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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Clotho Tessera, Venus: A fragment of Fortuna Tessera

Clotho Tessera, adjacent to southeast Lakshmi Planum, may provide additional evidence for lateral crustal motions, and a model for the origin of small tessera fragments. Clotho Tessera and Lakshmi Planum are so noticeably different, and in such close proximity, it is difficult to derive a reasonable model of their formation in situ. Squeezing of material out from beneath Lakshmi has been suggested as an origin for Moira Tessera, which is also adjacent to Lakshmi and 1400 km west of Clotho. However, a logical model of juxtaposition of the two different terrains, originally from points once distant, can be made for Clotho and Lakshmi (and perhaps other small tesserae as well). It is suggested that Clotho Tessera was once part of Fortuna Tessera, but was cut off by a transcurrent fault zone (the DLZ) striking perpendicular to the Sigrun rift and carried westward where it collided with Lakshmi Planum (forming Danu Montes). A gravity anomaly along the southern border of Lakshmi, in the area of Danu Montes, was interpreted as indicating subduction there, providing additional supporting evidence for the collision hypothesis. Diffusion of the DLZ with proximity to Sigrun Fossae may be due to either higher ductility near the postulated Sigrun rift, or to burial by flows away from the rift nearer to Valkyrie Fossae. Other possible examples of migrating tesserae occur elsewhere: small pieces of Ananke Tessera can be fit back together as though they had rifted apart, and the spreading apart of Ananke and Virilis Tesserae has been suggested because of their symmetric locations about the axis of an inferred spreading zone. Other tessera fragments appear to have been isolated by rifting, with little, if any, significant lateral motion (e.g., Meni and Tellus Tesserae, and Thethus and Fortuna Tesserae). The migrating terrain model for Clotho Tessera supports Sukhanov's interpretation of tesseral fragments as rafts of lighter crustal material.

Kozak, Richard C.↗
NASA NTRS1990

Erosional development of bedrock spur and gully topography in the Valles Marineris, Mars

Gully networks separated by resistant bedrock spurs are a common erosional feature along the escarpments that border the Valles Marineris. The resistant spur topography is best developed where the base of the slope is truncated by linear scarps interpreted as fault scarps. Regional variations in slope morphology imply that spur and gully topography undergoes a systematic progressive degradation through time associated with the erosional destruction of the basal fault scarps. The comparative morphometry of the divide networks indicates that the density of the spur networks and the number of first-order unbranched spurs decreases as the basal slope break becomes more sinuous. Abstraction of the spurs occurs through regolith storage in adjacent gullies at the slope base and the most degraded slope forms are entirely buried in talus. The basal fault scarps apparently control regolith transport by allowing debris to drain from the slope. As these basal scarps decay the slope base becomes increasingly sinuous and the slopes become transport limited. Dry mass-wasting may be the most important process acting on these slopes where a continually lowered base level is required to maintain the spur topography. In contrast to the Martian slopes, range front fault escarpments in the western U.S. show no systematic trend in spur network geometry as they are eroded. These weathering limited slopes are controlled by the more efficient removal of regolith through fluvial processes which rapidly create quasi-equilibrium drainage networks.

Patton, Peter C.↗
NASA NTRS1988

COD measurements at various positions along a crack

Load versus crack-opening-displacement (COD) was measured at various positions along the border of a fatigue crack as it grew from a small surface crack on the edge of an aluminum specimen into a through-the-thickness crack. Displacements were measured with a laser-based interferometric system with a gage length of 70 microns and a resolution of 0.01 micron. These load-COD curves can be used to determine opening loads and thereby investigate the effect of closure on the growth of small cracks. In general, the opening loads decrease as the crack grows.

Sharpe, W. N., Jr.↗
NASA NTRS1989

Doppler radar observations of the evolution of a small convective storm during Cohmex

Doppler radar observations of a deep convection that developed along the Alabama-Tennesse border on July 14, 1986 are analyzed. The evolution and structure of the convective storm are examined. Two convective cores are observed and both having a radius of about 1 km and maximum reflectivities of about 5 dBz; a third cell is also detected later in the region between the northern and southern cells. It is noted that the northern cell is the most dominant possessing an updraft through the region of maximum reflectivity. Diagrams of the vertical structure of the cells are provided.

