Wavelength dependence of polarization. IV. Volcanic cinders and particles

Five laboratory samples were measured for comparison with the moon, using a Wollaston photopolari meter with filters near 0.36, 0.53, and 0.97 , having the sun as light source. Three of the samples were porous dust layers of ground volcanic cinder particles smaller than 37 , in "fairy-castle" structures. The other two were: a porous but solid lava fragment, and the same fragment covered with a fairy-castle dust layer made from the same lava. The solid lava fragment is more highly polarized than the moon, and has essentially no wavelength dependence. The wavelength and phase dependence of the lunar polarization is closely matched by the fairy-castle structures. No dependence of polarization on sample orientation was found for the volcanic materials. Differential photometry was done with the same filters using a smoked MgO layer as comparison. All samples show a linear brightness increase (in magnitudes) from 50 to 20 phase, similar to the moon. The fairy-castle structures show an appreciable opposition effect in the ultra- violet which, however, almost disappears in the green and infrared.