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At least 505 records · Page 28

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NASA NTRS1981

Lunar floor-fractured craters - Evidence for viscous relaxation of crater topography

Viscous relaxation is evaluated quantitatively as a possible mechanism to account for the observed topographical modification of floor-fractured lunar craters. On the basis of a model of the moon as a viscous material of uniform Newtonian viscosity, the ultimate topographic profiles of fresh craters predicted taking into account viscous relaxation effects are compared with the observed profiles of floor-fractured craters of similar diameter. It is found that for a number of crater pairs, the agreement of the observations with the predictions of the simple viscous relaxation model is good, suggesting that isostatic adjustment of craters in areas of anomalously high temperatures at shallow depth may be a more important source of deformation than previously realized.

Hall, J. L.↗

NASA NTRS1982

Topography and stratigraphy of Martian polar layered deposits

The first samples of high resolution Viking Orbiter topographic and stratigraphic data for the layered polar deposits of Mars are presented, showing that these deposits are with respect to both slopes and angular relief similar to those in the south. It is also demonstrated that, in conjunction with stereophotogrammetry, photoclinometry holds promise as a tool for detailed layered deposit studies. The spring season photography, which lends itself to photoclinometric analysis, covers the entire area of the north polar deposits. Detailed tests of layered terrain evolution hypotheses will be made, upon refinement of the data by comparison with stereo data. A more promising refining technique will make use of averaging perpendicular to selected sections to enhance SNR. Local reliefs of 200-800 m, and slopes of 1-8 deg, lead to initial calculations of average layer thickness which yields results of 14-46 m, linearly correlated with slope.

Blasius, K. R.↗

NASA NTRS1982

The study of X-ray scattering to determine surface topography of smooth surfaces

The scattering of X-rays from state-of-the-art polished mirrors is discussed with reference to the requirements of the Advanced X-ray Astrophysics Facility telescope. An experimental set-up is described which allows information to be obtained with subarcsecond resolution. A sample of the data obtained is presented along with a possible theoretical model for its interpretation.

Williams, A. C.↗

NASA NTRS1981

Satellite laser altimeter for measurement of ice sheet topography

Results of a preliminary conceptual design study are presented for a laser altimeter that is capable of high-resolution mapping of the polar ice sheets and is proposed for inclusion in the National Oceanic Satellite System spacecraft or other polar-orbiting satellites. The instrument consists of a pulsed-laser transmitter operating at a wavelength of 532 nm and a duration of 5 nsec, a receiver telescope, a photomultiplier-tube detector, data processing and control electronics, and an attitude sensing and measurement system; a laser footprint diameter of 70 m is planned. The altimeter components are described, and the accuracy of both the altimetry measurement and the attitude measurement is evaluated. A range precision of about 10 cm is estimated, along with an attitude measurement accuracy of approximately 10 arcsec.

Bufton, J. L.↗

NASA NTRS1983

Interaction of the antarctic circumpolar current with bottom topography - An investigation using satellite altimetry

Theoretical studies on the interaction of a zonal current with a zonal ridge, an isolated bump, and a meridional ridge compare favorably with hydrographic observations within the eastward flowing Antarctic Circumpolar Current where it flows over similar topographic features. However, the existing hydrographic data are insufficient for examining the temporal stability and kinematic behavior of the resulting mesoscale structures. In this study, some of these transient features have been compared with patterns in sea surface variability, derived from collinear satellite altimetric data. When these features occurred near the crossing point of two satellite ground traces, it was possible to characterize their length scales, dynamic height relief, and translational and surface geostrophic velocities.

