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At least 433 records · Page 24

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NASA NTRS1985

Classification of Circular Features on Venus

Among the unanswered questions concerning Venus are the age of its surface and the mechanisms of lithospheric heat transfer (conduction, plate recycling, and hot spot volcanism). If there is a large population of impact craters, then the surface is ancient and Venus is characterized by conduction like the Moon, and Mercury, rather than plate recycling and hot spot volcanism. Alternatively, if there is a large population of volcanic craters, then the surface is younger and other mechanisms of heat transfer likely dominate. Previous studies have emphasized various aspects of the observational, theoretical, experimental, and comparative planetological studies of cratering on Venus, and several have reached divergent opinions concerning the age of the Venus surface. A major source of uncertainty in previous studies is the possible inclusion of circular features of nonimpact (volcanic or tectonic) origin in the so called impact crater population. A classification scheme of circular features on Venus is developed in order to further distinguish their origin and distribution.

Stofan, E. R.↗

NASA NTRS1984

Thermal infrared satellite data for the study of tectonic features

Multispectral thermal infrared data from the NOAA-7 Advanced Very High Resolution Radiometer (AVHRR) have been processed to evaluate their potential for mapping tectonic features. Day/night ratio data from 3 thermal bands between 3.7 and 12.0 microns were used to construct a thermal infrared color image that emphasizes heat retention differences in surface materials as a function of wavelength. The study area is an arid region of the Arabian Peninsula where lithologic units are widely exposed to satellite detectors. In spite of the coarse resolution of 1.1 km at nadir, the image of emitted radiation data contains prominent features that appear to relate to fault systems, unmapped extensions of fault systems, and drainage patterns that are lacking in AVHRR reflected radiation data.

Andre, C. G.↗

NASA NTRS1984

Use of Landsat-derived profile features for spring small-grains classification

The present model for the temporal behavior of agricultural greenness is applied to the extraction of Landsat-derived profile features, distinguishing small from large grain crops. An additional feature derivable from the temporal behavior of the ratio of greenness to brightness is noted which aids in the separation of crops from other vegetation. A limited training set of 20 pure pixels/class, obtained from ground data, is subjected to the Ho-Kashyap (1965) linear classifier. The initial correct classification value for pure pixels of about 85 percent drops to 75 percent for all Landsat pixels.

Badhwar, G. D.↗

NASA NTRS1983

Observation of an absorption feature in a gamma ray burst spectrum

A gamma ray burst was detected on March 25, 1978 by the High Energy X-ray and Low Energy Gamma Ray Experiment on HEAO-1. The burst spectrum shows an absorption feature at 55 + or - 5 keV with an equivalent width of 13 + or - 3 keV, values commensurate with those of similar features observed by the KONUS experiment. The burst spectrum also is characterized by a hard component extending from about 0.25-6 MeV. This component can be interpreted in terms of a fireball model for gamma ray bursts, which places the distance to the source at 1 kpc. The integrated fluence of the burst between 0.025 and 6 MeV is 1.5 x 10 to the -5th ergs/sq cm. The burst source has been localized to within a degree of RA = 237.5 deg and Dec = 76.2 deg.

Hueter, G. J.↗

NASA NTRS1985

Laboratory identification of the emission features near 3.5 microns in the pre-main sequence star HD 97048

This paper presents the results of a laboratory study of formaldehyde (H2CO) suspended in low temperature molecular matrices with compositions similar to what may be found in the 'dirty ice' mantles of grains. It is shown that the emission features near 3.5 microns in the pre-main-sequence star HD 97048 can be matched by a mixture of chemical complexes of H2CO with surrounding molecules in the grain. Furthermore, a discussion is presented of various possible excitation mechanisms for this emission. The conclusion is, that for the features near 3.5 microns in HD 97048, UV pumped IR fluorescence is the most likely mechanism.

Van Der Zwet, G. P.↗

NASA NTRS1985

Interstellar absorption features toward the compact infrared source W33A

A high-resolution (lambda/Delta lambda of approximately 2000) broad-bandwidth (3.3-5.0 microns) spectrum of the compact infrared source W33A is presented. Four absorption features are associated with compositionally distinct components in the interstellar medium: solid-phase C3, CN, or CH3OH at 4.9 microns, solid-phase CO and either solid or gaseous CH3NC near 4.6 microns and the well-known dirty ice feature near 3 microns. The analysis of this spectrum supports chemically complex models of grain mantle evolution in cold, quiescent molecular clouds.