Moore, Patrick D.↗
NASA NTRS1990

Aircraft microwave observations and simulations of deep convection from 18 to 183 GHz. I - Observations

Aircraft passive microwave observations of deep atmospheric convection at frequencies between 18 and 183 GHz are presented in conjunction with visible and infrared satellite and aircraft observations and ground-based radar observations. Deep convective cores are indicated in the microwave data by negative brightness temperature, T/(B) deviations from the land background (270 K) to extreme T(B) values below 100 K at 37, 92, and 183 GHz and below 200 K at 18 GHz. These T(B) minima, due to scattering by ice held aloft by the intense updrafts, are well correlated with areas of high radar reflectivity. For this land background case, T(B) is inversely correlated with rain rate at all frequencies due to T(B)-ice-rain correlations. Mean Delta-T between vertically polarized and horizontally polarized radiance in precipitation areas is approximately 6 K at both 18 GHz and 37 GHz, indicating nonspherical precipitation-size ice particles with a preferred horizontal orientation. Convective cores not observed in the visible and infrared data are clearly defined in the microwave observations, and borders of convective rain areas are well defined using the high-frequency (90 GHz and greater) microwave observations.

Adler, Robert F.↗
NASA NTRS1991

Design methodology for a community response questionnaire on sonic boom exposure

A preliminary draft questionnaire concerning community response to sonic booms was developed. Interviews were conducted in two communities that had experienced supersonic overflights of the SR-71 airplane for several years. Even though the overflights had ceased about 6 months prior to the interviews, people clearly remembered hearing sonic booms. A total of 22 people living in central Utah and 23 people living along Idaho/Washington state border took part in these interviews. The draft questionnaire was constantly modified during the study in order to evaluate different versions. Questions were developed which related to annoyance, startle, sleep disturbance, building vibration, and building damage. Based on the data collected, a proposed community response survey response instrument was developed for application in a full-scale sonic boom study.

Farbry, John E., Jr.↗
NASA NTRS1989

Lithology and structure within the basement terrain adjacent to Clark Mountains, California, mapped with calibrated data from the airborne visible/infrared imaging spectrometer

The Clark Mountains in eastern California form a rugged, highly dissected area nearly 5000 ft above sea level, with Clark Mountain rising to 8000 ft. The rocks of the Clark Mountains and the Mescal Range just to the south are Paleozoic carbonate and clastic rocks, and Mesozoic clastic and volcanic rocks standing in pronounced relief above the fractured Precambrian gneisses to the east. The Permian Kaibab Limestone and the Triassic Moenkopi and Chinle Formations are exposed in the Mescal Range, which is the only place in California where these rocks, which are typical of the Colorado Plateau, are found. To the west, the mountains are bordered by the broad alluvial plains of Shadow Valley. Cima Dome, which is an erosional remnant carved on a batholithic intrusion of quartz monzonite, is found at the south end of the valley. To the east of the Clark and Mescal Mountains is found the Ivanpah Valley, in the center of which is located the Ivanpah Play. Studies of the Clark Mountains with the airborne visible/infrared imaging spectrometer are briefly described.

Green, Robert O.↗
NASA NTRS1991

VLA observations of the supernova remnant Puppus A at 327 and 1515 MHz

Very Large Array radio images of Puppis A at 327 and 1515 MHz are presented. The observations were performed with the VLA in the C/D and B/C configurations, respectively. The angular resolution is about 77 arcsec x 43 arcsec. The observed radio shell shows signs of interaction between the expanding shock front and the inhomogeneous surrounding medium. An excellent correlation is found between radio and X-ray emission, mainly toward the NE border of the remnant. There is little correspondence between the optical and radio images, suggesting a different origin for the emission. A region of steeper radio spectral index is associated with the highly decelerated eastern periphery.

Dubner, G. M.↗
NASA NTRS1991

Chronology of early Archaean granite-greenstone evolution in the Barberton Mountain Land, South Africa, based on precise dating by single zircon evaporation

Precise Pb-207/Pb-206 single zircon evaporating ages are reported for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, as well as for granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group are shown to yield ages between 3445 + or - 3 and 3416 + or - 5 Ma and to contain older crustal components represented by a 3504 + or - 4 Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between 3259 + or - 3 Ma in age which are interpreted to reflect the time of crystallization. The comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites are keynoted. The data adduced show that the Onverwacht and Fig Tree felsic units have distinctly different ages and thus do not constitute a single, tectonically repeated unit as proposed by others. It is argued that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones, and that the BGB in the Kaapval craton of southern Africa and greenstones in the Pilbara Block of Western Australia may have been part of a larger crustal unit in early Archaean times.

