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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Some characteristic differences in the earth's radiation budget over land and ocean derived from the Nimbus-7 ERB experiment

Broad spectral band data derived from the Nimbus-7 Earth Radiation Budget experiment are analyzed for the top-of-the-atmosphere noon vs. midnight variations in the exitant longwave flux density, spectral variations in the regional albedos, and differences in land and ocean net radiation budgets. The data were studied for a year (June 1979 to May 1980) on a global scale and for five selected study areas. The annual global total, near-UV visible, and near-IR albedo values, obtained were 30.2, 34.6, and 25.9, respectively, with marked differences in behavior between oceanic and continental regions. Over the continents, clouds and snow sharply decreased the near-IR albedo. The over-the-continent noon-emitted flux density averages were 15-25 W/sq m larger than the midnight values, with large regional and seasonal variations. Over the oceans, the average noon and midnight outgoing longwave-flux densities were nearly identical, with regional aqnd seasonal differences of several watts per square meter.

Kyle, H. L.↗
NASA NTRS1986

Thermal-infrared and visual imaging of comet Giacobini-Zinner

Thermal-infrared and visual images obtained simultaneously of comet Giacobini-Zinner on UT 1985 August 4.4 are presented. The 10.8-micron map, which spans several arc minutes with 8-arcsec resolution, is the first ground-based infrared image of a comet. Considerable spatial structure is evident in the map, including a curved dust tail consisting primarily of dust grains larger than 100 microns. The width of this large-grain tail implies that these grains depart the nucleus with velocities less than 3 m/s. The distribution of grain albedo is determined for the central 1 arcmin. It is speculated that an observed increase in the albedo by a factor of 2 with distance from the nucleus and prolonged radius vector results from the confinement to the orbital plane of large fluffy grains previously considered for GZ by others; the lower albedo of this dust could result from multiple internal scattering increasing the probability of absorption of incident light. It is estimated that dust was departing the nucleus at the rate of about 8 x 10 to the 5th g/s, comparable to that derived for many other comets.

Telesco, C. M.↗
NASA NTRS1987

Satellite sensing of aerosol absorption

A method is developed for remote sensing of aerosol absorption from satellite images of the earth's surface. The method is based on the measurement of the change in the upward radiances between a clear and a hazy day over a varying surface reflectance. For a zero change balance between brightening due to scattering and darkening due to absorption and scattering is reached. This balance is utilized via a radiative transfer model to derive the aerosol single-scattering albedo. A sensitivity study is performed, and the method is tested against laboratory measurements. It is suggested that for the case of haze introduced on top of an existing background aerosol and with a fair estimate of the scattering phase function, the error in the remotely sensed single-scattering albedo is in the range of 0.03-0.05. The main errors in the method arise from variations in the surface reflectance between the clear and the hazy days, uncertainty in the scattering phase function, and variation of the aerosol and gaseous absorption between these two days. If the satellite calibration varies with time, the measurements of single-scattering albedo can be substantially affected.

Kaufman, Yoram J.↗
NASA NTRS1987

Spacecraft external heating variations in orbit

The results of an evaluation of variations in on-orbit external heating of spacecraft surfaces are presented. The analyzed time-varying components are the direct solar, the latitude-dependent shortwave reflected, and the latitude-dependent longwave planetary-emitted radiation. Differences in instantaneous heating rates at a given latitude among the different model environments are found to be much larger than are differences in orbital mean heating rates. The discrepancies are found to increase with the orbit inclination angle. The ratio of Northern Hemisphere winter-to-summer orbital mean heating rates for a nadir-facing node is near unity and shows only a slight dependence on large variations in node absorptance. Mean orbital albedo and emission values for low-inclination orbits are found to differ significantly from global values obtained in meteorological studies. Algorithms relating mean orbital albedos and emissions to orbit inclination are developed. The relative importance of more realistic nondiffuse over diffuse reflections is evaluated for spacecraft heating and found to be negligible. The sun-spacecraft-earth angular configuration is such that, for reception of the peak intensities in the nondiffuse reflectance angular distribution, the albedo heating rate is more than adequately suppressed by an extremely small view factor from the spacecraft to the sunlit 'crescent' earth.

