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Lumme, K.

Publications and source records attributed to Lumme, K..

34 records · Page 2
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NASA NTRS1981

Radiative transfer in the surfaces of atmosphereless bodies. I - Theory. II - Interpretation of phase curves

A generalized theory of radiative transfer is presented which includes single and multiple scattering of light in particulate surfaces, and is applicable to both the surfaces of atmosphereless bodies and to laboratory samples. Single scattering is described in terms of the effects of porosity and roughness, and is formulated by means of a probabilistic method. It is shown that, for low-albedo surfaces, the effects of porosity and roughness are separable, the opposition effect is caused by the former, and the slope of the linear part of the phase curve is mainly controlled by the latter. The theory, which is applicable to all albedos and phase angles, may be used in both surface brightness and integrated brightness studies. The limiting case of roughness and volume density tending to zero yields the results of classical radiative transfer theory.

Lumme, K.↗
NASA NTRS1981

Saturn's rings - Azimuthal variations, phase curves, and radial profiles in four colors

Four-color photographic photometry, at effective wavelengths of 5900, 5350, 4150, and 3560 A, obtained during the 1977, 1978, and 1979 Saturn apparitions, were analyzed to determine the ring brightness as a function of wavelength, solar phase angle, ring particle orbital phase (the azimuthal effect), declination of the earth relative to the ring plane (tilt effect), and radial distance from Saturn. Data were obtained using the International Planetary Patrol network when the ring tilt angle B was approximately 16.5, 11.5, and 6 deg, respectively. From the approximately 40,000 original images, 364 were scanned and digitized, along with corresponding calibration exposures. The smearing correction technique used achieved better signal-to-noise by utilizing such constraints as the known width of Cassini's Division on the major axis. The most important observational results were summarized, and included the following: the azimuthal brightness variations for the brighter portion of ring A increased as the ring tilt decreased from B = 26 deg to less than 16 deg, reaching the order of + or 20% with respect to the ansae; they are not detectable for ring B or the outer portion of ring A.

Thompson, W. T.↗
NASA NTRS1981

Theoretical interpretation of photometric properties of the Martian surface and atmosphere

Earth-based UBV photometry, photographs from the Lowell Observatory, and Mariner 9 data are combined with a new radiative transfer theory to derive physical parameters for the Martian surface and atmosphere, both before and during the 1971 dust storm. Storm dust particles had a single scattering albedo of 0.84 plus or minus 0.02 and an asymmetry factor of 0.35 plus or minus 0.10 in green (V) light. The geometric albedo of Mars was 0.15 and the phase integral 1.83, yielding 0.27 for the Bond albedo. The mean optical thickness of the 'clear' atmosphere averaged over the whole planet was 0.15 plus or minus 0.05 and was not dependent on wavelength. Geometric albedos for the surface are 0.25 (light areas) and 0.17 (dark areas) in V, 0.095 in B, and 0.060 in U. The soil particles are moderately backward scattering with an asymmetry factor of minus 0.20, and therefore rather opaque. The mean surface roughness is 0.57, representing the depth/radius ratio of an average hole, and being only one-half as large as values typical for the moon and asteroids.

Lumme, K.↗
NASA NTRS1979

A model for the azimuthal brightness variations in Saturn's rings

The paper presents a model to explain brightness variations in Saturn rings. These variations have been interpreted as implying the presence of large, synchronously rotating particles with elongated shapes or leading-trailing-edge differences although such synchronous rotation appears unlikely. The model given here assumes that the gravitational wakes or waves can be considered as elongated distributions of particles referred to as blobs, and the radiative transfer in the rings will involve both processes internal to the blobs and analogous processes among blobs. This model explains the existence of azimuthal variation in ring brightness, as well as the dependence of their amplitude on geometrical and photometric parameters. The probable photometric effects are outlined assuming the existence of density irregularities as a result of self-gravitation, and the results simplify the geometric discussion of Franklin and Colombo (1978).

Lumme, K.↗
NASA NTRS1979

International Planetary Patrol observations of Saturn's rings. I - Observations and data reduction

The paper describes the reduction procedure used for observations of Saturn's rings taken with the International Planetary Patrol Network in 1977. Corrections for atmospheric smearing are determined using an assumed ring model and performed by a two-dimensional Wiener filter developed at JPL's Image Processing Laboratory. These smearing corrections are shown to be consistent with the radial profile of the rings, not just on the major axis of the ring system, but over a range of azimuthal angle.

Lumme, K.↗
NASA NTRS1979

International planetary patrol observations of Saturn's rings. II - Four color phase curves and their analysis

New phase curves for Saturn's rings at an intermediate tilt angle B of about 17 deg are presented. Quantitative results for each of the A and B rings are reported in terms of the opposition effect, phase coefficient, and best logarithmic fit to the phase curve. There was no significant difference between the shape of the phase curves for the two rings in each of the four colors, and a four-parameter multiple scattering model of the rings was consistent with the observations. In this model, the difference in the phase curves for different colors can be explained by a variation in the single scattering albedo with wavelength. The observations allow the particles to have the same composition in the A and B rings, so that their different photometric behavior is explained by differences in optical depth and volume density in the two rings.

Esposito, L. W.↗
NASA NTRS1979

Azimuthal brightness variations of Saturn's rings. III - Observations at tilt angle B approximately equal to 11.5 deg

The brightness variation in Saturn's ring A with orbital phase of the ring particles does not change significantly as the declination of the earth, B, decreases from about 16.5 to 11.5 deg and is noticeably more pronounced than when the rings are fully open. The present data are also not inconsistent with earlier results suggesting that the amplitude of this azimuthal effect diminishes near opposition.