Kruener, Alfred↗
NASA NTRS1991

Late Noachian development of the Coprates rise, Mars

The Coprates rise forms a 900 km long, north to northeast trending ridge south of Coprates Chasma between long. 56 and 60 degs. Radar and stereo photogrammetric data indicate that the rise is 2 to 4 km above a neighboring trough to the east. The break in slope between the rise and this trough is well defined topographically and in Viking images. In turn, the trough is bordered to the to the east at long. 52 deg by a much gentler rise. West of the Coprates rise, the terrain dips about 0.2 deg to roughly long. 75 deg. The rise and flanking highs were previously interpreted to be tilted fault blocks formed by either Tharsis tectonism or an ancient impact. Results are now reported of a preliminary geologic study that documents Late Noachian growth of the Coprates rise as a asymmetric fold. More comprehensive work will lead to a mechanical analysis of the kinematic development of the rise. It is concluded that the Coprate rise formed during the Late Noachian by 2 to 4 km of asymmetric uplift (steeper on its east flank). The timing is inconsistent with an origin by an early impact, but it coincides in time with early Tharsis centered radial faulting at Syria Planum.

Tanaka, Kenneth L.↗
NASA NTRS1990

European user trial of paging by satellite

British Telecom conceived the idea of adapting their existing paging service, together with the use of existing terrestrial pagers, to yield a one way data (i.e., paging) satellite service to mobiles. The user trial of paging by satellites was successful. It demonstrated that services could be provided over a wide geographical area to low priced terminals. Many lessons were learned in unexpected areas. These include the need for extensive liaison with all users involved, especially the drivers, to ensure they understood the potential benefits. There was a significant desire for a return acknowledgement channel or even a return data channel. Above all there is a need to ensure that the equipment can be taken across European borders and legitimately used in all European countries. The next step in a marketing assessment would be to consider the impact of two way data messaging such as INMARSAT-C.

Fudge, R. E.↗
NASA NTRS1992

Amazonis and Utopia Planitiae: Martian Lacustrine basins

Amazonis and Utopia Planitiae are two large (greater than 10(exp 6) sq. km) basins on Mars having morphological features commonly associated with former lakes. The investigation of these areas is an extension of our previous paleolake studies in the Elysium basin. Using Viking images, we are searching for familiar geologic forms commonly associated with standing bodies of water on Earth. Like Elysium, the two basins exhibit terraces and lineations resembling shorelines, etched and infilled floors with channel-like sinuous markings in places, inflow channels along their borders, and other geomorphic indicators believed to be related to the presence of water and ice. In some areas these features are better displayed than in others where they may be very tenuous; their value as indicators can be justified only by their association with related features. Even though these postulated paleolakes are very young in the Martian stratigraphic sequence, their shoreline features are poorly preserved and they are probably much older than large Pleistocene lakes on Earth.

Scott, David H.↗
NASA NTRS1992

Physical properties of Oxia/Lunae Planum and Arabia-type units in the central equatorial region of Mars

Classification and mapping of surficial units in the central equatorial region of Mars Consortium data identified the relatively dark but 'red' materials that cover Lunae Planum and surround the Meridiani materials of Oxia Palus inertia region of Arabia. Oxia Province consists of the regions dominated by the characteristically dark 'red' Oxia materials, but it includes darker streaks and splotches of relatively 'blue' Meridiani materials and brighter 'red' deposits of dust belonging to Eos Province (the bright 'red' border between Oxia and regional Meridiani deposits to the south) and Xanthe Province (the moderately bright 'red' dust deposits in western Chryse Planitia and its vicinity, including the VL-1 landing site). Two Oxia units were recognized: a darker unit present on Lunae Planum and east of Oxia Palus that have intermediate to high inertias. Oxia Province surrounds the extremely low thermal inertia Arabia Province in the east part of the study area, and occurs as isolated patches within Arabia (often including splotches of Meridiani materials within the Oxia patches). Arabia Province's materials have been widely interpreted as unconsolidated dust deposits which are currently forming at this stage of Mars' precessional climate cycle, although the persistence of stable, moderately strong albedo contracts among Arabia materials has not been addressed in those models. A systematic interpretation of Oxia and Arabia Province materials based on currently available remote sensing data is given.

