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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Variations of Martian surface albedo: Evidence for yearly dust deposition and removal

The purpose is to determine the degree, spatial distribution and timing of the deposition and removal of dust storm fallout, and to relate the current patterns of dust deposition and removal to the long-term evolution of the Martian surface. Southern Hemisphere dark areas are found to quickly return to close to their pre-storm albedos, suggesting rapid removal of any dust that was deposited. Northern Hemisphere dark regions are brighter post-storm, but gradually darken to pre-storm levels over the Mars year. In doing so they act as local sources of dust during otherwise clear periods. Dust does not appear to be removed from bright regions, resulting in the 1 to 2 m thick deposits observed today.

Christensen, Philip R.↗
NASA NTRS1986

The geometric albedos of Uranus and Neptune between 2100 and 3350 A

Results are reported from large aperture IUE observations of the stratospheres of Uranus and Neptune. The data were collected using 16 Cyg A and B as solar analog stars. The spectra observed were similar to geometric albedos of the predicted in terms of Rayleigh-Raman scattering in the 2200-2800 A region. A falloff was observed near visible wavelengths, a factor which might be attributable to low-altitude haze absorbing in the 3000-4000 A interval. The data also indicated that the Uranus stratosphere does not strongly absorb UV radiation, and is not a good heating source. The stratospheric haze constituents over Uranus could therefore be condensates of CH4 and H2 photochemistry.

Wagener, R.↗
NASA NTRS1988

Ultraviolet geometric albedo of Uranus

The purpose was to obtain ultraviolet flux measurements of Uranus before and after the January 1986 flyby past Uranus by Voyager. These measurements were used to determine absolute flux distributions in the wavelength range 1950 to 3200 A. This wavelength range is an important region to observe since the model atmospheres for Uranus indicate that the albedo spectrum in this region can tightly constrain the height of the photochemical haze layer.

Neff, John S.↗
NASA NTRS1987

Spectral geometric albedos of the Galilean satellites from 0.24 to 0.34 micrometers - Observations with the International Ultraviolet Explorer

An 8-year IUE satellite spectrophotometric observation campaign's results for the Galilean satellites of Jupiter indicate low albedos for all four, consistent with the distribution of sulfurous materials escaping from the surface of Io to the surfaces of the three other bodies by means of magnetospheric processes. A previously reported UV spectral asymmetry between Europa's leading and trailing hemispheres is confirmed. The greater trailing than leading side opposition surge observed in Europa implies that the trailing side is the less compact of the two. Ganymede exhibits UV spectral asymmetry also, but only at wavelengths greater than 0.28 microns.

Nelson, Robert M.↗
NASA NTRS1987

The size, shape, density, and albedo of Ceres from its occultation of BD+8 deg 471

Photoelectric observations of BD+8 deg 471's November 13, 1984 occultation by Ceres show Ceres to be an oblate spheroid with a 479.6 + or - 2.4 km equatorial radius and 453.4 + or - 4.5 km polar radius. Despite the global appearance of a surface in hydrostatic equilibrium, real limb irregularities are distincly noticeable in the data. Visual geometric albedo is 0.073, and mean density 2.7 (+ or - 5 percent) g/cu cm.

Millis, R. L.↗
NASA NTRS1987

Azimuthal brightness variation and albedo measurements of the Uranian rings

The recent Voyager encounter with Uranus has produced new insights and observational constraints on the structure, composition, and evolution of the Uranian rings. Careful measurements have been made of the brightness variation as a function of longitude for the alpha, beta, gamma, delta, and epsilon rings. These measurements showed a substantial variation in azimuthal brightness because of the varying width of the rings. The single scattering albedo of the epsilon ring particles was found to be 3.9 + or - 0.6 percent (for the Lambert surface) or 2.3 + or - 0.4 percent assuming the 'lunar-type' phase function. The optical depth at apoapse of the epsilon ring was found to be 0.40 + or - 0.05.

Svitek, Tomas↗
NASA NTRS1988

Global color and albedo variations on Io

The present Voyager imaging data multispectral mosaics of Io include global mosaics from each of the Voyager 1 and 2 data sets and a high-resolution mosaic of the region centered on the Ra Patera volcano. The constancy of the disk-integrated color and albedo of Io over recent decades despite volcanic activity may be due to the regular occurrence of large Pele-type plumes with relatively dark, red deposits. Io's intrinsic spectral variability involves continuous variation among three major spectral end members. Attention is given to the mapping of the data into five spectral units for the purposes of comparison with laboratory measurements of Io surface material candidates.

Mcewen, Alfred S.↗
NASA NTRS1987

Mars low albedo regions: Possible map of near-surface

A freeze/thaw desorption mechanism is proposed in certain low albedo areas which could be the factor that instigated dust storms. It is widely accepted that the bulk of the episodic gas evolution (not necessarily the oxygen release) experienced during the humidification process in the Viking Gas Exhange Experiment (GEX) was caused by a familiar process in which more polar H2O molecules replace large quantities of other preadsorbed gas molecules on adsorption sites. The author suggests that a similar process could produce high pore pressures in soil that could disrupt the soil and eject dust at high velocity. The author also argued that association of sites of dust storms initiated with high thermal inertial areas may simply reflect repeated dust depletion.

Huguenin, R. L.↗
NASA NTRS1989

Wavelength dependence of IRAS asteroid diameters and albedos

The 1986 IRAS Asteroid and Comet Survey 'Final Data Product' compilation of albedos and diameters contains a systematic model diameter variation with wavelength for even the brightest asteroids; when averaged over the three bandpasses, this variation can lead to an overestimation of diameters of the order of 5-10 percent. No satisfactory determination has yet been made of the cause of wavelength dependence in the IRAS asteroid data; the present discussion aims only to call attention to this problem, so that data base users will take its limitations into account.

