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NASA NTRS1980

Solar flux incident on an orbiting surface after reflection from a planet

Algorithms describing the solar radiation impinging on an infinitesimal surface after reflection from a gray and diffuse planet are derived. The following conditions apply: only radiation from the sunny half of the planet is taken into account; the radiation must fall on the top of the orbiting surface, and radiation must come from that part of the planet that can be seen from the orbiting body. A simple approximate formula is presented which displays excellent accuracy for all significant situations, with an error which is always less than 5% of the maximum possible reflected flux. Attention is also given to solar albedo flux on a surface directly facing the planet, the influence of solar position on albedo flux, and to solar albedo flux as a function of the surface-planet tilt angle.

Modest, M. F.↗

NASA NTRS1980

The helium abundance of Jupiter from Voyager

Full disk measurements recorded 31 days before the Voyager 1 encounter with Jupiter by the radiometer of the infrared instrument, IRIS, indicate a geometric albedo of 0.274 + or - 0.013. Combining this measurement with the Pioneer derived phase integral of 1.25 and our error estimate of 0.1 yields a Jovian Bond albedo of 0.343 + or - 0.032. Infrared spectra recorded at the same time by the Michelson interferometer, along with a model extrapolation to low wave numbers not covered by the instrument, yield a thermal emission of (1.359 + or - 0.014) .001 W cm to the (-2) power. As in the case of the albedo measurement, the quoted errors in the emission measurement reflect estimates of systematic effects and are uncertain while the random component is negligible. From these measurements the internal heat flux of Jupiter is estimated to be (5.444 + or - 0.425) .0001 W cm to the (-2) power, and the energy balance defined as the ratio of emitted thermal to absorbed solar energy is 1.668 + or - 0.085.

Gautier, D.↗

NASA NTRS1980

Surface texture of Vesta from optical polarimetry

Polarimetric, photometric, and reflective spectroscopic properties of asteroid 4 Vesta are simulated in the laboratory by a preparation of eucrite Bereba consisting of a broad mixture of particle sizes (mainly greater than 50-micron) mixed and partially coated with particles of size 10 microns and less. Coarse grains are necessary for producing the same albedo and a very fine dust coating is necessary for producing the same polarization inversion angle as observed for Vesta. There are less small grains and fine dust in this sample than in lunar soils. Photometrically, if coating a sphere, this sample shows a constant brightness on the sunward half of the observed hemisphere, the brightness being given on the other half by the Minnaert reciprocity principle. With such a photometric behavior, the global geometric albedo and the sub-earth point geometric albedo differ by no more than 5%. The microscopic phase coefficient is 0.021 magnitude per degree for the sample; the larger value, 0.025, observed telescopically for Vesta, indicates that large-scale roughness is present on this asteroid.

Le Bertre, T.↗

NASA NTRS1980

Near-infrared colorimetry of J6 Himalia and S9 Phoebe - A summary of 0.3- to 2.2-micron reflectances

VJHK measurements of J6 Himalia and S9 Phoebe, using the new NASA IRTF telescope, show that these objects have carbonaceous chondritic type colors in the 0.5- to 2.2-micron region. For Phoebe, this is in contrast to the JHK colors published by Cruikshank (1980), which indicated that the satellite's surface was unlike the material found on asteroids and on the dark side of Iapetus. J6 is known to have a low albedo from thermal infrared studies (Cruikshank, 1977), and the new VJHK observations of S9 imply that it also has a low albedo. The H and K reflectances of S9 are slightly lower than those of J6, suggesting some slight difference in surface composition or a contamination by foreign material. The conjectured low albedo of S9 can be tested with measurements in the thermal infrared.

Degewij, J.↗

NASA NTRS1981

Near-infrared spectrophotometry of the satellites and rings of Uranus

New spectrophotometry from 1.5 to 2.5 microns is reported for the Uranian satellites Titania, Oberon, and Umbriel. A spectrum of the rings of Uranus from 2.0 to 2.4 microns is also reported. No evidence is found for frost covering the surface of the ring material, consistent with the low albedo of the rings previously reported by Nicholson and Jones (1980). The surfaces of the satellites are found to be covered by dirty water frost. Assuming albedos of the frost and gray components covering the Uranian satellites to be the same as the light and dark faces of Iapetus, radii are derived that are roughly twice those inferred from the assumption of a visual albedo of 0.5.

Soifer, B. T.↗

NASA NTRS1982

Low-latitude cloudiness and climate feedback - Comparative estimates from satellite data

Three studies of the relative albedo to the IR components of cloud amount feedback are reviewed, and an approach to the seasonal variability in low-level cloud numbers is presented. Comparisons are made and uncertainties calculated for the predictions of cloud amounts from satellite data sets for outgoing IR flux. IR data is used directly for a linear regression analysis and seasonal and latitudinal variations are considered in terms of monthly-annual means for particular latitude zones. Investigations of NOAA-NESS satellite data revealed that cirrus clouds are transparent in the 10.5-12.5 microns range and cloud albedos decrease at wavelengths greater than 0.7 micron, which suggests a possibility of cloud-sky and clear-sky albedo comparisons at 0.5-0.7 micron. The inclusion of Rayleigh scattering and atmospheric water vapor absorption of sunlight is recommended to test the effects on observed contracts.