Strickland, Edwin L., III↗
NASA NTRS1992

Diagnosing coupled jet-streak circulations for a northern plains snow band from the operational nested-grid model

On 17 March 1989, moderate to heavy snow developed in a 100- to 200-km-wide band extending from South Dakota to northern Michigan. The 4- to 8-inch snowfall within this band was not associated with major cyclogenesis, and developed 500 to 600 km north of a stationary surface front. A diagnostic analysis based on an application of the General Meteorological Package (GEMPAK 5.0) to a numerical simulation from the operational nested-grid model (NGM) is utilized to show that the development of this snow band is related to the interaction of two upper-tropospheric jet streaks and their associated transverse circulation patterns. The eastward propagation of a jet streak from the West Coast toward the middle United States and to the south of a slower-moving jet along the U.S.-Canadian border led to a merger of the ascent maxima associated with the direct and indirect circulations of the northern and southern jets, respectively. The snow band developed as the ascending branches of the jet-streak circulation patterns merged, with the eastward propagation of the heaviest snow linked to the motion of the coupled circulation pattern. The study also demonstrates the usefulness of the operational NGM for providing the higher-resolution datasets required to relate the evolution of jet-streak circulation patterns to the development of mesoscale precipitation bands.

Hakim, Gregory J.↗
NASA NTRS1992

New evidence of lacustrine basins on Mars - Amazonis and Utopia Planitiae

Amazonis and Utopia Planitiae are two large basins on Mars that have morphologic features commonly associated with former standing bodies of water. Like Elysium, the basins exhibit terraces and lineations resembling shorelines, etched and infilled floors marked by sinuous channels in places, inflow channels along their borders, and other geomorphic indicators believed to be related to the presence of water and ice. Moreover, most of the shoreline features have consistent elevations of about -1000 m, which suggests that the bodies of water thought to have occupied the basins may once have been connected. Although the concept of large paleolakes in the northern lowlands of Mars might be expanded to include inland seas, it is still premature to advance this hypothesis at the present stage of investigation. Even though these postulated paleolakes are very young in the Martian stratigraphic sequence, they are probably much older than large Pleistocene lakes on earth, and their shoreline features are less well preserved.

Scott, David H.↗
NASA NTRS1992

Preliminary analysis of ERS-1 SAR for forest ecosystem studies

An image obtained by the C-band VV-polarized ERS-1 SAR is examined with respect to potential land applications. A scene obtained near noon on August 15, 1991, along the U.S.-Canadian border near Sault Ste. Marie is calibrated relative to an array of trihedral corner reflectors and active radar calibrators distributed across the swath. Extensive contemporaneous ground observations of forest stands are used to predict the radar backscattering coefficient sigma at the time of the SAR overpass using a first-order vector radiative transfer model. These predictions generally agree with the calibrated ERS-1 data to within 1 dB. It is demonstrated that the dynamic range of sigma is sufficient to perform limited discrimination of various forest and grassland communities even for a single-date observation. Furthermore, retrieval of near-surface soil moisture is feasible for grass-covered soils when plant biomass is less than 1 tonne/ha.

Dobson, M. G.↗
NASA NTRS1992

Ablative subduction - A two-sided alternative to the conventional subduction model

The plausibility of a two-sided fluid-based model of lithospheric subduction that is based upon current views of lithospheric structure is examined. In this model the viscous lower lithosphere flows downward, and the brittle upper lithosphere deforms in passive response. This process is potentially double-sided, since it is found that even a buoyant plate can be dragged downward by a dense descending neighbor. Thus an apparent overriding plate may be worn away by a process of viscous ablation, with the rate of ablation a function of plate buoyancy. This process, called 'ablative subduction,' makes it possible to simply interpret observations concerning slab profiles, interplate seismicity, back arc tectonics, and complex processes such as double subduction and subduction polarity reversal. When experiments modeling the evolution of simple fluid 'slabs' are performed, slab profile is found to be strongly influenced by ablation in the overriding plate. When ablation is weak, as when a buoyant continent borders the trench, deformable slabs adopt shallow Andean-style profiles.

Tao, Winston C.↗
NASA NTRS1992

Mechanism of Muong Nong-type tektite formation and speculation on the source of Australasian tektites

The source crater of the youngest and largest of the tektite strewnfields, the Australasian strewnfield, has not been located. A number of lines of evidence indicate that the Muong Nong-type tektites, primarily found in Indochina, are more primitive than the much more abundant and widespread splash-form tektites, and are proximal to the source. In this study the spatial distribution of Muong Nong-type tektite sites and chemical character have been used to indicate the approximate location of the source. The variation of Muong Nong-type tektite chemical composition appears to be caused by mixing of two silicate rock end-members and a small amount of limestone, and not by vapor fractionation. The variation in composition is not random, and does not support in situ melting or multiple impact theories. The distribution of both Muong Nong and splash-form tektite sites suggest the source is in a limited area near the southern part of the Thailand-Laos border.

Schnetzler, C. C.↗