Colton, M. T.↗

NASA NTRS1983

Topography, surface properties, and tectonic evolution

Differences in atmospheric composition, atmospheric and lithospheric temperature, and perhaps mantle composition, suggest that the rock cycle on Venus is not similar to the earth's. While radar data are not consistent with a thick, widespread and porous regolith like that of the moon, wind-transported regolith could be cemented into sedimentary rock that would be indistinguishable from other rocks in radar returns. The elevation spectrum of Venus is strongly unimodal, in contrast to the earth. Most topographic features of Venus remain enigmatic. Two types of tectonic model are proposed: a lithosphere too thick or buoyant to participate in convective flow, and a lithosphere which, in participating in convective flow, implies the existence of plate tectonics. Features consistent with earth-like plate tectonics have not been recognized.

Mcgill, G. E.↗

NASA NTRS1984

Topography on Ganymede derived from photoclinometry

A fundamental limitation in the quantitative analysis of planetary geomorphology and structural geology results from the absense of topographic data. Among the studies that might be addressed, were such data available, are volumetric and slope relationships of Ionian volcanics, fracture and annealing processes in icy crusts on Europa and Ganymede, and evaluation of viscous relaxation as a means of degrading landforms on Europea, Ganymede, and Callisto. A technique for acquiring topographic data on the Galilean satellites using photoclinometry was developed and applied.

Danielson, G. E.↗

NASA NTRS1984

The Influence of Surface Topography on Polymer Friction

Current friction models for polymers are examined to determine their applicability in predicting the effects of roughness on friction. The two component, adhesion deformation model of friction predicts the variation of friction with the change in the sliding direction with respect to the lay direction on anisotropic rough surfaces. The two component model does not predict the change in friction with changes in roughness. A combination of an energy based model and a slip line deformation model may provide a friction model which could better represent measured friction.

Eiss, N. S., Jr.↗

NASA NTRS1984

Remote sensing of floe size distribution and surface topography

Floe size can be measured by several properties p- for instance, area or mean caliper diameter. Two definitions of floe size distribution seem particularly useful. F(p), the fraction of area covered by floes no smaller than p; and N(p), the number of floes per unit area no smaller than p. Several summertime distributions measured are a graph, their slopes range from -1.7 to -2.5. The variance of an estimate is also calculated.

Rothrock, D. A.↗

NASA NTRS1983

Gravity, topography, and crustal evolution of Venus

A method for estimating radial gravity anomalies from line-of-sight acceleration data, their interpolation, and the use of iteration for improved radial anomaly estimates is outlined. Results based on spherical harmonic coefficients for Venus' gravitational field are first discussed, and the procedure is then presented for determining radial acceleration estimates from the line-of-sight vectors and for interpolating the results at preselected locations using the method of objective mapping. A preliminary gravity anomaly map of much of the surface of Venus based on the first estimation step of this technique is presented, and the results are discussed.

Bowin, C.↗

NASA NTRS1984

Topography of the shield volcano, Olympus Mons on Mars

Olympus Mons, one of the largest known shield volcanoes in the solar system, covers an area of more than 3.2 x 10 to the 5th sq km and has a diameter of more than 600 km, excluding its vast aureole deposits. The structure is five times larger than the largest shield volcano on the earth. It is situated on the north-west flank of the Tharsis volcanic region, a broad topographic rise on the Martian surface. The volcano has three physical subdivisions: the summit caldera, the terraced upper flanks, and the lower flanks, which terminate in a scarp 2-10 km high that nearly surrounds the structure. A large block of images of the Tharsis region, including Olympus Mons, was obtained by the Viking mission. A topographic map of Olympus Mons is presented here, which has been compiled using various combinations of stereo pairs of these images, together with stereoscopic perspective views generated by image processing techniques.

Wu, S. S. C.↗

NASA NTRS1984

SIR-B interferometric topography

An interferometric method of using the shuttle imaging radar-B (SIR-B) data for high resolution elevation measurements is to be developed. It is proposed that signals from separate SIR-B passes be combined to provide the equivalent of an interferometer. Data acquisition and handling are considered along with expected results.

