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NASA NTRS1981

Voyager photometry of surface features on Ganymede and Callisto

Photometric properties of selected surface features on Ganymede and Callisto are studied, using Voyager images over phase angles from 10 to 124 deg, taken with a clear filter (effective wavelength of approximately 0.5 microns). Normal reflectances on Ganymede average 0.35 for the cratered terrain, and 0.44 for the grooved terrain; the ubiquitous cratered terrain on Callisto is 0.18. The photometric properties of these regions are described by a simple scattering function, where the function of the phase angle is qualitatively similar to that of the moon, i.e., concave upward. By contrast, bright craters on both satellites have functions of the phase angle which are concave downward. The scattering function is not Lambertian, and may be due to an admixture of a small amount of dark, opaque silicate grains with the frost deposits. The brightest craters on Callisto have reflectances which are 10% lower than the brightest craters on Ganymede, and both have similar scattering laws.

Squyres, S. W.↗

NASA NTRS1981

Photometry of HD 45088 - A new bright BY Draconis variable

The K-dwarf SB2 system HD 45088, predicted to be a BY Dra variable on the basis of its orbital period and emission-line characteristics, is found to be photometrically variable. The amplitude is significantly variable, having ranged between 0.02m during February-April 1980 to 0.045m during September-October 1980. The photometric period, shown to be 7.36 days, differs appreciably from the 6.99-day orbital period. Possible explanations for this difference are (1) nonsynchronization in a young binary, (2) spot development at high latitudes on a differentially rotating star, and (3) imperfect synchronization in a binary with an eccentric orbit.

Bopp, B. W.↗

NASA NTRS1981

A simple radiation model of cometary dust for P/Halley

Algebra, parameter values, and results for a model of the radiation from the dust environment of Comet P/Halley are presented. The model includes absorption and single scattering of sunlight and IR emission, for the dust and nucleus. It employs dust particle photometric properties which are independent of wave length, particle size, and position within the coma, separately in the visible and IR regions. Values for these properties are obtained from published photometric data for several comets, and are applied to a numerical model for the dust distribution near Halley. A calculation for a spacecraft 1081 km sunward of Halley's nucleus (at 1.05 AU, postperihelion) leads to omnidirectional, integral flux values of 1235 W/sqm for direct sunlight, 0.24 W/sqm for dust IR emission, 0.095 W/sam for sunlight scattered from the dust, 0.0015 W/sqm for sunlight reflected from the nucleus, and 0.00033 W/sqm for nucleus IR emission.

Divine, N.↗

NASA NTRS1982

Spectra of cosmic X-ray sources

X-ray measurements provide the most direct probes of astrophysical environments with temperatures exceeding one million K. Progress in experimental research utilizing dispersive techniques (e.g., Bragg and grating spectroscopy) is considerably slower than that in areas utilizing photometric techniques, because of the relative inefficiency of the former for the weak X-ray signals from celestial sources. As a result, the term "spectroscopy" as applied to X-ray astronomy has traditionally satisfied a much less restrictive definition (in terms of resolving power) than it has in other wavebands. Until quite recently, resolving powers of order unity were perfectly respectable, and still provide (in most cases) the most useful spectroscopic data. In the broadest sense, X-ray photometric measurements are spectroscopic, insofar as they represent samples of the overall electromagnetic continua of celestial objects.

Holt, S. S.↗

NASA NTRS1982

Results of the IUE program of monitoring the ultraviolet emission line fluxes of four binary systems: HR 1099, 2 Peg, AR Lac and By Dra

A program to obtain IUE spectra and optical photometry and spectra of three RS CVn-type binaries (HR 1099, II Peg, and AR Lac) and the prototype BY Dra system is reported. The systems were monitored for at least one orbital phase, and periodic variations in emission line flux from II Peg and HR 1099, indicative of notational modulation of an active region on these stars were detected. It is found that for II Peg the active region is in phase with photometric minimum as expected, but for HR 1099 ultraviolet emission maximum occurs at the time of photometric maximum.

Marstad, M.↗

NASA NTRS1980

Orbiting solar observatory 8 high resolution ultraviolet spectrometer experiment

Oscillations, physical properties of the solar atmosphere, motions in the quiet solar atmosphere, coronal holes, motions in solar active regions, solar flares, the structure of plage regions, an atlas, and aeronomy are summarized. Photometric sensitivity, scattered light, ghosts, focus and spectral resolution, wavelength drive, photometric sensitivity, and scattered light, are also summarized. Experiments are described according to spacecraft made and experiment type. Some of the most useful data reduction programs are described.

