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R.M. Nelson (Jet Propulsion Laboratory), Yu. Shkuratov, A.A Ovcharenko (Kharkov State University), W.D. Smythe (Jet Propulsion Laboratory), B.W. Hapke (Univ. of Pittsburgh)
The reflectance phase curves at very small phase angle can potentially give important information on structure of the planetary regoliths. This is because the phenomenon of coherent backscatter is particle size dependent and influences the shape of the phase curve at small phase angles. Two instruments, one at JPL and the other at the Kharkov University, are currently active in providing laboratory data on the opposition effect to the planetary science community. Both instruments are capable of making measurements at phase angles of less than a degree. Studying the same samples, we find that the reflectance phase curve measurements with both instruments agree reasonably rather well. We have started joint measurements to understand how the slope of the phase curve at small phase angles depends quantitatively on particle size of powdered samples. For this purpose we use highly reflective powders of quartz and aluminium oxide with different size of particles. These samples have approximately the same optical constants (n=1.5 and 1.7 respectively) and albedo(s) ~95 at the phase angle 2 deg. We present here results obtained by the Kharkov goniometer. It used an incandescent light source with an R wavelength filter. It can measure from 0.1 to 3.5 degs. We used 7 samples of aluminium oxide powder with the particle sizes 0.1-30 um and 3 quartz samples with the particle size 0.01-150 um. For all these samples the phase dependences were measured. Then, these data were normalized. For each curve the phase ratio 0.2/2.0 degs. was found. The table shows that this ratio has a minimum at 0.7 um.
| particle size | 0.01 | 0.1 | 0.3 | 0.7 | 1.0 | 2.1 | 5.8 | 30 | 53 | 150 |
| ratio 0.2/2.0 | 0.952 | 0.853 | 0.831 | 0.828 | 0.830 | 0.850 | 0.860 | 0.887 | 0.925 | 0.963 |
Our preliminary explanation of this particle-size resonance is based on an assumption that at the mean distance between scatterings is minimum for particles approximately equal to the wavelength in size.