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At least 1,243 records · Page 69

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NASA NTRS1986

Speckle interferometry of asteroids. III - 511 Davida and its photometry

Photometric interferometry data on the fifth largest minor planet, 511 Davida, are used to generate orbital elements and physical dimensions. The Seward 2.3 m telescope was employed to obtain the speckle interferometric data. The asteroid was assumed to be a uniform-surfaced triaxial ellipsoid rotating about its shortest axis. A simultaneous amplitude-magnitude aspect method is defined for determining the pole position of the object, which is calculated to be within 4 deg of (307 deg; +32). A weighted amplitude aspect relation provides similar data. The albedos are estimated to lie within the range 0.24-0.42, and an a/b axial ratio of 0.97-1.66 is projected. The variations in the photometric albedos are taken as evidence of a nonuniform surface, a conclusion similar to that reached after earlier photometric measurements of the asteroid Herculina.

Drummond, J. D.↗

NASA NTRS1986

Oceanic cloud feedbacks on earth radiation budget parameters

Oceanic cloud variability and sensitivity to sea surface temperature (SST) were examined using radiometer data gathered with instrumentation on the Nimbus-7 satellite. The study area was a region of the Pacific Ocean north of Hawaii. SST and albedo data for the period 1978-1983 were compared and similar trends were found for minimal values of the albedo and SST.

Chertock, B.↗

NASA NTRS1986

Comparison of Nimbus 7 and ERBE radiation budget measurements

Correlations were sought between data sets on exitance radiance and albedos sensed by the Earth Radiation Budget Experiment (ERBE) and the Nimbus-7 spacecraft instruments. The emitted exitance (W/sq m) and the albedos measured over the ocean and the land and diurnal global averages are compared. The 5 W/sq m average difference between noon-midnight recorded by the ERBE spacecraft showed that noon-midnight averages can not be used with ERBE data as they have been with Nimbus-7 data.

Campbell, G. G.↗

NASA NTRS1986

Cloud cover estimation using bispectral satellite measurements

An algorithm has been developed for cloud cover estimation using bispectral satellite measurements. Based on the distribution of pixels in albedo-brightness temperature space, a number of threshold values are applied to identify those pixels which are most likely totally cloud filled. Mean cloudy-column albedo in a region much larger than a single pixel is then estimated and cloud cover computed. The algorithm has been applied to the International Satellite Cloud Climatology Project Geostationary Meteorological Satellite B2 data. Locations of tropical convective cells and the passage of fronts in higher latitudes are identified. Since these features represent the states of large-scale atmospheric circulations, it is concluded that the algorithm can yield consistent cloud data sets useful for climate studies.

Chou, M.-D.↗

NASA NTRS1987

Recent mafic volcanism on Mars

The evidence for volcanism on Mars is commonly accepted, but none has been documented in the Valles Marineris equatorial rift system. A recent survey of the troughs in this valley revealed dark patches that are interpreted to be volcanic vents. The configuration and association of these patches with tectonic structures suggest that they are of internal origin; their albedo and color ratios indicate a mafic composition; and their stratigraphic position, crispness of morphologic detail, and low albedo imply that they are young, perhaps even recent.

Lucchitta, B. K.↗

NASA NTRS1986

Narrow- and broad-band satellite measurements of shortwave radiation - Conversion simulations with a general circulation model

The Oregon State University/Lawrence Livermore National Laboratory general circulation model has been employed as a vehicle for suggesting and exploring various means of converting narrow-band measurements of reflected solar radiation from the earth-atmosphere system to broad-band quantities. For purely illustrative purposes within the model's solar radiation routine, a narrow-band filter function consisting of a square-wave window extending from 0.5 to 0.9 microns is adopted. A limitation of the model, for this sort of endeavor, is that it does not include the wavelength dependence of surface albedos. Nevertheless, the model simulations tend to mimic the calibration of a narrow-band instrument, utilizing reflected solar radiation from the earth-atmosphere system as simultaneously measured by a collocated broad-band instrument; for the model, however, this is done in terms of fluxes, in contrast to instrument-measured radiances. The model results suggest that it might be preferable to perform narrow- to broad-band conversions in terms of planetary albedo (or an equivalent quantity), rather than in terms of reflected fluxes or radiances. Further improvement is achieved if, for instruments that can differentiate between clear and overcast conditions, separate clear and overcast calibrations are performed.

