and by electron microprobe and INAA. Lithologic abundances support the idea (Korontev, 1981) that the variation of soil composition at Apollo 16 results from mixing between a component represented by station five and components much like either the dimict breccias or feldspathic fragmental breccias in composition. Pyroxene, olivine, and coexisting plagioclase compositions from within the anorthosite portions of dimict breccias bridge the gap between the Mg-rich and ferroan anorthosite fields. Analyses from associated cumulate and granulitic clasts indicate that they are the source of the intermediate material. Dimict breccias formed about 3.92 b.y. ago, the nectaris event occurred 3.84-3.92 b.y. ago, and the Cayley plains were deposited as a result of the Imbrium event sometime later than 3.84 b.y.

Mckinley, J. P.↗
NASA NTRS1983

Space law - Current status and issues

The evolution of space law over the past 25 years is surveyed, with attention also given to the procedures that were followed. The treaties now in existence are given, as are issues currently before the United Nations Committee on the Peaceful Uses of Outer Space. Projections that were made 25 years ago are discussed in the light of subsequent developments. It is noted that nearly all the technological advances in space activities forecast 25 years ago have come to pass. Various provisions of the 1967 Outer Space Principles Treaty relating to stricture against weapons and the militarization of space are discussed.

Hosenball, S. N.↗
NASA NTRS1984

Constraints on lunar origin: Evidence preserved in Precambrian stromatolites

The existence of subaqueous unicellular algae and bacteria from the Precambrian period is evidenced by strongly abundant fossilized structures consisting of many layers of usually darker algae-bacterial growth alternating with layers of usually lighter sediment-precipitate. The earliest of these are dated to 3.5 billion years ago. A form of these stromatolites, Anabaria juvensis was analyzed and a sinusoidal columnar growth pattern was interpreted to be a response of stromatolite forming microbes to the changing inclination of the Sun over the seasons, with microbe growth rate positively related to solar intensity. Additional specimens are being used to develop a systematic methodology for extracting data evidencing Earth-Moon-Sun dynamics at the time of stromatolite formation. In particular, stromatolites span the time from 1 to 2 billion years ago, critical for several theories of lunar formation and/or Earth/Moon near encounter. Such cataclysmic events would influence stromatolite formation.

Vanyo, J. P.↗
NASA NTRS1985

Precambrian evolution and the rock record

The Precambrian time which refers to geological time prior to the first appearance of animals with mineralized hard parts was investigated. Best estimates for this event are around 570 million years ago. Because the rock record begins some 3,800 million years ago the Precambrian encompasses about 84% of geologic time. The fossil record for this immense span of time is dominated by prokaryotes and the sedimentary structures produced by them. The first fossil remains that are considered eukaryotic are found in 1,000 million year old rocks. The first animals may be as old as 700 million years. The fossil records of the first 84% of the Earth's history are collected and described.

Awramik, S.↗
NASA NTRS1985

Chronology and petrogenesis of a 1.8 g lunar granitic clast:14321,1062

Geochronological, isotopic, and trace element data for a pristine granite clast from Apollo 14 breccia 14321 obtained using Rb-Sr, Sm-Nd, and (Ar-39)-(Ar-40) methods are presented. Trace element data for a possibly related evolved rock, the quartz-monodiorite clast from breccia 15404 are also presented, and the relationship between these two rock types is discussed. The concordancy of the Rb-Sr and Sm-Nd internal isochron ages and especially the Rb-Sr model age strongly suggest that the granite clast formed 4.1 AE ago. It probably crystallized slowly in the crust and was later excavated and brecciated about 3.88 AE ago, as indicated by the Ar-Ar age. A two-stage model involving crystal fractionation followed by silicate liquid immiscibility is proposed for the lunar granite genesis.