Cess, R. D.↗

NASA NTRS1982

The effect of brokenness on cloud-climate sensitivity

A study has been made of the effect of brokenness on the infrared and albedo feedback of clouds in climate models using a simplified treatment of broken cloudiness. It is shown that the individual feedback terms computed using the plane-parallel assumption differ markedly from the computations performed for a regular array of cuboidal clouds. If the cloudiness changes such that clear areas become cloudy but the nature of the broken cloud field remains unchanged, then both the feedbacks are magnified, the infrared more so than the albedo. However, if individual elements in the cloud field are increased horizontally in a constrained area, the feedbacks can be enhanced or diminished depending on the cloud fraction and aspect ratio of the elements. It is also shown that, in the global average, the ratio of the change in outgoing infrared flux to global albedo, as deduced from satellite measurements, will be larger than model simulations using planiform clouds.

Harshvardhan, MR.↗

NASA NTRS1982

The sensitivity of numerically simulated climates to land-surface boundary conditions

Eleven sensitivity experiments that were made with general circulation models to see how land-surface boundary conditions can influence the rainfall, temperature, and motion fields of the atmosphere are discussed. In one group of experiments, different soil moistures or albedos are prescribed as time-invariant boundary conditions. In a second group, different soil moistures or different albedos are initially prescribed, and the soil moisture (but not the albedo) is allowed to change with time according to the governing equations for soil moisture. In a third group, the results of constant versus time-dependent soil moistures are compared.

Mintz, Y.↗

NASA NTRS1982

The scattering phase function of interstellar grains - The case of the reflection nebula NGC 7023

IUE observations of the reflection nebula NGC 7023 and the illuminating star, HD 200775, in the spectral range from 1300 to 3100 A are combined with ground-based measurements of the nebular brightness distribution at 3500, 4100, 4700, and 5500 A to determine the scattering properties of the nebular dust grains. Total nebular fluxes are derived from existing data obtained by satellite and ground-based observations. The relevance of far-IR fluxes from the nebula is considered, and models are derived that are applicable to NGC 7023 both with respect to the ratio of nebular to stellar fluxes and with respect to the nebular surface brightness distribution. It is shown that the average grain albedo in the UV is about 0.54 and that the albedo increases to a level of about 0.6 at 1400 A after reaching a minimum of approximately 0.4 near 2200 A. The results suggest that isotropically scattering particles of high albedo make a significant contribution to interstellar scattering in the far-UV.

Witt, A. N.↗

NASA NTRS1982

Daily temperature variations on Mars

It is noted that for approximately 32% of the Martian surface area no values of thermal inertia or albedo can fit the thermal observations. These temperature anomalies do not correlate with elevation, geologic units, morphology, or atmospheric dust content. All regions having a Lambert albedo less than 0.18 can be well fit with the standard thermal model, but all areas with albedo greater than 0.28 are anomalous. A strong inverse correlation is seen between the magnitude of the anomaly and the thermal inertia. This correlation is seen as indicating that some surface property is responsible for the anomaly. In the anomalous region the temperatures are observed to be warmer in the morning and cooler late in the afternoon and to decrease more slowly during the night than the Viking model temperatures. It is believed that of all the physical processes likely to occur on Mars but not included in the Viking thermal model, only a layered soil can explain the observations. A possible explanation of the layering deduced from the infrared thermal mapper observations is a layer of aeolian deposited dust about one thermal skin depth thick (1 to 4 cm), covering a duricrust.

Ditteon, R.↗

NASA NTRS1982

A parameterized model for global insolation under partially cloudy skies

A simple and efficient parameterization of insolation under partially cloudy skies is discussed and compared with a set of exact radiative transfer results for clear skies, an empirical equation and observations. The parameterization is physically based and requires, as input variables, the ozone path length, precipitable water, Angstrom turbidity, surface air pressure and albedo, fractional cloud-cover and cloud thickness. Multiple reflection between the surface and the overlying atmosphere, and clouds are considered. The albedo of the earth-atmosphere system is also formulated and compared with a set of exact radiative transfer results. As compared to the exact radiative transfer results, the errors in the insolations are generally less than 1 percent, and in the albedo of the earth-atmosphere system less than 10 percent. The errors in the calculated insolations using climatological data are 2-3 percent when compared with many years averaged observations at Maudheim (Antarctica) and at Rockville (U.S.A.). A parametric equation for calculating directly the daily total insolation is also given.