Cess, Robert D.↗

NASA NTRS1987

The bare nucleus of comet Neujmin 1

Simultaneous visible and infrared observations of comet P/Neujmin 1 1984c are presented which show that the comet has a large (mean radius 10 km), dark (geometric albedo 2-3 percent) nucleus with a surface which is mostly inert material but which still shows a low level of gaseous activity. This is the first physical evidence that cometary nuclei can leave behind an inert body after the coma activity ceases. No asteroid or asteroid class has been found to match the reflectance and albedo of this comet except possibly some D asteroids.

Campins, Humberto↗

NASA NTRS1987

Determination of the scaled optical thickness of clouds from reflected solar radiation measurements

A method for determination of scaled optical thickness from reflected solar radiation measurements is described in which measurements of the reflected function are compared with asymptotic expressions for the reflection function of optically thick layers. Analytic formulas are derived which show the dependence of the reflection and transmission functions of nonabsorbing atmospheres on cloud optical thickness, ground albedo, and the asymmetry factor. An expression is derived which shows that the ground albedo produces a constant bias in the derived optical thickness, regardless of the value of the measured reflection function. Results from an analysis of high-resolution images, obtained in Oklahoma, of the reflection function of clouds shows the impact of details of the single scattering phase function on the derived optical thickness.

King, Michael D.↗

NASA NTRS1987

Estimation of X-band scattering properties of tree components

An X-band FM-CW very fine range resolution scatterometer was used to acquire backscattering data for a number of tree species. Using a model to describe the scattering source function and an experimental procedure for selected removal of plant parts allows the estimation of the volume backscatter coefficient and the volume extinction coefficient. It is found that: (1) leaves are strong attenuators as well as scatterers; (2) the albedo at a given angle of incidence is nearly independent of the tree type; (3) the tree limbs are good attenuators but rather poor scatterers; and (4) the albedo changes as a function of the angle of incidence and for deciduous trees is also a function of the season.

Pitts, D. E.↗

NASA NTRS1987

Estimating scattered and absorbed radiation in plant canopies by remote sensing

Several research avenues are summarized. The relationships of canopy characteristics to multispectral reflectance factors of vegetation are reviewed. Several alternative approaches for incorporating spectrally derived information into plant models are discussed, using corn as the main example. A method is described and evaluated whereby a leaf area index is estimated from measurements of radiation transmitted through plant canopies, using soybeans as an example. Albedo of a big bluestem grass canopy is estimated from 60 directional reflectance factor measurements. Effects of estimating albedo with substantially smaller subsets of data are evaluated.

Daughtry, G. S. T.↗

NASA NTRS1988

Angular radiation models for Earth-atmosphere system. Volume 1: Shortwave radiation

Presented are shortwave angular radiation models which are required for analysis of satellite measurements of Earth radiation, such as those fro the Earth Radiation Budget Experiment (ERBE). The models consist of both bidirectional and directional parameters. The bidirectional parameters are anisotropic function, standard deviation of mean radiance, and shortwave-longwave radiance correlation coefficient. The directional parameters are mean albedo as a function of Sun zenith angle and mean albedo normalized to overhead Sun. Derivation of these models from the Nimbus 7 ERB (Earth Radiation Budget) and Geostationary Operational Environmental Satellite (GOES) data sets is described. Tabulated values and computer-generated plots are included for the bidirectional and directional modes.

Suttles, J. T.↗

NASA NTRS1987

Recent comet impacts on the moon - The evidence from remote-sensing studies

Multispectral images and near-infrared reflectance spectra were obtained for the Reiner Gamma Formation, a swirllike albedo and magnetic anomaly in the western portion of the lunar near side, as well as for nearby geologic units in Oceanus Procellarum, using the University of Hawaii 2.24-m telescope at the Mauna Kea Observatory. The spectra of the high albedo swirl material are closely matched by models incorporating 90-95 percent fresh mare basalt with 5-10 percent highlands debris.

Bell, Jeffrey F.↗

NASA NTRS1987

IRAS observations of the Pluto-Charon system

High-signal-to-noise-ratio observations of the Pluto-Charon system at 25, 60, and 100 microns using IRAS are combined with visual-magnitude and mutual-eclipse constraints to evaluate thermal models of Pluto and Charon. These models are consistent with eclipse observation by Dunbar and Tedesco (1986) but not with Reinsch and Pakull (1987). The most likely model for Charon is the standard asteroid model, typical for the icy Galilean and Saturnian satellites. Charon models with a significant atmosphere can be ruled out. Based on currently available radius and albedo constraints, no significant numerical distinction is possible between Pluto models ranging from isothermal spheres with surface emissivity between 0.4 and 0.9. Concerns regarding the viability of an emissivity as low as 0.4 favor the higher-emissivity models. The globally uniform surface temperature of Pluto may thus at present be as low as 45 K, with a methane column abundance of 6.7 cm atm. The most likely models are centered on radii of 1180 and 747 km and albedos of 0.47 and 0.26 for Pluto and Charon, respectively.

