Two different mechanisms can cause an opposition effect: shadow-hiding and coherent backscatter. Because each mechanism samples different properties, it is important to the interpretation of earth and planetary remote sensing observations to understand which dominates in a given medium. This question can be answered by measuring the circular polarization ratio, the ratio of the intensity of circularly polarized light reflected with the same helicity as incident to that reflected with the opposite helicity. If the opposition surge is caused primarily by shadow-hiding, the circular polarization ratio decreases with decreasing phase angle, while if it is caused primarily by coherent backscatter, the circulation polarization ratio increases. We have measured the circulation polarization ratio as a function of phase angle for lunar and terrestrial soils, vegetation and H2O frost. Typical samples of our results are shown in the figure, which plots reflectance, r, and the circulation polarization ratio against phase angle. The reflectances of all samples possess opposition effects 5-10 wide. The circulation polarization ratio increases with decreasing phase angle for the frost and dry terrestrial soil, showing that the opposition effects in these media are dominated by coherent backscatter. However, the circulation polarization ratio decreases for broad-leafed vegetation and moist terrestrial soil, showing that shadow-hiding is the important mechanism for these media.
Lunar and Planetary Science Conference
- Pub Date:
- March 1996
- COHERENT BACKSCATTER;
- OPPOSITION EFFECT;