Wavelength dependance of polarization. III. The lunar surface.

Photoelectric measurements of brightness and polarization were made on various lunar regions, using diaphragms about 10 sec of arc in diameter. In addition to UB V a UCI photometric system was used, where I has effective wavelength at 0.94 . The color index of lunar regions increases with phase angle a typical relation is B - V = +0.84+0.0017 a . Brightness-phase relations all show a nonlinear surge close to zero phase; this "opposition effect" may be as much as a factor of 2 brightness increase from +5 to 0?0 phase. If so, the full moon has V= - 13.35 mag.+0.06 (p.e.), and the geometric albedo at 5400 A is 21% with the maria having 13% and the highlands 27%; the uncertainty is mostly due to luminescence. Luminescence was detected in the photometry and independently confirmed by the polarimetry. In 1956/59 the lunar surface was 10-20% brighter than in 1963 November/1964 January. The effect was fairly constant from day to day; it probably varies with the solar cycle. Polarization-phase curves, with a filter at 0.54 M, were found to be similar to those of Lyot, while at 0.36 M the polarization generally was greater and at 0.94 smaller. Gradual rotation of the polarization position angle, as reported in the literature, was not found; to within +3 precision, the electric vector maximum was either perpendicular to the plane of scattering, or lay in that plane. The opposition effect is reproduced by the theoretical photometric functions derived by Hapke (J. Geophys. Res. 68, 4571,1963) for a tenuous surface texture, but a volume density of only 0.5% is indi- cated. The texture is remarkably uniform over the lunar globe, and it is similar to that of the asteroids. The lunar surface appears to be covered with a thin, about 0.06 mm, cloud of particles that have radius a-0.8 , and separation of about 8 M They presumably are accreted interplanetary particles. They are ionized and their charges, of the order of 10- esu, keep them separated and suspended, while partial recombination causes the observed luminescence. It appears that this surface cloud explains all photometric and polarimetric observations on the moon and asteroids.