Source record↗

NASA NTRS1981

Mars atmospheric opacity effects observed in the Northern Hemisphere by Viking orbiter imaging

Viking orbiter television camera observations of Mars contrasts show variations of atmospheric opacity in the Southern Hemisphere. The study is extended into the Northern Hemisphere, over a longer time period, and results in a detailed description of photometric changes at the lander sites, as seen from orbit. During the period of January 1977 to April 1978, a series of four storms at 202 deg, 264 deg, 40 deg and 86 deg latitudes have provided photometric changes and planetwide aerosol distributions for 18 months. It is found that the large, forward-scattering particles did not enter the atmosphere at the storm's onsets. A reduction of atmospheric transmission by over 20% at visual wavelengths remained for nearly one Martian year.

Thorpe, T. E.↗

NASA NTRS1981

BE Ursae Majoris /PG 1155 + 492/ - A unique cataclysmic-variable-like object

The variable spectrum of BE Ursae Majoris, which is similar to that of the cataclysmic variables (CVs), shows (1) an emission-line spectrum with the high-excitation CN 4650 A blend as the strongest optical feature, weaker H and He lines, and a strong Balmer jump, and (2) an absorption spectrum dominated by He. Although the object shows no evidence of photometric variability on either short or long time-scales, it has been shown by Kurochkin (1964, 1971) to have a 2.29-day, sinusoidal, 1.5-mag variation. The luminosity is dominated by a UV power-law component, though the 4000-10000 A continuum is flat. The hypothesis that this object is an active mass transfer binary is not supported by its photometric behavior, energy distribution and long period, despite the spectroscopic variability. Nevertheless, three components whose nature is not understood remain: (1) an ultraviolet source whose energy distribution is explainable neither by a uniform temperature primary star nor by an accretion disk; (2) a reprocessed component powered by the UV star; and (3) a component similar to a CV disk, producing the optically thick Balmer lines.

Ferguson, D. H.↗

NASA NTRS1982

Effects of body shape on disk-integrated spectral reflectance

The effects of scattering geometry on the spectral reflectance curves of various powdered substances representing possible planetary surfaces are examined. Reflectance measurements were made as a function of incidence angle and emission angle for ground samples of a slightly oxidized basalt, the C3 carbonaceous chondrite Allende, and the ordinary chondrite Bruderheim, and compared with various photometric functions. The best fit to the observations is obtained with the photometric function of Goguen (1981) and Hapke (1981), and with a simpler function representing a linear combination of a Hapke-Irvine law and Lambert's law. The wavelength dependence of the adopted functions implies that the spherical reflectance of a flat sample will differ from that of a spherical planet of the same material, and that there will be a difference between the spectral reflectance curves of spherical and elliptical planets.

Gradie, J.↗

NASA NTRS1981

Spectral investigations of Mars from Viking

In connection with the Viking mission, spectral and photometric data of Mars were obtained with the aid of camera systems located either on the planetary surface (Lander I and II) or in Martian orbit (Orbiter I and II). Operations from Lander I are scheduled to continue through 1990. Comparisons made of the surface and atmospheric spectral and photometric properties at various times throughout the Mission were probably the most important studies conducted by the Lander cameras. The variations in atmospheric optical density due to clouds, dust storm activity (both local and global), and even a partial eclipse of the sun by the small satellite Phobos, were observed by the Lander imaging systems. Using Viking Orbiter II images, the surface materials of the equatorial region have been classified and mapped on the basis of spectral reflectance properties. Attention is also given to some details regarding the characteristics of the Viking imaging systems.

Evans, N.↗

NASA NTRS1981

The RS Canum Venaticorum binaries

This review presents a brief up-to-date summary of observational properties of RS CVn binaries, with emphasis on the photometric. A table of the 69 systems known at this time give the orbital period, mean apparent V magnitude, spectral classification, range of amplitude determinations for the photometric wave, and an indication of known period variations and eclipses. Points considered are (1) the underlying cause of the extreme surface activity, (2) the evolutionary status, (3) the geometric configuration, (4) the temperature, size, shape, location, and lifetimes of the dark spot regions, (5) synchronization of rotational and orbital motion, (6) spectroscopic features in the optical and far ultraviolet, (7) the wave migration, (8) the search for spot cycles, and (9) the orbital period variations produced by the enhanced stellar wind.

Hall, D. S.↗

NASA NTRS1982

Photoclinometric determination of the topography of the Martian north polar cap

Photoclinometry is useful for the determination of topography for areas which have a uniform albedo. The technique is applied to early spring Viking images of the Martian north polar cap, taken when the surface was covered by a nearly uniform frost cover. Unlike earlier approaches, the topographic profiling can be used for surfaces with any photometric function, but the strike of the planetary surface relative to the illumination angle must be specified along the profile. The resultant profiles are relatively insensitive to misestimation of the photometric function and slope orientation, but are quite sensitive to the assumed values of the reflectance of an equivalent level surface and the atmospheric opacity (if it is large).

