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Figure 8.—Normalized polarizations of Icarus, Eros, and the Moon as functions of the inverse of wavelength. (Measurements for Eros are unpublished; observations of B. H. Zellner.)

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Figure 9.-Curve of polarization of Flora obtained in 1968-69 by J. Veverka at Harvard Observatory. (Photoelectric polarimeter.)

function of the inverse of the wavelength, with Mare Crisium of the Moon and unpublished results on Eros.

In 1968-69, J. Veverka (1971), using a photoelectric polarimeter with the Harvard 155 cm telescope, derived the polarization curve for Flora reproduced in figure 9.

COMPARISON WITH OTHER CELESTIAL OBJECTS

It is relevant to compare these polarization curves of Ceres, Pallas, Vesta, Iris, Icarus, and Flora with those of other celestial bodies practically devoid of atmospheres.

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Figure 10 shows the polarization curves of the four major satellites of Jupiter obtained by A. Dollfus (unpublished) with a photoelectric polarimeter attached to the 60 cm Meudon reflector and the 107 cm Pic-du-Midi reflector. The negative branches are systematically less pronounced, and the inversion angle smaller, than for asteroids.

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Figure 10.-Curves of polarization of the four Galilean satellites of Jupiter obtained by A. Dollfus with the 60 cm reflector of Meudon Observatory and the 107 cm reflector of Pic-du-Midi Observatory. (Photoelectric polarimeter; previously unpublished results.)

Figure 11 shows the complete polarization curve in orange light (580 nm) for Mercury based on observations by B. Lyot (1929) and A. Dollfus (unpublished). The inversion angle occurred at 24° and the negative branch is well pronounced.

Figure 12 shows in great detail the negative branch for the polarization curve of the Moon. This curve is almost identical to the case of Mercury; the minimum of polarization occurs near a = 12° with the value Pmin = -1.2 percent. The inversion angle occurs at oo = 24° and is almost independent of the albedo and the area on the Moon. Then, the polarization is positive and the steepness of the curve increases as albedo and wavelength decrease.

Figure 13 illustrates the polarization curves of planet Mars at three wavelengths. The observations, collected by A. Dollfus and J. Focas (1969), only include measurements selected when the atmosphere of Mars was apparently free of aerosols. The negative branch for X = 0.61 pum is almost identical to the case of the Moon and Mercury.

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Figure 11.-Curve of polarization of planet Mercury in orange light (580 nm). Measurements by B. Lyot and A. Dollfus of Meudon and Pic-du-Midi Observatories. (Visual and

photoelectric polarimeters.)

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Figure 12.-Detailed observations of the negative branch of the polarization curve for the Moon (orange light). The negative branch is independent of wavelength and area on the Moon. The dashed curves a, b, and c are from Oceanus Procellarum, Mare Serenitatis,

and Hipparchus, respectively.

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Figure 13.—Curves of polarization of the light area at the center of Mars' disk for wavelengths of 0.61, 0.55, and 0.51 um. Observations by J. Focas and A. Dollfus at Meudon and Pic-du-Midi Observatories. The measurements are selected for absence of detectable aerosols in the Martian atmosphere. •: visual polarimetry (clear regions); o: photoelectric polarimetry.

INTERPRETATION OF THE POLARIZATION CURVES OF THE MOON, MARS, AND MERCURY

The negative branches of the polarization curves on the Moon, Mercury, and Mars are strikingly identical, when compared to those of other celestial bodies; they are very well developed with a minimum near -1.2 percent and an inversion angle of about 24°. These properties correspond to an extreme case reached only by very specific types of materials; these criteria fortunately provide a discriminative identification of the nature and structure of these planetary surfaces. The behavior of these negative branches was found by B. Lyot (1929) to be reproduced on volcanic ashes. Since this pioneering work, extensive studies were developed by A. Dollfus (1955, 1956, 1962), who found this characteristic shape of the curve to be typical of fine powders mixed together and made of very absorbing material of different grain sizes as small as a few micrometers. Laboratory measurements (Dollfus, 1956) proved that multiple scattering between the absorbing grains is responsible for the negative branch. Other optical criteria, like the spectral variation of the albedo (for Mars), or the spectral variation of the polarization maximum (for the Moon), enable one to discriminate the nature of the absorbing material from which the powder originates. For the Moon, the best optical laboratory simulation was found in finely pulverized basalts (Dollfus et al., 1971a). For the planet Mars, the small grains should be ferreous oxides, like goethite or limonite, or coatings by these oxides (Dollfus, 1956; Dollfus et al., 1969). The remote identification was recently successfully confirmed, in the case of the Moon, by the lunar samples returned to Earth by the Apollo missions. Figure 14(a) displays a microphotograph of a pulverized basalt selected in 1954 as being the best simulation of the lunar surface for the optical properties concerned. This picture was published 16 yr ago and again in 1962 by A. Dollfus (1955, 1962). Figure 14(b) is a microphotograph of a sample of lunar

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Figure 14.-(a) Microphotograph of a pulverized basalt from lava flow simulating the optical properties of the lunar surface. Original size: 3 by 3 mm. This picture was obtained in 1954 and published in 1955 (Dollfus, 1955). (b) Microphotograph of an Apollo lunar fines sample obtained in 1970 in the same conditions as (a).

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