LABORATORY STUDY OF LIGHT SCATTERING BY
ROUGH SURFACES
AKIHIDE. KAMEI, TETSUNORI MINATO, AND AKIKO M.
NAKAMURA
Graduate School of Science and Technology, Kobe University
Laboratory photo-polarimetric measurements of terrestrial rocks and
meteorites have been performed to better understand the surface materials and
structure of asteroids from the data of ground-based observations. The
intensity and the state of polarization of light scattered from a solid surface
depend on the scattering geometry and the physical and geological properties
of the surface: the light scattering by asteroid surfaces is a complicated
problem and it is not established yet.
Recently, space missions have provided data of asteroids' scattering properties
at large phase angles, but less laboratory work that covers a wide range of
phase angle has been done. We performed laboratory measurements of
bidirectional reflectance of rough surfaces of terrestrial rocks and meteorites
over phase angles from 2 to 155 deg., by using a goniometric photopolarimeter
at Kobe University. The light source is a He-Ne laser at the wavelength of
632.8 nm. The detector is a photomultiplier and the signal is digitized by an
electrometer. Both the light source and the detector can be moved from 0 to 90
deg. of the zenith angle independently. Therefore, it is possible to perform
measurements at a wide range of phase angle. The samples were plates and
particulate surfaces consisting with different size distributions ranging from
tens to hundreds m.
We found that, in general, the particulate surfaces show forward scattering
lobes as expected from the forward single scattering character of natural soil
particles. We analyzed our bidirectional reflectance data by Hapke’s
photometric model and obtained positive values of the asymmetry parameter
for most of particulate surfaces, although the values determined for asteroids
are usually negative. We further discuss a comparison of our results of the
other Hapke parameters with those of asteroids.