Interpretation of the surface brightness of Phobos

Analysis of disk resolved images of Phobos obtained by the Phobos 2 spacecraft makes it possible to study the surface scattering law and albedo variations. Low phase angle images reveal variations in local geometric albedo about 10 percent, with a correlation length of about 1 km. The scattering law is reasonably well matched by the recently proposed LPI (Lumme et al., 1990) model, which makes it possible to deduce a small scale (1 mm) surface roughness (0.5), defined here as the rms tangent of the local surface normal relative to the mean surface normal in the Duxbury (1991) model of Phobos. This value is very close to what has been found for Mercury and the moon.