Lunar surface imaging polarimetry: 1. Roughness and grain size

Imaging polarimetry characterizes the physical nature of the lunar surface by the median size of the grains and by the roughness of the surface. A video-polarimeter was designed at Observatoire de Paris to produce at the telescope images of planetary surfaces in intensity, degree of linear polarization, flux of polarized light, and other relevant photo-polarimetric images. With these images, the median grain size Md and mean roughness slope angle theta have been derived and mapped over characteristic lunar features. The method is explained. Terrains of anomalous surface textures have been discovered. The Imbrian and Eratosthenian mare surfaces, worked by the long term accumulation of impacts and by space exposure, produce a "mature" morphology characterized by Md = 47 mu m and theta = 10 degrees, irrespective of the composition of the material. Ejecta blankets around craters of Copernican age or younger show smoother surfaces and larger grains. Pyroclastic deposits have the same roughness, theta = 10 degrees, but consist of grains too small to have been produced by mechanical impact comminution. Very specific terrain types have been discovered along the cliff of Rupes Recta and the crater Birt, which indicate clean rocks depleted of dust. These anomalies may result from seismic action when an impact formed the recent nearby crater Thebit A. (C) 1998 Academic Press.