Shih, C.-Y.↗
NASA NTRS1986

Paleosols and the chemical evolution of the atmosphere

The chemistry and mineralogy of soils reflects the chemistry of the atmosphere below which they develop. Today the presence of a cover of land plants can obscure the signature of a highly oxidizing atmosphere. Prior to the advent of higher land plants ca. 400 m.y. ago the obscuring effect of a biological interface should have been minor. It can be shown that under such conditions the behavior of ferrous iron during weathering depends on the relationship between the ratio of the net concentration of oxidants to the concentration of acids in rainwater, and on the ratio of the oxygen demand to the acid demand for complete weathering of the parent rock. An analysis of the behavior of iron in some 15 paleosols between 1.0 and 2.9 b.y. in age showed some time ago that iron was retained, or largely retained, in paleosols developed on rocks with a low ratio of oxidant demand to acid demand (R value) and was lost from paleosols with a high R value. The results indicate that between 1.1 and 2.9 b.y.b.p. the ratio of the net concentration of oxidants to CO2 in rainwater was less than ca. 1/600 of the present-day value. Work during the past year has sought to extend the data base for paleosols and to define more precisely the relationships between the composition of paleosols and the chemistry of the atmosphere below which they developed.

Holland, H. D.↗
NASA NTRS1986

Perspective on the local interstellar medium

Theoretical local-interstellar-medium (LISM) models designed to explain the observed soft-X-ray background (SXRB) are reviewed. Consideration is given to the LISM in the immediate solar vicinity (the Local Fluff, extending 3-10 pc), the local bubble (a large irregularly shaped cavity filled with very-low-density gas at 10 to the 6th K and surrounded by denser material), the three-dimensional structure of the SXRB emitting region, the thermal nature of the SXRB, and the implications of process time scales for LISM theories. A number of possible mechanisms are discussed in the context of a model in which the LISM hot gas is a remnant from the creation of the local bubble about 20 Myr ago and/or the product of supernova reheating as recently as 150 kyr ago.

Cox, D. P.↗
NASA NTRS1987

The large crater origin of SNC meteorites

A large body of evidence strongly suggests that the shergottite, nakhlite, and Chassigny (SNC) meteorites are from Mars. Various mechanisms for the ejection of large rocks at Martian escape velocity (5 km/sec) have been investigated, but none has proved wholly satisfactory. This article examines a number of possible ejection and cosmic-ray exposure histories to determine which is most plausible. For each possible history, the Melosh (1984, 1985, 1987) spallation model is used to estimate the size of the crater required to produce ejecta fragments of the required size with velocities not less than 5 km/sec and to produce a total mass of solid ejecta consistent with the observed mass flux of SNC meteorites. Estimates of crater production rates on Mars are then used to evaluate the probability that sufficiently large craters have formed during the available time. The results indicate that the SNC meteorites were probably ejected from a very large crater (greater than 100 kilometers in diameter) about 200 million years ago, and that cosmic-ray exposure of the recovered meteorites was initiated after collisional fragmentation of the original ejecta in space at much later times (0.5 to 10 million years ago).

Vickery, A. M.↗
NASA NTRS1988

The growth of continents and some consequences since 1.5 Ga

The budget of Earth's oceanic sediment masses was discussed in terms of crustal growth and recycling. Based on estimates of the volume of oceanic sediments and the average age of oceanic crust, a continental denudation rate of 1.65 cu km/yr was computed. This crudely balances estimated crustal production rates of about 1 cu km/yr, but the efficiency of sediment loss via subduction, for example, must be considered. It was argued, on the basis of earthquake focal solutions, imagery of subduction zones, and plate kinematic reconstructions that little, if any, sediment was lost in this way. This yields a present day crustal growth rate of about 1 cu km/yr. The volume of continents to 1.5 Ga ago was discussed, assuming constant continental thickness and freeboard, and a constant hydrosphere volume. It was concluded that ocean ridge length was a factor of about 1.75 greater 1.5 Ga ago, but a major uncertainty is the average spreading rate in the past.

Howell, David G.↗