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fines returned to Earth by the Apollo 11 mission and photographed under the same conditions as figure 14(a). The striking similarity is convincing as regards the validity of the polarimetric criteria from remote identifications. (See also Geake et al., 1970.)
LABORATORY POLARIZATION STUDIES RELEVANT TO
For the purpose of interpreting the asteroid's polarization curves, we are developing, at Meudon Observatory, a program of laboratory measurements on samples expected to simulate the asteroidal conditions. This survey is still in progress. Among the likely candidates for the simulations of the superficial properties of asteroids are frost deposits, aggregated cosmic dust, impact-generated regolith from lunar or meteoritic material, and bare rocks superficially pitted and etched by impacts and possibly coated by adhesive grains. Laboratory measurements on the polarization of frost deposits were obtained by A. Dollfus (1955); the amount of polarization is very low for all phase angles. Optical measurements on deposited cosmic dust are difficult, although some preliminary indications are obtained (see below). Particularly relevant to the asteroidal problems are the lunar surface samples returned to Earth by the Apollo missions. These minerals were exposed to the etching, weathering, or disaggregation by long space exposure and meteoritic impacts; these processes should reproduce those operating on asteroidal surfaces. Of special interest are the samples of lunar fines collected at the surface of the regolith layer and generated by impact pulverization of the lunar surface. Figure 15 shows the polarization curves for Apollo lunar fines sample 10084,6 from Mare Tranquillitatis. The albedo in orange light is 0.075. The curves of polarization for the full range of phase angles are given in five wavelengths. The negative branches of the polarization curves, given in figure 15, bottom right, are identical to those of the lunar surface measured through telescopes, proving that the physical structure of the powder in the laboratory retains the original configuration it had on the lunar surface. Electron microscope scanning images of the grains are given in figure 16, up to a scale releasing details smaller than the wavelength. These documents were obtained at Manchester by Dr. J. E. Geake. The texture is very complex with grains of all sizes, showing many shock features. It is the multiple scattering between all these grains that is responsible for the deep negative branch of polarization with inversion angle as high as 24°. The lunar rocks should also be compared to asteroids because, as a result of the low value of their escape velocities, the small asteroids hardly retain the powder ejected by impacts (although cohesion and adherence due to vacuum
may replace the gravitational retention); this is precisely the case for the rocks collected at the surface of the lunar regolith. Figures 17 and 18 correspond to a lunar crystalline rock, Apollo sample 12051,51. The albedo in orange light is 0.24 and the color is reddish; further information is found in Dollfus et al. (1971a). The maximum of polarization (fig. 17) is higher than on the fines (fig. 15) and occurs for larger phase angles. The negative branch, reproduced on a larger scale in figure 18, is definitely less pronounced than for the lunar fines (fig. 16), with a minimum of -0.4 percent only (against - 1.2 percent for the fines) and an inversion angle of 16° (against 24° for the fines). The explanation is given by the electron scanning microscope images obtained for the purpose by Dr. J. E. Geake and reproduced in figure 18; the texture is definitely smoother than the samples of fines. The picture at top right, with the highest magnification, characterizes the average texture and can be compared with the picture of the lunar fines taken with the same magnification; the multiple scattering is less efficient in these structures; furthermore, an appreciable fraction of the surface releases glassy aspects due to splashes of melted silicates reproduced in the picture at center; these areas are almost devoid of appreciable multiple scattering. Finally, because of the smaller effect of the multiple scattering, this typical lunar crystalline rock gives a small negative branch of polarization. Figure 19 characterizes a particularly glassy lunar rock, Apollo sample 12002,102. The albedo is 0.13 in orange light. The largest fraction of the
Figure 17.-Curves of polarization of Apollo lunar crystalline rock 12051,51. The photoelectric measurements were made in five wavelengths. (Meudon Observatory.)
surface appears to be almost smooth, as strikingly seen in the image on top right. A still greater enlargement is shown in the picture at center. The multiple scattering should be limited and the negative branch of polarization very small; the curve, reproduced at bottom, has a minimum of -0.2 percent only and an inversion angle of 10°. Figure 20 belongs to a lunar breccia, Apollo sample 10059,36, with albedo 0.095 and an almost gray color. Each of the three images is centered on the same area, with increased enlargement, by factors of about 3. This is a cohesive mixture of grains, exceptionally rough in all scales; some glassy grains are incorporated; the picture at center shows a glassy fragment cemented (in the upper half) with a clump of very small cohesive grains (lower half). But the multiple scattering is dominant, and the negative branch of polarization is almost as pronounced as in the case of the lunar fines. Additional results on polarization properties of lunar rocks and fines are found in the two papers by Geake et al. (1970) and Dollfus et al. (1971b).