Jay D. Goguen

Six-color photometry of lapetus, Titan, Rhea, Dione and Tethys

Abstract Six-color photometric observations made during Saturns 1972/73 opposition enable us to separate the solar phase and orbital phase contributions to the observed light variations of Iapetus, Titan, Rhea, Dione and Tethys. Titan shows no orbital variations, but has phase coefficients which range from negligible values in the infrared to 0.014mag/deg in the ultraviolet. Rhea has a bright leading side, a light curve amplitude of about 0.2mag, which increases toward short wavelengths, and surprisingly large phase coefficients, which increase from 0.025mag/deg in the red to 0.037mag/deg in the ultraviolet. Combined with other available information, this behavior suggests a very porous, texturally complex surface layer. Dione also has a leading side which is a few tenths of a magnitude brighter than the trailing side, but the light curve amplitude has little wavelength dependence and the phase coefficients are significantly smaller than those of Rhea, suggesting a less intricate surface texture. The leading side of Tethys is probably a few tenths of a magnitude brighter than the trailing side. Our Iapetus observations generally supplement the earlier work by Millis. The phase coefficients of the bright (trailing) side are typically ∼0.03mag/deg and are not strongly wavelength dependent; the dark (leading) side coefficients are large (∼0.05 mag/deg) and increase at shorter wavelengths, indicating a very porous and intricate surface texture. The light curve amplitude shows a slight increase at shorter wavelengths, suggesting an increasing contrast between the dark and bright materials. The spectral reflectance curves we derive for the satellites are in agreement with the spectrophotometry of McCord, Johnson, and Elias.

Scattering of light from particulate surfaces: I. A laboratory assessment of multiple-scattering effects

Abstract A convenient photometric function for many particulate surfaces is the generalization of the Lommel-Seeliger law derived by Hapke (1963) and Irvine (1966) . This generalization accounts for the effects of mutual shadowing among particles, but still assumes that multiple scattering within the surface layer can be neglected—an assumption which is evidently valid for dark surfaces. We describe a series of laboratory measurements which test the range of validity of this basic assumption, and the applicability of the Hapke-Irvine photometric function, for particulate surfaces whose normal reflectances ranges from 0.04 to 1.04. We find that multiple-scattering effects can be neglected, and that the Hapke-Irvine function can be used, for particulate surfaces whose normal reflectance is about 0.3, or less. The function is definitely inapplicable to surfaces whose normal reflectance exceeds 0.4.

Lunar occultation of Saturn. I. The diameters of Tethys, Dione, Rhea, Titan, and Iapetus

Abstract The diameters of Tethys, Dione, Rhea, Titan and Iapetus were determined from observations of their March 30, 1974, lunar occultations, made with the Mauna Kea 224 and 61 cm telescopes. Light curves were obtained simultaneously in four colors, and the difference between the time of occultation at the two telescopes provided a direct measurement of the slope of the lunar limb, found to be small in all cases. The satellite diameters were determined by least-squares fits of model occultation light curves to the data. In these fits the diameter and degree of limb darkening of the satellite are correlated variables, requiring the limb darkening to be specified before the diameter can be determined, or vice versa. However, for Titan the signal-to-noise ratio is sufficiently high to allow some assessment of the amount of limb darkening, which was found to be substantial. Titans diameter must be at least 5800 km, much larger than the currently accepted value of 5000 km, making it the largest satellite in the solar system. This larger diameter implies a low mean density. For the other four satellites arguments are presented in favor of accepting the occultation diameters corresponding to limb darkened disks. Except for Titan, the lunar occultation diameters generally agree with previous diskmeter and radiometric determinations.

Near-opposition limb darkening of solids of planetary interest

Abstract This paper presents a laboratory study of the limb darkening near opposition, of particulate materials of planetary interest and concentrates on the wavelength dependence of this limb darkening. We find that near zero phase the scattering properties of most particulate materials can be described adequately by Minnaerts law. However, there are materials for which such a representation is totally inadequate. Examples are bronzite and graphite, materials that tend to fracture into flakes having mirrorlike surfaces. In addition, there are materials, such as olivine, whose scattering properties within deep absorption bands show definite departures from Minnaerts law at large angles of incidence or emission. Our Minnaert parameters, k and B 0 , were measured at a phase angle of α = 4°. For samples of comparable surface texture and roughness, k and B 0 are approximately linearly related, k increasing as B 0 increases. Very dark materials tend to have k ∼ 0.5 to 0.6, while very bright materials tend to have k ∼ 1. The linear relationship between k and B 0 can be explained in terms of the varying importance of multiple scattering in the surface layer. Thus for materials for which B 0 is strongly wavelength dependent, so is k . For example, for olivine, k varies from 0.73 to 0.87 between 0.4 and 1.2 μm. These variations are closely correlated with those in B 0 : the value of k is relatively high outside of absorption bands and relatively low within them. For bright materials, k is very sensitive to surface roughness. For example, for BaSO 4 powder, k can be changed from ∼1.0 to ∼0.8 by this effect alone, a fact which has relevance to the photometry of frost-covered satellites. For dark materials, the effects of surface roughness on k are smaller and more subtle.

