  The visible reflectance spectrum of many solar system bodies changes with
changing viewing geometry for reasons not fully understood. It is often
observed to redden (increasing spectral slope) with increasing solar phase
angle, an effect known as phase reddening. Only once, in an observation of the
Martian surface by the Viking 1 lander, was reddening observed up to a certain
phase angle with bluing beyond, making the reflectance ratio as a function of
phase angle shaped like an arch. However, in laboratory experiments this
arch-shape is frequently encountered. To investigate this, we measured the
bidirectional reflectance of particulate samples of several common rock types
in the 400-1000 nm wavelength range and performed ray-tracing simulations. We
confirm the occurrence of the arch for surfaces that are forward scattering,
i.e. are composed of semi-transparent particles and are smooth on the scale of
the particles, and for which the reflectance increases from the lower to the
higher wavelength in the reflectance ratio. The arch shape is reproduced by the
simulations, which assume a smooth surface. However, surface roughness on the
scale of the particles, such as the Hapke and van Horn (1963) fairy castles
that can spontaneously form when sprinkling a fine powder, leads to monotonic
reddening. A further consequence of this form of microscopic roughness (being
indistinct without the use of a microscope) is a flattening of the disk
function at visible wavelengths, i.e. Lommel-Seeliger-type scattering. The
experiments further reveal monotonic reddening for reflectance ratios at
near-IR wavelengths. The simulations fail to reproduce this particular
reddening, and we suspect that it results from roughness on the surface of the
particles. Given that the regolith of atmosphereless solar system bodies is
composed of small particles, our results indicate that the prevalence of
monotonic (...)