Larson, H. P.↗

NASA NTRS1986

The use of multidate multichannel radiance data in urban feature analysis

Two images were obtained from thematic mappers on Landsats 4 and 5 over the Washington, DC area during November 1982 and March 1984. Selected training areas containing different types of urban land use were examined,one area consisting entirely of forest. Mean digital radiance values for each bandpass in each image were examined, and variances, standard deviations, and covariances between bandpasses were calculated. It has been found that two bandpasses caused forested areas to stand out from other land use types, especially for the November 1982 image. In order to evaluate quantitatively the possible utility of the principal components analysis in selected feature extraction, the eigenvectors were evaluated for principal axes rotations which rendered each selected land use type most separable from all other land use types. The evaluated eigenvectors were plotted as a function of land use type, whose order was decided by considering anticipated shadow component and by examining the relative loadings indicative of vegetation for each of the principal components for the different features considered. The analysis was performed for each seven-band image separately and for the two combined images. It was found that by combining the two images, more dramatic land use type separation could be obtained.

Duggin, M. J.↗

NASA NTRS1986

Airborne spectroradiometry: The application of AIS data to detecting subtle mineral absorption features

Analysis of Airborne Imaging Spectrometer (AIS) data acquired in Australia has revealed a number of operational problems. Horizontal striping in AIS imagery and spectral distortions due to order overlap were investigated. Horizontal striping, caused by grating position errors can be removed with little or no effect on spectral details. Order overlap remains a problem that seriously compromises identification of subtle mineral absorption features within AIS spectra. A spectrometric model of the AIS was developed to assist in identifying spurious spectral features, and will be used in efforts to restore the spectral integrity of the data.

Cocks, T. D.↗

NASA NTRS1986

Atmospheric-water absorption features near 2.2 micrometers and their importance in high spectral resolution remote sensing

Selective absorption of electromagnetic radiation by atmospheric gases and water vapor is an accepted fact in terrestrial remote sensing. Until recently, only a general knowledge of atmospheric effects was required for analysis of remote sensing data; however, with the advent of high spectral resolution imaging devices, detailed knowledge of atmospheric absorption bands has become increasingly important for accurate analysis. Detailed study of high spectral resolution aircraft data at the U.S. Geological Survey has disclosed narrow absorption features centered at approximately 2.17 and 2.20 micrometers not caused by surface mineralogy. Published atmospheric transmission spectra and atmospheric spectra derived using the LOWTRAN-5 computer model indicate that these absorption features are probably water vapor. Spectral modeling indicates that the effects of atmospheric absorption in this region are most pronounced in spectrally flat materials with only weak absorption bands. Without correction and detailed knowledge of the atmospheric effects, accurate mapping of surface mineralogy (particularly at low mineral concentrations) is not possible.

Kruse, F. A.↗

NASA NTRS1986

Moderate spectral resolution observations of 3 micron absorption features in highly obscured objects

The 3 micron absorption spectra of sources seen in or behind molecular clouds generally show a variety of absorption features. Three separate absorptions are used to explain these features. The cooled-grating array spectrometer (CGAS) at the NASA Infrared Telescope Facility was used to obtain spectra of the late-type mass-loss star OH 0739-12 and the protostars MonR2 IRS-2 and IRS-3 (solid circles). The differences between the spectra are discussed.

Smith, R. G.↗

NASA NTRS1986

Features and technologies of ERS-1 (ESA) and X-SAR antennas

Features and technologies of planar waveguide array antennas developed for spaceborne microwave sensors are described. Such antennas are made from carbon fiber reinforced plastic (CFRP) employing special manufacturing and metallization techniques to achieve satisfactory electrical properties. Mechanical design enables deployable antenna structures necessary for satellite applications (e.g., ESA ERS-1). The slotted waveguide concept provides high aperture efficiency, good beamshaping capabilities, and low losses. These CFRP waveguide antennas feature low mass, high accuracy and stiffness, and can be operated within wide temperature ranges.