Choudhury, B.↗

NASA NTRS1984

Dark streaks on talus slopes, Mars

High-resolution pictures of talus slopes on Mars show small, dark streaks that characteristically widen downward. These streaks are different from the thin and even streaks of various albedos that stream from cliffs on talus slopes, but gradations between the two streak types occur and not all streaks can be classified with confidence. In order to study the nature and origin of the small, widening, dark streaks, all Viking pictures with a resolution of less than 100 m/pixel were surveyed. To date several hundred streaks were located, but only few are of high enough resolution to be confidently identified as widening downwards. The approximate dimensions of the streaks were measured and their shapes, numbers, position, and spacing on slopes were noted. They were plotted on a topographic map, and their relation to topography, geologic units, and regions of distinct thermal inertia and albedo were studied. Also noted was the season at which images containing streaks were acquired and the direction of illumination. Albedo measurements are in progress. Several streaks can be seen stereoscopically, but none are observed on color images. The observation of small dark streaks on talus slopes on Mars is compatible with an interpretation of their origin as eruptions of small masses of wet debris in places where steep walls intersect aquifers or where seasonal equatorial warming permits the local melting of ground ice.

Ferguson, H. M.↗

NASA NTRS1983

Saturn's small satellites - Voyager imaging results

Voyagers 1 and 2 provided images of sufficient resolution for morphologic and photometric studies of Saturn's small satellites. These objects, all very difficult to observe from earth, orbit Saturn at distances of 2.3 to 6.3 Saturn radii (just outside the A ring to the orbit of Dione) and range in mean diameter from 22 to 188 km. All are irregularly shaped (long/short axis ratios range from 1.4 to 2.0) and probably heavily cratered. While impacts have apparently been important in shaping these objects, observed crater densities suggest that the present forms may have survived for about 4 billion years. Geometric albedos vary from 0.4 to at least 0.8. These albedos and the few color data available are similar to those of larger Saturn satellites which are known to have surfaces made predominantly of water ice. The range of observed albedos could be explained by minor variations in the amount of dark, opaque contaminants.

Thomas, P.↗

NASA NTRS1984

The sensitivity of numerically simulated climates to land-surface boundary conditions

Mintz, Y.↗

NASA NTRS1984

Voyager photometry of Rhea, Dione, Tethys, Enceladus and Mimas

Voyager imaging observations provide new photometric data on Saturn's satellites at large phase angles (up to 133 deg in the case of Mimas) not observable from earth. Significant new results include the determination of phase integrals ranging from 0.7 in the case of Rhea to 0.9 for Enceladus. For Enceladus an average geometric albedo 1.04 and a Bond albedo of 0.9 are found. The data indicate an orbital lightcurve with an amplitude of 0.2 mag, the trailing side being the brighter. For Mimas, the lightcurve amplitude is probably less than 0.1 mag. The value of the geometric albedo of Mimas reported here, is definitely higher than the currently accepted value of about 0.5. For Dione, the Voyager data show a well-defined orbital lightcurve of amplitude about 0.6 mag, with the leading hemisphere brighter than the trailing one.

Buratti, B.↗

NASA NTRS1984

Surface of Miranda - Identification of water ice

Near-infrared spectrophotometry at 5-percent resolution shows Miranda to have a water-ice surface. Estimates of Miranda's albedo made from the depth of its 2.0-micron absorption band suggest that its visual geometric albedo is likely to be between 10 and 70 percent which when combined with the satellite's visual magnitude, yields a diameter of 500 + or 225 km. There is some evidence that suggests the visual geometric albedo of Miranda may be greater than or equal to 0.3, which implies that its diameter may lie near the lower end of the estimated range. With these results all the Uranian satellites are now known to have water-ice surfaces.

Brown, R. H.↗

NASA NTRS1984

The Nimbus 7 ERB data set - A critical analysis

An analysis of the first year of the Nimbus 7 earth radiation budget data set reveals that there are systematic differences between wide and narrow field of view measurements. The larger differences appear in the albedo data and are due primarily to a bias introduced by the processing method. There are smaller differences, associated with the outgoing longwave radiation, which are probably due to errors in calibration. The bias in albedo originates in the factors used for converting the shortwave intensity measured at satellite altitude into a mean daily albedo at the top of the atmosphere. The factor that proves most troublesome is the one that converts measured intensity to the total instantaneous outgoing flux. That factor is determined by angular distribution models, which depend upon the type of scene within the target area. An alternative method of processing the data, which is model independent, is shown to have practically no bias and yields better over-all agreement with the wide field of view measurements.

Arking, A.↗

NASA NTRS1984

Voyager disk resolved photometry of the Saturnian satellites

The scattering properties of the icy satellites of Saturn are studied. Scans of intensity along the photometric equator near opposition are analyzed, and disk-resolved measurements of intensity are fitted to simple photometric functions. Histograms of the albedo distribution across the satellite disks are constructed, and the geometric albedos are determined as a function of wavelength. It is found that, at low phase angles, Rhea and Dione closely follow lunar behavior with almost no limb darkening. Mimas, Tethys, and especially Enceladus show significant limb darkening at low phase angles, which suggest multiple scattering is important for their surfaces. No satellite's surface can be described by Lambert's law. The darker satellites are redder, and the darker areas of each satellite are redder. A longitudinal dark stripe which represents a 15 percent decrease in albedo is situated near the center of the trailing side of Tethys. Mimas and Enceladus both have remarkably flat spectral reflectances into the ultraviolet.

Buratti, B. J.↗