Aumann, H. H.↗

NASA NTRS1987

Spectrophotometry of Pluto-Charon mutual events - Individual spectra of Pluto and Charon

Time-resolved spectra of the March 3 and April 4, 1987 mutual events of Pluto and Charon, obtained with spectral coverage from 5500 to 10,000 A with 25-A spectral resolution, are discussed. Charon has a featureless reflectance spectrum, with no evidence of methane absorption. Charon's reflectance appears neutral in color and corresponds to a geometric albedo of about 0.37 at 6000 A. The Pluto reflectance spectrum displays methane absorption bands at 7300, 7900, 8400, 8600, and 8900 A and is red in color, with a geometric albedo of about 0.56 at 6000 A.

Sawyer, S. R.↗

NASA NTRS1988

Speckle interferometry of asteroids. IV - Reconstructed images of 4 Vesta

The images of 4 Vesta obtained by means of speckle interferometric observations on November 16 and 17, 1983, support a 5-hr 20.5-min rotational period. The 4 Vesta visible light curve was approximately matched by the placing of circular 'spots' with 135-km diameter over the three dark and three bright features, followed by the assigning of albedos of 0 to the dark spots and 2 to the bright. The light curve is found to be primarily defined by albedo structure, rather than shape; this results in one maximum and one minimum/rotation, rather than the two of each that would be associated with 4 Vesta's triaxial ellipsoid morphology.

Drummond, J.↗

NASA NTRS1987

Satellites of Uranus - Disk-integrated photometry from Voyager imaging observations

Voyager 2 imaging observations over a wide range of phase angles are used to determine the fundamental photometric parameters for the five largest satellites of Uranus. Over the spectral range covered by Voyager cameras (approximately 350-600 nm) the disk-averaged colors are moderately gray (no redder than the spectrum of Saturn's satellite Phoebe). Geometric albedos range from 0.19 for Umbriel to 0.40 for Ariel. Phase coefficients determined generally between phase angles of 10 and 60 deg vary from 0.021 mag/deg for Ariel to 0.028 mag/deg for Miranda. Phase integrals lie in the range of 0.5-0.65. The Bond albedos are about 0.1 for Umbriel and about 0.2 for the other satellites.

Veverka, J.↗

NASA NTRS1987

Voyager 2 photopolarimeter observations of the Uranian satellites

The surfaces of the principal Uranian satellites are characterized on the basis of UV and IR geometric albedos, phase curves, and phase coefficients obtained in full-disk photopolarimetric observations during the Voyager 2 encounter with Uranus in January 1986. The data are presented in tables and graphs and found to be consistent with a heavily cratered terrain and loosely packed regolith. The Bond albedos are calculated as 0.22 + or - 0.1 for Ariel, 0.07 + or - 0.05 for Umbriel, 0.16 + or - 0.12 for Titania, and 0.19 + or - 0.22 for Oberon. The characteristics of the Uranian satellites indicate compositions (and probably formation conditions and surface-modification mechanisms) distinct from those of the Saturnian and Jovian satellites.

Nelson, Robert M.↗

NASA NTRS1988

Characterization of the Nimbus-7 SBUV radiometer for the long-term monitoring of stratospheric ozone

Precise knowledge of in-orbit sensitivity change is critical for the successful monitoring of stratospheric ozone by satellite-based remote sensors. This paper evaluates those aspects of the in-flight operation that influence the long-term stability of the upper stratospheric ozone measurements made by the Nimbus-7 SBUV spectroradiometer and chronicles methods used to maintain the long-term albedo calibration of this UV sensor. It is shown that the instrument's calibration for the ozone measurement, the albedo calibration, has been maintained over the first 6 yr of operation to an accuracy of approximately + or - 2 percent. The instrument's wavelength calibration is shown to drift linearly with time. The knowledge of the SBUV wavelength assignment is maintained to a 0.02-nm precision.

Cebula, Richard P.↗