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At least 1,243 records · Page 69
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Mars topography harmonics and geophysical implications

The paper describes an improved model of Martian global topography which has been obtained by fitting a sixteenth-degree harmonic series to occultation, radar, spectral, and photogrammetric measurements. Empirical elevation data based on photographic data are used to supplement the observations in areas without data. Values for the mean radius, the mean density, and the displacement of the center of the figure from the center of mass are presented. The reported geometric flattening is too great and the reported dynamic flattening is too small for Mars to be homogeneous and hydrostatic. Maps of the data distribution, global topography, and Bouguer gravity anomaly are interpreted in terms of a crustal thickness map which is consistent with gravity, topography, and recent preliminary Viking seismic results.

Bills, B. G.↗
NASA NTRS1975

The system anorthite-forsterite-fayalite-silica to 2 kbar with lunar petrologic applications

Near-liquidus phase relations in the magnesium-rich portion of the system anorthite-forsterite-fayalite-silica have been determined from synthesis experiments at 1 atm and 2 kbar at oxygen fugacities near the iron-wuestite buffer. The major-element chemistries of KREEP basalts are similar to peritectic liquids, L/ol,px,an/, produced experimentally. The reaction L/ol,px,an/ is well-located at low pressures. It intersects the join anorthite-enstatite-ferrosilite at Fe/(Fe+Mg) equals 0.45 (atomic ratio), and is peritectic only for Fe/(Fe+Mg) not greater than 0.45. Although positions of the equilibria, L/an,ol/ and L/ol,px/, differ significantly at 1 atm from their positions at 2 kbar, the position of the piercing point, L/ol,px,an/, on the anorthite-enstatite-ferrosilite join is not sensitive to pressure. The composition of liquids produced by partial melting of troctolites and pyroxenites in the lunar crustal environment will be affected by pressure, whereas liquids derived from gabbroic assemblages will not.

Merrill, R. B.↗
NASA NTRS1975

Origin of 78235, a lunar norite cumulate

A chemical and petrographic study is reported of the phases from the rock 78235 which was returned on the Apollo 17 mission. Petrographic analysis of the thin sections from the bounder confirm its cumulate origin. In order to develop further the crystallization history for 78235, its subsequent shock history, and its relationship to other lunar crustal rocks, orthopyroxene, plagioclase, glass, and whole-rock samples were prepared and analyzed for major, minor, and trace elements. It is speculated that an early fractional crystallization event producing a layer of orthopyroxene-plagioclase cumulate with varying amounts of trapped liquid took place within 20 km of the surface of the moon.

Winzer, S. R.↗
NASA NTRS1978

Spaceborne ranging system

A spaceborne laser ranging system is described that could survey a large network of ground reflectors, and provide their relative locations to a precision of + or - 1 cm. This performance is believed realizable for networks covering up to 10 to 6th power square kilometers and from only a few days of observations. This system could be used to monitor crustal movements in many areas of the world and has the potential to provide an almost real-time system for detecting precursory ground motions before large earthquakes.

Smith, D. E.↗
NASA NTRS1978

Local precision nets for monitoring movements of faults and large engineering structures

Along Bocono Fault were installed local high precision geodetic nets to observe the possible horizontal crustal deformations and movements. In the fault area there are few big structures which are also included in the mentioned investigation. In the near future, measurements shall be extended to other sites of Bocono Fault and also to the El Pilar Fault. In the same way and by similar methods high precision geodetic nets are applied in Venezuela to observe the behavior of big structures, as bridges and large dams and of earth surface deformations due to industrial activities.

Henneberg, H. G.↗
NASA NTRS1978

Review of Canadian experience in precise gravimetry

Results of gravity observations made in Canada from 1974 to 1978 reviewed, in order to estimate the true accuracy of present-day gravimetry and thereby assess the potential capability of the method for detecting crustal movements. The standard error of the mean of ties is 15-20 nm/s squared. Inter-instrument comparisons and other tests show, however, that a more realistic estimate of D meter accuracy is 30-40 nm/s squared. This accuracy can only be maintained over the long term where uncertainties in gravimeter calibration curves are minimized by resetting to the same dial reading on the resurveys. A further deterioration in accuracy to 40-50 nm/s squared occurs where reliance is placed on presently available D meter calibration curves. Despite the present accuracy limitations significant time variations in gravity of 100-150 nm/s squared are seen over spatial scales of 10-100 kilometers in Canada over a period of several months.

Lambert, A.↗
NASA NTRS1979

Strain buildup and release, earthquake prediction and selection of VLB sites for the margins of the North Pacific

Projects studying different aspects of crustal deformation are discussed. The rifting process at the plate boundary in northern Iceland was investigated using combined seismic, tilt, and displacement measurements. Stresses acting on the Alpine fault, New Zealand, were modeled based on observations of regional variation in metamorphism and argon loss. The relationship between the stress pattern shown by intraplate seismicity and possible stresses on plates arising from litohspheric motion over the asthenosphere was investigated. Small earthquakes in Southern California were studied to determine if subregions of uniform strain release exist and if interaction occurs between faults.