Goldstein, R. M.↗

NASA NTRS1985

Effect of counterface material type and its topography on the tribological properties of polyimide composites

Graphite fiber reinforced polyimide composite pins were slid against seven different counterfaces to determine the effect of material type on the tribological properties of polymer composites. In addition, the effect of sliding a new pin on a pre-established transfer film was investigated. The results indicated that almost a five order of magnitude difference in composite wear rate can occur just by varying the counterface material. An attempt to make all surfaces as smooth as possible was made, but due to differences in material composition this was not possible and a range of surface roughnesses were obtained. The results indicate that the smoother the surface, the lower the composite wear rate; but that small protrusions (not discernible with arithmetic surface roughness measurements) can markedly increase wear rates. A pre-established transfer film improved both run in and steady state wear rates.

Fusaro, R. L.↗

NASA NTRS1984

Modeling crater topography and albedo from monoscopic Viking Orbiter images. I - Methodology

The methodology for a new photoclinometric technique for extracting topographic data from planetary images is described. The technique fully compensates for oblique viewing geometry prevalent in many spacecraft images. Brightness data are taken from a pair of profiles simultaneously, overcoming the problem of albedo variations in the application of photoclinometry to planetary surfaces. An additional procedure is described for eliminating the requirement of topographic symmetry along a pair of profiles. Test results show that both procedures have an accuracy of about two degrees for slopes of typical bowl-shaped craters, or about 5 percent for depths.

Davis, P. A.↗

NASA NTRS1987

Sound propagation over uneven ground and irregular topography

The acoustic impedance of the surface coverings used in the laboratory experiments on sound diffraction by topographical ridges was determined. The model, which was developed, takes into account full wave effects and the possibility of surface waves and predicts the sound pressure level at the receiver location relative to what would be expected if the flat surface were not present. The sound pressure level can be regarded as a function of frequency, sound speed in air, heights of source and receiver, and horizontal distance from source to receiver, as well as the real and imaginary parts of the surface impedance.

Berthelot, Yves H.↗

NASA NTRS1987

Subsidence of topography on Io

The underlying roots of Io's topographic features are softened and eroded by contact with the hot mantle, resulting in a subsidence which is analogous to the progress of a butter pat on a frying pan. This process would be offset by crustal thickening due to continuing volcanism if the rate for this phenomenon were more than the observed 1 cm/year or less. Because the crustal thinning would occur at about 50 cm/year if the material underneath were a pure magma ocean, Io cannot have a global magna ocean, and interior viscosities greater than about 10 to the 10th P are implied.

Webb, Erik K.↗

NASA NTRS1988

Streaking images that appear only in the plane of diffraction in undoped GaAs single crystals: Diffraction imaging (topography) by monochromatic synchrotron radiation

Streaking images restricted to the direction of the diffraction (scattering) vector have been observed on transmission through undoped GaAs. These disruption images (caused by the reduction of diffraction in the direction of observation) appear both in the forward and in Bragg diffracted directions in monochromatic synchrontron radiation diffraction imaging. This previously unobserved phenomenon can be explained in terms of planar defects (interfaces) or platelets which affects the absorption coefficient in anomalous transmission. Such regions of the crystal look perfect despite the presence of imperfections when the scattering vector is not perpendicular to the normal of the platelets. The observed crystallographic orientation of these interfaces strongly indicates that they are antiphase boundaries.

Kuriyama, Masao↗

NASA NTRS1988

Applications of synchrotron radiation to materials science: Diffraction imaging (topography) and microradiography

Synchrotron radiation sources are now available throughout the world. The use of hard X-ray radiation from these sources for materials science is described with emphasis on diffraction imaging for material characterization. With the availability of synchrotron radiation, real-time in situ measurements of dynamic microstructural phenomena have been started. This is a new area where traditional application of X-rays has been superseded. Examples are chosen from limited areas and are by no means exhaustive. The new emerging information will, no doubt, have great impact on materials science and engineering.

Kuriyama, Masao↗