Schuessler, R.↗

NASA NTRS1986

A detected feature at the expected wavelength for the HD R5(0) line in Jupiter's and Uranus' atmospheres

A feature was detected at the expected wavelength for the HD R5(0) line in Jupiter's and Uranus' atmospheres. An upper limit for Neptune was also found. Added to the earlier detection of a similar feature for Saturn, it is proposed that all evidence from this type of measurement can be interpreted as arising from a D/H ratio of about .0004 for all the major planets. This value is not in agreement with measurements from CH3D transitions, and is at least fifty times the accepted interstellar medium value of 5 x .000001, implying deuterium enhancement in the solar system via fractionation in the proto-solar nebula.

Smith, Wm. Hayden↗

NASA NTRS1986

The Saturnian ribbon feature: A baroclinically unstable model

Using measurements made by the Voyager spacecraft, an oscillatory feature in the northern midlatitudes of Saturn is examined. Measurements made by the imaging and infrared instruments are used to estimate its horizontal wavelength and vertical extent. Some of these characteristics suggest that the feature could be due to baroclinic instability. A numerical model is described of such an instability with parameters based upon the Voyager observations, and using the lower boundary condition developed by Gierasch et al for the Jovian planets.

Godfrey, D.↗

NASA NTRS1986

Field study of the effect of orographic features on planetary boundary layer models

A flight program to measure the influence of orographic features on turbulent momentum, heat, and moisture fluxes was carried out. Characteristic wake flow patterns were observed in laboratory studies. While the upper level winds (above 10,000 ft msl) blow from west to east (perpendicular to the mountain range), flow patterns similar to flow from a backward-facing step are observed. When the upper level winds blow at 301 and 312 deg from true north (slightly oblique to the mountain range), experimental evidence suggests flow patterns typical of a vortex shed from the mountain range similar to a leading edge vortex from a delta wing aircraft. The observed flow patterns are analyzed. It is concluded that wake flow very similar to those observed in laboratory facilities can occur due to winds over mountain ranges. Results of studies in the laboratory may shed considerable insight to atmospheric boundary layer winds obstructed by orographic features.

Frost, Walter↗

NASA NTRS1987

Lithospheric structure on Venus from tectonic modelling of compressional features

In previous studies, extensional models were used that incorporated realistic rheologies in order to constrain lithospheric structure. Lithospheric modelling is considered herein from the standpoint of compressional deformation. Features of presumed compressional tectonic origin are reviewed and a model for compressional folding based on lithospheric strength envelopes are presented that include the effects of both brittle and ductile yielding as well as finite elastic strength. Model predictions are then compared with the widths and spacings of observed tectonic features and it is concluded that the results are consistent with a thin crust overlying a relatively stronger mantle, with thermal gradients probably in the range of 10 to 15 deg/km.

Banerdt, W. B.↗

NASA NTRS1986

General features of the Comet Halley-solar wind interaction from plasma measurements

The Giotto Complete Positive ion, Electron and Ram Negative Ion measurements near Comet Halley plasma experiment identified regions in which the solar wind interaction with the cometary plasma displayed characteristic features. Beginning 4.6 million km from the comet there is an upstream region with sporadic connection to the comet. An electron foreshock is present up to 250,000 km away from the bow shock. A bow shock is detected at 1.15 million km. Between the bow shock and the cometopause the outer regions can be divided into three parts. A cometopause is found at 135,000 km, and a density decrease is detected at 45,000 km from the comet. The detailed plasma features associated with these regions are described.

Reme, H.↗

NASA NTRS1986

A Fourier-based textural feature extraction procedure

A procedure is presented to discriminate and characterize regions of uniform image texture. The procedure utilizes textural features consisting of pixel-by-pixel estimates of the relative emphases of annular regions of the Fourier transform. The utility and derivation of the features are described through presentation of a theoretical justification of the concept followed by a heuristic extension to a real environment. Two examples are provided that validate the technique on synthetic images and demonstrate its applicability to the discrimination of geologic texture in a radar image of a tropical vegetated area.

Stromberg, W. D.↗

NASA NTRS1986

Linear volcanic features at Alba Patera, Mars - Probable spatter ridges

A number of extensive linear constructional features occur on the north and east flanks of Alba Patera. They are found in a 150-km-wide zone and comprise aligned ridge elements that are locally associated with short volcanic flows. They are among the youngest volcanic features in the region and are believed to be Martian analogs of terrestrial spatter ridges. On the basis of measurements made on Viking images, it appears that active volcanic fissures on Mars were larger than their terrestrial counterparts.

Cattermole, Peter↗