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Figure 23.-Log A versus log Pmax plot for 38 volcanic samples of pulverized basalts, ignimbrites, and ashes, and five Apollo lunar fines samples, measured at five wavelengths. The left and right ends of each segment correspond respectively to the wavelengths 354 and 580 nm.
Dollfus, A. 1955, Étude des Planètes par la Polarisation de la Lumière. Doctoral Thesis,
Lunar Material From Apollo 11. Proc. Apollo 11 Lunar Sci. Conf. Geochim. Cosmochim. Acta 34, suppl. 1, vol. 3, 2127-2147. Gehrels, T., Roemer, E., Taylor, R., and Zellner, B. 1970, Minor Planets and Related Objects. IV. Asteroid (1566) Icarus. Astron. J. 75, 186-195. Lyot, B. 1929, Recherches sur la Polarisation de la Lumière des Planètes et de Quelques Substances Terrestres. Doctoral Thesis, Univ. of Paris. (1964, NASA TT F-187.) Lyot, B. 1934, Polarisation des Petites Planètes. C.R. Acad. Sci. 199,774. McCord, B., Adams, J. B., and Johnson, T. V. 1970, Asteroid Vesta: Spectral Reflectivity and Compositional Implications. Science 168, 1445-1447. Provin, S. 1955, Preliminary Observations of the Polarization of Asteroids. Publ. Astron. Soc. Pac. 67,115. Veverka, J. 1971, Photopolarimetric Observations of the Minor Planet Flora. Icarus 15(3), In press.
ARRHENIUS: For the case of Vesta, on the basis of the high albedo you rule out lunar dust and powder; probably you have in mind Apollo 11 or 12 dust because the new data on Apollo 14 indicate a much higher albedo. DOLLFUS: We measured lunar fines on Apollo 12 on the lightest area available and none of them were higher than 0.25. ARRHENIUS: How about pure feldspar? DOLLFUS: Pure feldspar is too transparent to produce the curve of polarization observed. It cannot be basalt because basalts are too absorbent. ANDERS: I do not think we should dismiss the possibility of a regolith on asteroids. True, the escape velocity is low, but impact ejecta have a velocity distribution starting at zero, and so some fraction of the dust and debris will remain on the body. It does not matter that more material is lost than is gained in such an impact, because the loss is all in one place, the crater. The remainder of the body gets showered with low-velocity debris. Moreover, a few percent of the stony meteorites in each class contain a noble-gas component that seems to represent trapped solar wind. Wänke suggested some years ago that implantation of solar-wind ions might happen in the regoliths of bodies without atmospheres, and this has been beautifully confirmed by Apollo 11 and 12 lunar soils and breccias. Now, meteorites apparently do not come from the Moon, and if the asteroids are too small to have a regolith, we will be hard pressed to find larger bodies without atmospheres where these gas-rich meteorites might form. Comets are too small and too far away and in a region where the solar-wind flux is very low. The moons of Jupiter are too large to permit escape of ejecta and cannot send material into terrestrial space with any efficiency. Thus we are left with the asteroids. CHAPMAN: Anders has correctly noted that low escape velocity for asteroids need not imply that asteroids are not covered with a regolith or dust layer. The cratering and steady-state loss and accretion processes that govern the development of asteroidal regoliths are complex and depend on many factors. For example, in the case of a large population index for the impacting debris, a “sandblasting” effect may continually remove most or all of the regolith that otherwise would develop in the manner Anders mentioned. Dr. Dollfus, in the final part of his presentation, has described how measurements of the maxima of asteroid polarization curves have some compositional implications. He argued that such measurements would be an important goal for a space mission. Ground-based spectral reflectivity studies are much cheaper, and they are far more diagnostic of mineralogical composition. HAPKE: The relation between surface texture and the negative branch of polarization that Dr. Dollfus has described is certainly true. However, another property of materials that also affects the negative branch should be mentioned, and that is albedo. For a powder with a high albedo, such as pure feldspar or enstatite, the negative branch will be less pronounced than for a dark powder, such as Moon soil, so that there is some ambiguity in the interpretation of the negative polarization branch if the albedo is not known.
VEVERKA: I just want to note that the Lyot polarization curves for Ceres and Vesta were obtained by a rather insensitive photographic method and do not agree too well with more recent photoelectric measurements. One therefore should not base any inferences on them.
PHOTOMETRIC OBSERVATIONS AND REDUCTIONS OF LIGHTCURVES OF ASTEROIDS
ROWALD. C. TAYLOR
The brightness-time variation (the lightcurve) of an asteroid is observed to obtain the rate of rotation, some indication of the shape, and the orientation of the rotation axis in space. The brightness-phase relations are observed for the study of surface texture.
This paper deals specifically with observing routines and reductions, including discussions of lightcurves, rotation periods, absolute magnitudes, phase coefficients, opposition effects, and pole determinations. This paper supplements the review chapter by Gehrels (1970). Only photoelectric techniques are considered because the visual and photographic ones are, nearly without exception, not precise enough.
Photoelectric observations of asteroids were made as early as 1935 by Calder (1935). Calder already observed comparison stars, chosen for their proximity to the asteroid and for similarity in color and magnitude. The comparison star observations allow correction for photometric changes in the quality of the night and to remove extinction effects from the lightcurve. (This, of course, assumes that the comparison star does not vary during the night.) A value for scatter of the comparison readings can be computed as an indication of the quality of the night. Because it is impossible—with a single detector—to observe the comparison star and the asteroid at the same time, interpolation of comparison star readings is necessary. The deviation of the comparison star readings from a smooth secant Z curve is, at good sites as the McDonald and Kitt Peak Observatories, on the order of 0.01 mag; such effects remain uncorrected if no comparison star is observed (for instance, by Miner and Young, 1969, 1971). With careful comparison star corrections, the precision of the lightcurves generally is +0.003 mag (for the mean of three measurements). With a two-detector photometer this may be improved to +0.001 mag. (See Gehrels, 1970, fig. 4.)
With slight variations, the generally adopted observing routine is as follows: At the beginning and end of each run a red and a blue standard star are observed for B, V, and U. The order of the lightcurve observations is AASAA, CC, AASAA, CC, etc., where A represents the asteroid, C the comparison star, and S the sky readings. Automatic lightcurve reductions during the night of observing have not as yet been made. As a step toward that goal we have a preliminary automated lightcurve reduction program. Standard stars (not to be confused with the comparison star) are observed to determine the magnitude of the asteroid; generally, the UBV system of Johnson and Morgan (Johnson, 1963) is used. If one has a fast-moving telescope, the transfers can be performed during the lightcurve run rather than at the beginning and end of the night. Figure 1 illustrates the transfer routine. In certain circumstances, one may find it desirable to forsake standard star observations during an asteroid run. In those cases, the comparison stars—and more than one should be observed because of possible variations—can be tied in to standard stars on subsequent evenings.