Lumme, K.↗
NASA NTRS1979

Low tilt angle photometry and the thickness of Saturn's rings

Nine photographic plates taken by Focas and Dollfus (1969) at the moment of the 1966 passage of the earth through the ring plane of Saturn have been remeasured. The value 0.8 (+2.3, -0.8) km is obtained for the ring thickness. The observed transmitted radiation through the rings at two distances from the planet suggests that there are density fluctuations in Ring A with the low density areas having an optical thickness less than 0.13. The radiation reflected by the outermost part of the ring layer can be explained in terms of particles similar to those in the bulk of the rings. The time of the passage of the earth through the ring plane was found to be December 18 at 07h plus or minus 4h UT in 1966.

Lumme, K.↗
NASA NTRS1979

Colorimetry and magnitudes of asteroids

In the present paper, 1500 UBV observations are analyzed by a new rather general multiple scattering theory which provided clear insight into previously poorly-recognized optical nature of asteroid surfaces. Thus, phase curves are shown to consist of a surface-texture controlled component, due to singly scattered light, and a component due to multiple scattering. Phase curve shapes can be characterized by a single parameter, the multiple scattering factor, Q. As Q increases, the relative importance of the opposition effect diminishes. Asteroid surfaces are particulate and strikingly similar to texture, being moderately porous and moderately rough on a scale greater than the wavelength of light. In concequence, Q (and also the phase coefficient) correlate well with geometric albedo, and there exists a purely photometric means of determining albedos and diameters.

Bowell, E.↗
NASA NTRS1978

Five-color photometry of Saturn and its rings

Analysis of 206 high-quality plates from three recent apparitions taken in five colors has yielded several photometric parameters for Saturn and its A and B rings. Phase curves and geometric albedos are derived for two regions on Saturn and for each ring. The phase coefficients of the rings are found to be independent of the ring-plane inclination angle. A comparison of the phase curves shows that the particles of ring A exhibit a larger phase coefficient than do those of ring B. When examined with a multiple-scattering model using Henyey-Greenstein phase functions, the observations of the ring tilt effect indicate that the particles of ring A may also have lower single-scattering and geometric albedos. The color dependence of the geometric albedo of the particles in ring B is shown to be very similar to that of Europa (J II). Optical thicknesses of 0.50 for ring A and 0.018 for the Cassini division are found.

Lumme, K.↗
NASA NTRS1978

Azimuthal brightness variations of Saturn's rings

It is now well established that near maximum inclination and in the visual band Saturn's A ring exhibits brightness variations with orbital phase of the ring particles. Constraints on possible models to explain this behavior are provided by the variation in amplitude of this effect with changing declination of the earth with respect to the ring plane, by the variation of the effect with solar phase angle, and by its dependence on wavelength. In particular, the amplitude of the azimuthal effect increases as earth's declination with respect to the ring plane decreases from 26 to 16 deg. The effect diminishes at opposition and decreases with decreasing wavelength (and hence with decreasing particle albedo).

Esposito, L. W.↗
NASA NTRS1977

The tilt effect for Saturn's rings

Multiple-scattering computations are carried out to explain the variation of the observed brightness of the A and B rings of Saturn with declination of the earth and sun. These computations are performed by a doubling scheme for a homogeneous plane-parallel scattering medium. A range of choices is tested for the phase function, albedo for single scattering, and optical depth of both the rings. Isotropic scattering and several other simple phase functions are ruled out, and it is found that the phase function must be moderately peaked in both the forward and backward directions. The tilt effect can be explained by multiple scattering in a homogeneous layer, but, for ring B, this requires a single-scattering albedo in excess of 0.8. The brightest part of ring B must have an optical depth greater than 0.9. It is found that the tilt effect for ring A can be reproduced by particles having the same properties as those in ring B with the optical depth for the A ring in the range 0.4 to 0.6.

Esposito, L. W.↗
NASA NTRS1976

Photometry of Saturn's rings

A total of 202 photographic plates of Saturn taken from four different locations and in seven different colors have been studied in order to define better the variation in ring brightness with solar phase angle, declination of the earth with respect to the ring plane, orbital phase angle of the ring particles, and wavelength. A major effort has been made to investigate possible differences in the photometric behavior of the A and B rings.

Lumme, K.↗
NASA NTRS1976

On the surface brightness and geometric albedo of some Martian areas

High-quality photographs of Mars (red, yellow, green, and blue) are used to analyze the surface, limb, and south-polar-cap brightness of Mars. The surface brightness can be fitted with the Lommel-Seeliger reflection law. For the limb and polar-cap brightness, the method suggested by Lumme (1974) has been used to correct for smearing effects. It is found that the brightness increases noticeably when approaching the limb, that the upper limit to optical thickness of the atmosphere in the blue is 0.16, and that the corresponding single-scattering albedo is 0.55, both with uncertainties of about 15%. Values for the geometric albedo and the phase function (at 37-deg phase angle) are also obtained for both the atmosphere and the ground for a central meridian of 0 deg. The south polar cap in September 1973 was nearly circular, with a radius of about 8 deg (heliocentric longitude of 0 deg) and geometric albedos of 0.68 (red), 0.68 (yellow), 0.60 (green), and 0.53 (blue).

Lumme, K.↗
NASA NTRS1976

Azimuthal brightness variations of Saturn's rings

Saturn's ring A exhibits a variation in brightness with azimuth (with orbital phase of the ring particles). Maximum brightness occurs at true orbital phase angles of about 45 and 225 deg; minimum brightness-occurs at about 135 and 315 deg, where the eastern ansa is taken as zero deg. No such effect appears to exist for ring B.

Lumme, K.↗