Leffler, J. M.↗
NASA NTRS1987

Climatic consequences of anthropogenic vegetation changes from 1880-1980

The changes in the surface albedo associated with land-use alterations from 1880-1980 and the climatic implications of these changes are discussed. Changes in atmospheric CO2, the hydrologic cycle, atmospheric particulates and aerosols are reviewed. A global survey of the growth of agriculture and forestry data is presented. The anthopogenic vegetation changes in western Africa are examined in detail. Maps and tables of anthopogenic vegetation and albedo changes are presented for western Africa, South America, and worldwide. It is found that the mean regional albedo increased from 17.4 in 1880 to 17.8 in 1980. No apparent secular decrease in precipitation linked to vegetation clearing is observed. It is suggested that disruption of the hydrologic cycle as a result of devegetation may have led to reductions in soil moisture.

Gornitz, Vivien↗
NASA NTRS1988

Study of the consistency of climatological products of Nimbus-7

The study, in addition to investigating the consistency of climatological products from Nimbus-7 Earth Radiation Budget and Temperature Humidity Infrared Radiometer experiments, focussed on the climatological analysis of the specified regions of the Earth. The climatological study consisted of the effects of various types of clouds on the net radiation, albedos, and emitted radiation. In addition to a correlational study for determining consistency level of data, a population study of the regions was formulated and conducted. The regions under this study were formed by clustering the target areas using the criteria of climatological conditions such as geography, ocean, and land. Research is limited to tropics from 18 deg north to 18 deg south. A correlational study indicates that there is high positive correlation between high clouds and albedo, and a reduced negative correlation between albedo and net radiation.

Dhuria, Harbans L.↗
NASA NTRS1988

Physical properties of asteroids

Infrared photometry at 1.2, 1.6 and 2.2 micrometer provides a relatively rapid and accurate method for the classification of asteroids and is important for comparison with laboratory measurements of meteorites and other possible compositional analogues. Extension beyond the visual is espicially useful for minerals which have strong characteristic infrared colors such as olivine in the A class asteroids. Radiometry at long infrared wavelengths is important for deriving basic physical parameters (via thermal models) such as size and albedo which in turn enables the conversion of relative colors to absolute reflectances. In particular, albedos are the only way to distinguish among the otherwise ambiguous E, M and P classes of asteroids. Infrared observations of 15 asteroids were made at the NASA infrared Telescope Facility (IRTF) on Mauna Kea in 1987. Researchers completed the analysis of 22 Aten, Apollo and Amor asteroids. Results include albedos and diameters for these objects as well as the identification of the first known class M and Class E near-Earth asteroids. The standard thermal model appears to be inadequate for some of these small asteroids because of their coarse regolith, so researchers constructed a rotating thermal model for such asteroids. They have identified a subtle systematic difference between the sub-populations of large and small IRAS asteroids as well as several anomalous objects.

Veeder, Glenn J.↗
NASA NTRS1989

Infrared photometry of comet Wilson (1986l) at two epochs

1-20 micron filter photometry of comet Wilson (1986l) was obtained on May 29 - June 2, 1987 (r = 1.36 AU) and January 11 -14, 1988 (r = 3.75 AU) at the NASA Infrared Telescope Facility. The thermal emission at 1.36 AU has been fit with a model for a size distribution of small absorbing grains; the size and the optical properties of the dust (color, albedo, and thermal emission spectrum) appear typical of short period comets at similar r. No 10 micron silicate feature was evident. The dust-production rate on June 1 was about 5 x 10 to the 5th g/s. The geometric albedo of comet Wilson at r = 3.75 AU appears to be higher than that of comets measured at smaller r, after phase effects have been accounted for. Such albedos seem typical of comets at larger heliocentric distances.

Hanner, Martha S.↗
NASA NTRS1989

V photometry of Titania, Oberon, and Triton

The phase angle and orbital brightness variations of Titania, Oberon, and Triton are presently obtained through analysis of V filter photometry obtained at Mauna Kea in 1982-1983. While Titania and Oberon exhibit magnitude variations with phase angle comparable to those of low-to-moderate albedo asteroids observed within several deg of opposition, Triton's phase variation is distinctly different from these and has a phase coefficient consistent with either a high-albedo regolith or an optically thick nonparticulate scattering layer (perhaps an atmosphere, or an ocean). A low-albedo regolith cannot on the strength of these data be ruled out, however.