Lebofsky, Larry A.↗
NASA NTRS1989

The surface albedo distribution of Pluto

Two distinct configurations have been established for the surface albedo distribution of Pluto on the basis of a model fitting absolute photometry data obtained over the 1954-1986 period. The two sets of model parameters primarily differ in the equatorial spots; these generate the rotational lightcurve, while the polar caps yield the orbital lightcurve. While the two models predict similar event lightcurves through the end of 1987, they begin to markedly differ in 1988. Mutual event observations made on January 13, 1988 exhibit agreement with only one of the equatorial spot configurations, having one bright and one dark spot.

Buie, Marc W.↗
NASA NTRS1989

Albedo patterns on Triton

A model for seasonal taransport of volatiles based on Trafton (1984) is coupled to considerations of methane photochemistry to infer albedo patterns on Triton's surface at the time of the Voyager encounter. The relatively large southern subsolar latitude at present is predicted to cause the deposition of an extensive fresh frost layer covering approximately the northern half of the planet. Sublimation of Triton's south polar ice has been occurring since 1930, but has been preferentially occurring near the pole. The sublimation of ices in this region may be concentrating dark organic matter on the surface of the ice or exposing layers of this material which have built up in the ice over many seasonal cycles. The south polar cap may be distinctly darker near the pole than around the equatorward edge.

Stansberry, John A.↗
NASA NTRS1989

Phase relations of high albedo asteroids - The unusual opposition brightening of 44 Nysa and 64 Angelina

The present photoelectric lightcurve observations of 44 Nysa at one aspect, and 64 Angelina at both polar aspects, indicate the presence of an opposition spike or brightening of approximately 0.25 mag within several deg of zero-phase angle. Since the three curves are identical within the bounds of observational scatter, the opposition spike cannot be judged anomalous within the asteroid taxonomic class E. The general similarity of the observed phase curves to those of the Uranian satellites and the rings of Saturn supports the status of the spike as an ordinary property of moderate-to-high albedo atmosphereless surfaces.

Harris, A. W.↗
NASA NTRS1989

Cosmic-ray neutron albedo dose in low-earth orbits

An earth albedo neutron environmental model is proposed which provides a way to estimate neutron exposure in low-earth orbit. It is shown that, in the predominantly low inclination orbits (i=28.5 deg) used in the U.S. space program, the neutron exposures are relatively low (0.7 cSv/y). The neutron exposures are more significant for polar orbital missions and even high inclination missions, such as Skylab (i=57 deg).

Wilson, J. W.↗
NASA NTRS1989

Miranda - Color and albedo variations from Voyager photometry

Voyager 0.48-micron images of Miranda covering phase angles of 13-39 deg, together with 0.35-micron 0.41-micron, and 0.56-micron data at 16-deg phase angle, are the bases of the Minnaert parameters from which the present albedo/color maps are derived. Color ratios are found to be essentially constant over all terrain types and albedo ranges on the satellite; spectrally, Miranda is gray. The color data are noted to conform with a model in which varying amounts of materials spectrally resembling the asteroidal F-material are mixed with an icy component.

Hillier, J.↗
NASA NTRS1990

The nature of low-albedo asteroids from 3-micron multi-color photometry

The present broadband and narrowband 1.2-3.5 micron spectrophotometry of 16 low-albedo asteroids encompasses C-, F-, P-, D-, and T-class asteroids, and has been used in an effort to identify low-temperature minerals on their surfaces. Attention has been given to the identification of water of hydration. While G-class asteroids have water-rich surfaces, and I asteroids possess some hydrated silicates, not all class Cs do. Finally, the P-, F-, and T-class asteroids do not seem to possess hydrated-silicate surfaces.

Lebofsky, Larry A.↗
NASA NTRS1990

Color and albedo of the south polar layered deposits on Mars

Five color/albedo units, including polar frost, have been recognized and mapped in the southern layered deposits on Mars. Atmospheric dust scattering was measured in shadows and modeled in order to remove the component of brightness in Mars images due to the atmosphere and quantify the albedo and color of the surface. The layered deposits appear to be mantled by red dust, except where eolian stripping has exposed the underlying bedrock. Dark material has been deposited in topographic depressions in much of the south polar region, including the layered deposits. The available observational data suggest that the layered deposits are composed of bright dust, ice, and a small amount of dark material. If the dark material is sand, a periodic change in polar winds seems required in order to transport the sand poleward into the layered terrain. In any case, the observations are not consistent with the layered deposits being composed only of bright dust and ice.

Herkenhoff, Ken E.↗
NASA NTRS1989

Asteroid magnitudes, UBV colors, and IRAS albedos and diameters

This paper lists absolute magnitudes and slope parameters for known asteroids numbered through 3318. The values presented are those used in reducing asteroid IR flux data obtained with the IRAS. U-B colors are given for 938 asteroids, and B-V colors are given for 945 asteroids. The IRAS albedos and diameters are tabulated for 1790 asteroids.

Tedesco, Edward F.↗
NASA NTRS1989

The low energy atmospheric antiproton albedo

The flux of albedo antiprotons in the 100-1000 MeV kinetic energy range produced by the cosmic ray primaries in the atmosphere is calculated. It is shown that this is not a significant background to measurements of the low energy anti-proton cosmic ray flux.

Cole, J. B.↗