A statistical study of crater-associated wind streaks in the North Equatorial Zone of Mars

Abstract A statistical study of all crater-related wind streaks visible on Mariner 9 A-camera frames between latitudes 0 and 30°N has been completed. Of the 2325 streaks identified 1914 (82%) are light tone streaks, 189 (8%) are dark tone, and the remaining 222 (10%) are of mixed tone. Nine parameters characterizing each streak and its associated crater were measured and intercorrelated. Because of the large number of light streaks in our sample fir findings for this type of streak are most significant statistically: light tone streaks occur preferentially in Pc terrain (heavily cratered plains); they are preferentially associated with fresh craters; the surface density of light streaks is not a strong function of elevation; a significant latitude effect does emerge—the density of light tone streaks reaches a maximum between 10 and 15°N, and drops off appreciably both toward the equator and toward higher latitudes; the mean angular width of light streaks is about 25°—long light streaks are significantly narrower than short ones; about 50% of streaks have streak length/crater diameter ratios of ⩽4; light streak directions conform closely to the wind regime expected at the season of global dust storms (southern summer). Generally speaking, the results for dark and mixed tone streaks in the northern equatorial zone are similar, with the following possible exceptions: dark streaks may show a slight preference to form at higher elecations; dark streaks may be slightly wider on average than light or mixed tone streaks; mixed tone streaks do not share the preference for sharp craters exhibited by light and dark streaks; in general, the directions of dark streaks do not conform to the general circulation pattern expected at the season of global dust storms as well as do those of the light streaks.

How to compare the surface of Io to laboratory samples

Since one does not know the photometric functions of various parts of Io, one cannot convert the observed geometric albedo of the satellite to a parameter more directly measurable in the laboratory. One must therefore convert laboratory reflectances to geometric albedos before quantitative comparisons between Ios surface and a laboratory sample are made. This procedure involves determining the wavelength dependence of the samples photometric function. For substances such as sulfur, whose reflectance varies strongly with wavelength, it is incorrect to assume that the photometric function, and hence the ratio (laboratory reflectance/geometric albedo) is independent of wavelength. To illustrate this point, measurements of the color dependence of this ratio for sulfur are presented for the specific case in which the measured laboratory reflectance is the samples normal reflectance. In general, unless the laboratory reflectance is precisely the geometric albedo, a wavelength-dependent correction factor must be determined before the laboratory sample can be compared quantitatively with Ios surface.

Lunar occultation of Saturn. III - How big is Iapetus

Abstract By considering both the orbital lightcurve of Iapetus and data obtained during the March 30, 1974, occultation of the satellite by the Moon, we obtain information about the brightness distribution on the bright face of Iapetus and derive an accurate value for the satellites radius. From the observed orbital lightcurve we find that the trailing face of Iapetus must consist predominantly of a single bright material with an effective limb-darkening parameter of k = 0.62 −0.12 0.10 . Given this result the occultation observations imply a radius of 718 −78 +87 km. If the patchy albedo model proposed by Morrison et al . represents the surface of Iapetus accurately (as far as the relative albedo distribution is concerned) then the radius of Iapetus is 724 ± 60 km. Both estimates are consistent with the radiometric radius of 835 (+50, −75) km derived by Morrison et al . Combining our results with the value of 0.60 ± 0.14 for the normal reflectance (in V) of the material at the center of the bright face derived by Elliot et al . we find that the normal reflectance of the dark side material is 0.11 −0.03 +0.04 . These values are higher than the corresponding values of 0.35 and 0.05 quoted by Morrison et al .

Marsshine on Phobos

The Viking Orbiters have obtained several images of Phobos at large phase angles in which the portion of the satellite not directly illuminated by the sun is faintly visible. A photometric analysis of one such image is presented to prove that the phenomenon is real and can be explained by Marsshine (i.e., the illumination of Phobos by sunlight reflected from Mars). Such images provide cross sections of Phobos and are useful in determining the true shape and size of the satellite. The cross section observed in Picture 111A03 agrees closely with that predicted by triaxial ellipsoid model of Phobos developed by Duxbury (1974).

The lightcurve and rotation period of asteroid 139 Juewa

Abstract Results of broad-band photoelectric photometry of 139 Juewa during 5 consecutive nights in March 1974 are presented. The synodic period found is 20.9 hr. A linear phase coefficient, β = 0.080 ± 0.004, is determined between phase angles of 0.9° to 1.5°. This value is similar to that for the lunar highlands and for three other asteroids (4 Vesta, 20 Massalia, 110 Lydia) at similar phase angles, indicating that these surfaces have comparable porosities. The composite lightcurve presented covers 80% of the rotational period; short timescale features in the lightcurve are seen which correspond to topography a few kilometers in size.

On matching the spectrum of Io: Variations in the photometric properties of sulfur-containing mixtures

Abstract The problem of comparing laboratory spectra of sulfur-containing binary mixtures with the spectrum of Io is discussed. For the satellite, the observable is the geometric albedo as a function of wavelength, whereas in the laboratory one often measures some other type of albedo. In a previous paper we demonstrated that for pure sulfur the multiplicative factor which converts the laboratory albedos to geometric albedos can be strongly wavelength dependent. The present paper demonstrates that this is also true for binary sulfur-containing mixtures. Furthermore, there is no universal conversion factor applicable to all binary mixtures, nor can the factor be interpolated for a particular mixture from the conversion factors of the two end members. The conversion factor is a function not only of the specific composition of a binary mixture, but of the relative particle size distributions of the two components, and must be measured specifically for each individual sample if a quantitative comparison between a laboratory sample and Ios surface is desired.