Scholz, C. H.↗
NASA NTRS1978

Period variations in pulsating X-ray sources. II - Torque variations and stellar response

A statistical description of variations in the torque acting on a rotating neutron star is developed in terms of stationary random processes and is used to calculate the torque power spectrum, stellar response power spectrum, and the total mean square variation in the crustal angular velocity of the star. The response of a star with a finite frequency internal mode is calculated with the aid of phenomenological equations which correspond to a generalized two-component model of the star. The form of the stellar response functions is examined in a number of limiting cases of physical interest, and the dependence of the response power on both the period of observation and the time over which fluctuating torques have been acting is discussed.

Lamb, F. K.↗
NASA NTRS1979

Past obliquity oscillations of Mars - The role of the Tharsis uplift

The paper deals with possible ancient variations of the obliquity of Mars. Consideration of data on internal differentiation, Tharsis uplift and crustal phenomena, mantle convection, and eigenfrequencies and mean motion suggest that (1) differentiation with core formation decreased the primeval spin precession constant (alpha) by 4-5%, driving it through resonance with the j = 2 orbital term; (2) a combination of geophysical processes caused alpha to drift back and forth through resonance with the j = 2 orbital term during part of Mars' history; (3) this situation was turned off by the Tharsis uplift, which drove alpha away from j = 2 resonance and possibly through the j = 26 resonance. The consequences of such events for the planet's obliquity are analyzed. Various types of motions are calculated, demonstrating the sensitivity of the obliquity to passage through resonance with such minor orbital terms.

Ward, W. R.↗
NASA NTRS1979

Martian canyons and African rifts - Structural comparisons and implications

The resistant parts of the canyon walls of the Martian rift complex Valles Marineris have been used to infer an earlier, less eroded reconstruction of the major troughs. The individual canyons are compared with individual rifts of East Africa. When measured in units of planetary radius, Martian canyons show a distribution of lengths nearly identical to those in Africa, both for individual rifts and for compound rift systems. A common mechanism which scales with planetary radius is suggested. Martian canyons are significantly wider than African rifts. This is consistent with the longstanding idea that rift width is related to crustal thickness: most evidence favors a crust on Mars at least 50% thicker than that of Africa. The overall patterns of the rift systems of Africa and Mars are quite different in that the African systems are composed of numerous small faults with highly variable trend. On Mars the trends are less variable; individual scraps are straighter for longer than on earth. The basement and lithosphere of Mars are inferred to be simple, reflecting a relatively inactive tectonic history prior to the formation of the canyonlands.

Frey, H.↗
NASA NTRS1979

Inversion of satellite magnetic anomaly data

A method of finding a first approximation to a crustal magnetization distribution from inversion of satellite magnetic anomaly data is described. Magnetization is expressed as a Fourier series in a segment of spherical shell. Input to this procedure is an equivalent source representation of the observed anomaly field. Instability of the inversion occurs when high frequency noise is present in the input data, or when the series is carried to an excessively high wave number. Preliminary results are given for the United States and adjacent areas.

Mayhew, M. A.↗
NASA NTRS1979

Geoid anomalies and the thickness of the lithosphere

By using accepted crustal density distributions and either error function or linear temperature distributions the difference in geoid height between stable continental areas and deep ocean basins has been determined as a function of the continental lithospheric thickness. If the continental lithosphere were greater than 200 km thick, the geoid anomaly over the continents would be systematically negative in relation to that over the ocean basins. By using the GEM 9 satellite geoid the mean geoid anomalies over ocean basins and stable continental areas have been obtained. No systematic difference between continental and oceanic geoids is observed. It is concluded that the thickness of the continental lithosphere is near 180 km. This is in good agreement with various interpretations of the surface heat flow observations.

Turcotte, D. L.↗
NASA NTRS1978

Clast assemblages of possible deep-seated /77517/ and immiscible-melt /77538/ origins in Apollo 17 breccias

Breccia samples 77517 and 77538 are composed of abundant mineral and lithic clasts set in porous, poorly sintered matrices. Clast assemblages in the two rocks are of contrasting composition and origin. Breccia 77517 has Mg-rich olivine and pyroxene and calcic plagioclase clasts, indicating limited, almost exclusively ANT-suite parentage. A significant feature is the presence of an assemblage (aluminous enstatite, forsterite, anorthite, aluminous spinel) corresponding to spinel cataclasite, a rock type of deep-seated (about 60 km) crustal origin. Breccia 77538 contains Fe-rich pyroxene and rather sodic plagioclase clasts, indicative of predominantly KREEP and/or mare derivation. An important feature is the occurrence of high-K and high-Fe lithic clasts whose compositions resemble those of immiscible-melts produced during late-stage magmatic crystallization, and which probably originated via silicate liquid immiscibility in a KREEP or mare basalt magma. Both rocks contain numerous fine-grained breccia clasts which represent material that has been modified by impact processes at or very near the moon's surface.

