Martian surface materials: effect of particle size on spectral behavior.

The presence of abundant limonite on Mars has long been the subject of controversy. Some advocates of abundant limonite also suggest that the albedo differences between Martian light and dark areas are caused by different sizes of particles in those areas. We show that the relative albedo is reversed from the blue to the red for samples of limonite with particles of different sizes. Observations of Mars reveal no blue-red albedo reversal between the light and dark areas. Consequently, the hypothesis of particle size control of albedo is incompatible with the presence of abundant limonite on Mars.