Because the opposition effect is a major remote sensing tool, it is important to understand its nature in the bidirectional reflectances of planetary regloliths. The opposition effect of the moon has traditionally been ascribed to shadow hiding (e.g., Hapke, 1986, Icarus, 67, 264). However, more recently, it has been suggested that the primary cause of the phenomenon is coherent backscatter, based on the upturn in the circular depolarization ratio in the reflectances of lunar soil samples as zero phase angle is approached (Hapke et al, 1993, Science, 260, 509). Further analysis suggests that, although coherent backscatter is important, shadow hiding also plays a major role. Evidence for this conclusion includes the following: (1) Increasing the albedo, either by changing the wavelength in the same lunar sample or by changing the sample, decreases the circular depolarization ratio. (2) The angular width of the brightness curve has little wavelength dependence. (3) Very close to zero phase angle the moon exhibits no polar darkening. All three observations are predicted if shadow hiding dominates the phase curve, but are not consistent with coherent backscatter as the sole cause of the opposition effect.
AAS/Division for Planetary Sciences Meeting Abstracts #28
- Pub Date:
- September 1996