Warner, R. D.↗
NASA NTRS1978

The magnetization of the lunar crust as deduced from orbital surveys

A two-dimensional magnetic anomaly map of a region of intense crustal magnetization near Van de Graaff crater has been constructed from Apollo 15 and 16 subsatellite vector magnetometer observations. The mapping method used limits coverage to relatively low altitudes (65-80 km) over the Van de Graaff region along the Apollo 15 track and over the central far side highlands along the Apollo 16 track; a simple differencing technique was used to remove external noise contributions. An approximate inversion of this map using a surface plate model for source region geometry has revealed at least four different sources. No correlation between the lateral locations of these source regions and surface morphology was found; the directions of magnetization seem to be unrelated except for a strong north-south depletion. A rough antipodal correlation of the two most strongly magnetized regions yet mapped on the moon with the Imbrium and Orientale basins gives mild support to the ejecta deposit hypothesis suggested by Strangway et al. (1973) for formation of anomaly source regions.

Hood, L. L.↗
NASA NTRS1978

Composition and origin of the Apennine Bench Formation

Integration of stratigraphic relationships, geologic setting, surface morphology and remote sensing data suggest that the Apennine Bench Formation is composed of post-Imbrium basin volcanic KREEP basalt flows. This KREEP volcanism occurred shortly after the time of Imbrium basin formation and may have been triggered by the large basin impact which could have provided the structural 'plumbing' through which magma could reach the surface. There are indications that the Apennine Bench Formation may represent the largest surface exposure of highland (nonmare) volcanism preserved on the lunar surface and, as such, provides valuable insight into early lunar crustal processes.

Spudis, P. D.↗
NASA NTRS1978

Seismic structure of the lunar mantle - An overview

The direct P and S wave arrival times from natural lunar seismic events are the most complete and reliable data set for determining the structure of the lunar mantle. A total of 40 events provide sufficiently well-observed arrivals to permit the extraction of structural information. Using this arrival time data set, the average velocities in a two-layered mantle with an assumed crustal structure (from Toksoz et al., 1974) have been obtained. Reflected phases arriving after direct S are then used to calculate the depth of the boundary between the two mantle layers, and to demonstrate that it is probably a complex transition zone approximately 80 km thick between 400 and 480 km depth. To determine velocity gradients in the upper mantle it is required that the model explain the pronounced decrease in shear wave amplitudes and accompanying delay in shear wave arrival times beyond a distance of about 90 deg. The final model is well-constrained.

Goins, N. R.↗
NASA NTRS1979

Preliminary geological mapping of Io

A preliminary summary of information gained by Voyager 1 on the colored, terrain and landform surface units of Io and their global distribution is presented. Colored units are classified as white to bluish-white regions which may be sulfur or sulfur dioxide deposits, red, orange, or yellow regions thought to contain various sublimates or alterations of sulfur, brownish regions limited to the polar areas and dark brown areas surrounding some vents. Terrain features observed include plains broken by scarps, isolated mountainous regions and volcanic vents resembling terrestrial caldera or pit craters. Maps of the distribution of these features, compiled by photogeological mapping techniques developed for terrestrial volcanic mapping, are presented, and the implications of the surface unit distributions for the volcanology, crustal composition, internal convection patterns and surface age of Io are discussed.

Masursky, H.↗
NASA NTRS1979

Comparison of Magnetic Anomalies of Lithospheric Origin Measured by Satellite and Airborne Magnetometers over Western Canada

Crustal magnetic anomaly data from the OGO 2, 4 and 6 (Pogo) satellites are compared with upward-continued aeromagnetic data between 50 deg -85 deg N latitude and 220 deg - 260 deg E longitude. Agreement is good both in anomaly location and in amplitude, giving confidence that it is possible to proceed with the derivation and interpretation of satellite anomaly maps in all parts of the globe. The data contain a magnetic high over the Alpha ridge suggesting continental composition and a magnetic low over the southern Canada basin and northern Canadian Arctic islands (Sverdrup basin). The low in the Sverdrup basin corresponds to a region of high heat flow, suggesting a shallow Curie isotherm. A ridge of high field, with two distinct peaks in amplitude, is found over the northern portion of the platform deposits and a relative high is located in the central portion of the Churchill province. No features are present to indicate a magnetic boundary between Slave and Bear provinces, but a trend change is evident between Slave and Churchill provinces. South of 60 deg latitude a broad magnetic low is located over very thick (40-50 km) crust, interpreted to be a region of low magnetization.

Langel, R. A.↗