Goguen, J. D.↗
NASA NTRS1989

The first UV spectrum of Triton - IUE observations from 2600 to 3200 A

The results of the first observations of Triton's spectrum below 3300 A are reported. Triton's 2700 A geometric albedo is found to be 0.28 + or - 0.04. The albedo increases monotonically from 2600 to 3200 A, with a slope of 0.13 + or - 0.03 per 1000 A. This positive slope is qualitatively similar to, but shallower than, Triton's visible albedo slope and argues against a strong Rayleigh scattering signature. Triton's integrated flux is found to be 1.5 times Pluto's, indicating Triton to be either more reflective in the UV, or bigger, or both. The redness of Triton's visible and UV colors argues against an extremely bright surface and for a larger radius than Pluto's. A lower limit on the radius of 1240 + or - 90 km is derived. The spectrum observed by IUE is consistent with either an icy surface or a suspended aerosol layer which preferentially absorbs UV light.

Stern, S. Alan↗
NASA NTRS1989

Dark surfaces of asteroids and comets - Evidence for macromolecular carbon compounds

The presence of very low-albedo material on the surfaces of some classes of asteroids (C,P,D) and most comets appears to be related to macromolecular carbon compounds on these bodies. Preliminary detections of an organic spectral signature have been reported for some asteroids. The complex refractory organics that occur in some meteorites are probably the material that makes low-albedo asteroids dark. Comet surfaces and ejected dust particles are also very low albedo, apparently resulting from the radiation processing of organic ices.

Cruikshank, D. P.↗
NASA NTRS1989

The UV spectrum of Pluto-Charon - IUE observations from 2600 to 3100 A

Observations of the UV spectra of the Pluto-Charon system were obtained between 1987 and 1988 using the large aperture of the IUE long-wavelength prime spectrograph. At the resolution and SNR of the data, no gas-phase spectral identifications were made. The present data indicate a geometric albedo of 0.35 + or - 0.05. Curve fitting the geometric albedo from 2600-3100 A, a blue slope is found which corresponds to a decrease of about 12 percent per 1000 A. An albedo reversal or trough between about 3000 A and the shortest ground-based data (at about 3500 A) is noted which may be due to a spectral absorption feature on the surface.

Stern, S. Alan↗
NASA NTRS1989

Radiative transfer modeling of Saturn's outer B ring

The present investigation of Voyager images of the Saturn outer B ring's light-scattering behavior gives attention to the four radial regions formed by brighter and darker areas within and outside the 'spokes'. The B-ring particles are found to have a strongly backscattering phase function and a large-particle scattering albedo of 0.54 in the dark regions and 0.58 in the bright ones; these albedos are interpretable as due to multiple scattering among very pure ice grains covering the ring particle surfaces. It is shown that dark micrometeoroid impacts into the B ring would have darkened the particles to their current spherical albedo in as little as 100 to 200 million years.

Doyle, Laurance R.↗
NASA NTRS1990

Inhomogeneities of stratocumulus liquid water

There is a growing body of observational evidence on inhomogeneous cloud structure, most recently from the extensive measurements of the FIRE field program. Knowledge of cloud structure is important because it strongly influences the cloud radiative properties, one of the major factors in determining the global energy balance. Current atmospheric circulation models use plane-parallel radiation, so that the liquid water in each gridbox is assumed to be uniform, which gives an unrealistically large albedo. In reality cloud liquid water occupies only a subset of each gridbox, greatly reducing the mean albedo. If future climate models are to treat the hydrological cycle in a manner consistent with energy balance, a better treatment of cloud liquid is needed. FIRE concentrated upon two cloud types of special interest: cirrus and marine stratocumulus. Cirrus tend to be high and optically thin, thus reducing the effective radiative temperature without increasing the albedo significantly, leading to an enhanced greenhouse heating. In contrast, marine stratocumulus are low and optically thick, thus producing a large increase in reflected radiation with a small change in emitted radiation, giving a net cooling which could potentially mitigate the expected greenhouse warming. The FIRE measurements in California stratocumulus during June and July of 1987 show variations in cloud liquid water on all scales. Such variations are associated with inhomogeneous entrainment, in which entrained dry air, rather than mixing uniformly with cloudy air, remains intact in blobs of all sizes, which decay only slowly by invasion of cloudy air. Two important stratocumulus observations are described, followed by a simple fractal model which reproduces these properties, and finally, the model radiative properties are discussed.