Howard, A. D.↗

NASA NTRS1981

The International Ultraviolet Explorer

The instrumentation and functioning of the IUE are discussed. The design of the telescope, fine error sensors, echelle spectrographs, and television cameras, as well as their integration, is described. The optical characteristics of the IUE are listed and the schematic arrangement of the detector is depicted. Sources of background noise are assessed, including the thermionic effects in the UV converters and television tubes, radiation from the earth's trapped electron belts, wide angle white light scattering from the earth and sun, scattering of dispersed light caused by low dispersion gratings, photometric calibration, and photometric stability. IUE sensitivity changes during the first three years of operation are shown together with sensitivity curves for short and long wavelengths. Modes of wavelength determination are considered.

Boggess, A.↗

NASA NTRS1983

Astronomical and statistical algorithms used in the Space Telescope guide star selection system

The construction of the Space Telescope Guide Star Catalog from digitized Schmidt survey plates covering the entire sky is described. In order to provide sufficient pointing information, the Guide Star Selection System has to contain a catalog of 20 million guide star candidates in the range of 9.0 to 14.5 visual magnitudes. An image inventory process extracts the relevant object data; after photometric and astrometric calibration the data are screened to arrive at suitable guide star candidates. The selection process for a given target takes into account geometric and photometric parameters of the field, scheduling information, and acquisition probabilities for each guide star pair.

Jenkner, H.↗

NASA NTRS1984

Photometry of icy satellites: How important is multiple scattering in diluting shadows?

Voyager observations have shown that the photometric properties of icy satellites are influenced significantly by large-scale roughness elements on the surfaces. While recent progress was made in treating the photometric effects of macroscopic roughness, it is still the case that even the most complete models do not account for the effects of multiple scattering fully. Multiple scattering dilutes shadows caused by large-scale features, yet for any specific model it is difficult to calculate the amount of dilution as a function of albedo. Accordingly, laboratory measurements were undertaken using the Cornell Goniometer to evaluate the magnitude of the effect.

Buratti, B.↗

NASA NTRS1984

Voyager Feature-fixed Photometry of Iapetus

Iapetus is one of the most mysterious objects in the Solar System. As indicated by previous ground-based studies and dramatically confirmed by the Voyager images, the surface of this satellite has a strikingly bimodal distribution of albedo and color. While the underlying object seems to be a largely icy satellite similar to Rhea, the leading hemisphere of Iapetus is dominated by a very dark patch of hemispheric scale. The symmetry of this dark material with respect to the orbital motion of the satellite strongly suggests an external control, if not necessarily an external origin, for the dark material. Quantitative photometric techniques are used to map the photometric function of the surface of the material over Iapetus. The approach is to study clearly defined surface areas that appear on a number of Voyager images obtained under different spacecraft and lighting geometries.

Goguen, J.↗

NASA NTRS1984

Theoretical aspects of photoclinometric terrain profiling on the Galilean satellites

For large parts of the surfaces of the Galilean satellites there is no stereoscopic high resolution imagery, and so for shadow-free regions far from the terminator, the only method of obtaining topograpic information is photoclinometry, the technique of converting brightness variations into local surface tilts and hence into topographic profiles and contour maps. The conversion from brightness changes to slopes require knowledge of the angular photometric function for the type of planetary surface terrain being analyzed, i.e., the relationship describing the brightness of the surface as a function of the angles of incidence (I) and emergence (E) of light at a surface element. In order to investigate the effects of these changes on the shapes of topographic profiles deduced from the functions, an area of grooved terrain (lines 500-505, samples 550-650 on frame 20640.27 0060J2) using wide ranges of values of all the adjustable parameters in Hapke's new photometric function were analyzed.

Wilson, L.↗

NASA NTRS1984

Facular Limb-darkening Functions for Irradiance Modeling

The limb darkening function of faculae is an important factor in estimating facular contributions to solar irradiance variations. The existing photometric data is reviewed and a synthetic limb darkening function for faculae was generated which was compared with the limb darkening functions currently in use for irradiance modeling. The excess facular flux ranges from 0.017 to 0.0349 of the solar photospheric flux for the various representations. The present limitation appears to be the lack of comprehensive photometric data. The representations are especially varied near the limb, where photometry is most difficult. Synoptic data on calcium plage areas is used as a substitute for facular areas in modeling. Based upon the existing data, the present best value for the effect of faculae on the solar constant is given by delta s/s = 3.9 x 10(-8) A (sub plage prime) for an active region near disk center, where A (sub plage) is the calcium plage area in millionths of a solar hemisphere.

Hirayama, T.↗