Can weak localization of photons explain the opposition effect of Saturn's rings?

Abstract

Weak localization of photons in discrete disordered media (or the coherent backscattering mechanism) is shown to be a likely explanation of the opposition effect exhibited by Saturn's rings. Specifically, we assume that the particles of Saturn's rings are covered with small H2O ice grains and compute theoretically the opposition effect produced by these grains via the coherent backscattering mechanism. Both the width and amplitude of the observed opposition effect at visible wavelengths are consistent with theoretical calculations for effective grain radii of about 0.1 – 1 µm. Such grains are known to be present in the outer B ring of Saturn and give rise to the so-called 'spokes'. Thus, we demonstrate that the opposition effect of Saturn's rings may be due to the surface properties of the individual ring particles rather than to interparticle shadowing, as is usually assumed.