Cahalan, Robert F.↗
NASA NTRS1990

Photoclinometric analysis of wrinkle ridges on Lunae Planum, Mars

Wrinkle ridges are common morphologic features on Mars. Both volcanic and tectonic mechanisms were suggested to explain their origin; recent work has focused on a compressional origin. Analysis of terrestrial analogs has greatly influenced and aided the understanding of wrinkle ridge formation. An important aspect necessary to intrepret structure is topography. Topographic profiles across ridges can provide important constraints for models of internal structure and analyzing deformation associated with ridges. Topographic maps of Mars are too coarse to resolve the topography of individual ridges; therefore, monoscopic photoclinometry was used to derive topographic profiles for the ridges. Profiles spaced a few kilometers apart were obtained for each ridge, the number depended on ridge length, morphology, and albedo variation. Photoclinometry relies on pixel brightness variations which results from topography, albedo, or both. Because of the albedo variations, photoclimometric profiles can not be extended across large distances, such as between adjacent ridges (about 20 to 80 km). However, the technique is applicable to shorter distances, such as the distance across typical ridges. Profiles were measured across the ridge and extended a few kilometers on either side, including all visible components of the ridge. The results of these measurements and the use of internal structure and topographic profile models for estimating the shortening due to folding and faulting are discussed.

Plescia, J. B.↗
NASA NTRS1991

Shortwave shape factor inversion of earth radiation budget observations

Results are presented on an investigation of the effects of various assumptions (including assumptions of a constant flux field, a constant albedo field, and a variable albedo field) used for deriving the shortwave shape factor on the estimates, from radiometric measurements, of the albedo at the top of the atmosphere. The accuracies and the resolutions of the shape-factor-flux estimates obtained using these assumptions are determined by simulating the shape factor inversion technique with scanner data from the Earth Radiation Budget Experiment (ERBE). The resulting biases and variances are given for both the ERBE medium-field-of-view and wide-field-of-view radiometers.

Green, Richard N.↗
NASA NTRS1991

Variable features in the Valles Marineris region of Mars

Transient phenomena on Mars have long been recognized in Mariner and Viking images as well as in decades of Earth based telescopic observations. These events are of interest because of the information they present on currently active meteorological and geological processes. Changes in surface albedo patterns and atmospheric conditions can also affect the analysis and interpretation of data based on spectral or morphological properties of geologic units on the surface. The mechanism responsible for albedo pattern change is currently under investigation. Generation and subsequent transportation and deposition of dark sands has been interpreted in the Valles. However, the removal of a bright dust layer is more consistent with the rapid time period of the change (about two months) and with preliminary multispectral mapping results which suggest that the dark streak south of Eos and Coprates Chasmata is spectrally distinguishable from the dark saltating materials found elsewhere in the canyon system. If a layer of bright dust was removed to affect the albedo change, questions concerning how such micron-sized particles are mobilized by the winds during a normally quiescent season (Southern Hemisphere Autumn) should be addressed.

Geissler, Paul E.↗
NASA NTRS1992

Soil texture and granulometry at the surface of Mars

The microtexture of the near-surface Martian soil was sensed with three diagnostic parameters: (1) the albedo A at normal incidence and phase angle 5 degrees, which relates to the composition of the top surface exposed layer; (2) the polarization parameter b characterizes the texture of the top surface layer in terms of grain size; and (3) the thermal inertia parameter I which refers to the soil compaction through the first few decimeters below the top surface sensed by polarimetry, in terms of size for the pieces making a granular regolith. Parameter b was derived from instrument VPM on board the Soviet spacecraft MARS-5, inertial I is from IRTM on the American Viking, and albedo A from both. The polarimetric scans racked strips covering two contrasted regions, the dark hued Mare Erythraeum, and the adjacent bright orange Thaumasia. Erythraem is characterized everywhere by a same type of terrain, despite the large geomorphological diversity of the surface. There is an ubiquitous coating or mantling with small dark grains, of both albedo 12.7 percent and particle size 10 to 20 microns, above a subsurface dislocation in pieces around 300 to 600 microns. A simple model is with sand-size particles completely coated with 15 micron dark grains.

Dollfus, A.↗