Grain Size Dependence of Brightness Phase Curves of the Lunar Surface

Abstract

AbstractThe lunar opposition effect is the increase of brightness on the lunar surface when the phase angle approaches 0°, which is caused mainly by shadow‐hiding and coherent backscattering. However, the contribution of coherent backscattering is not yet well established. This study analyzes the contribution of coherent backscattering to the opposition effect using the correlation between the contribution of increased brightness of the theoretical coherent backscattering and the grain size of the reflecting surface. We analyze the correlation between the two using data from the Lunar Reconnaissance Orbiter Wide Angle Camera and median grain size d calculated by linear polarization observations. The size of effective scatterers of coherent backscattering is similar to the observation wavelength. In this case, the effective scatterer size for coherent backscattering is 0.6 μm. The results of our analyses show that there is no correlation between d and reflectance phase curve in the phase angle range from 0° to 6°. In addition, we analyze the distribution of grain size of lunar samples collected during the Apollo and Luna missions. We find an inversely proportional correlation between mean grain sizes and submicron grain content in the lunar samples. Therefore, d seems to indicate the content of effective scatterer. The strength of the coherent backscattering opposition effect seems to be affected by the content of the submicron grains. Thus, the lack of correlation between d and the opposition effect seems to imply minimal contribution of coherent backscattering to the lunar opposition effect.