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J. L. Piatek, B. W. Hapke (U. of Pittsburgh), R. M. Nelson, W. D. Smythe, A. S. Hale (JPL)
The physics of scattering of electromagnetic waves by planetary regoliths and other media in which the particles are in contact has been thought to be relatively well understood when the particles are larger than the wavelength. However, this is not true when the particles are comparable with or smaller than the wavelength. We have measured the scattering parameters of well-sorted suites of planetary regolith analogs whose particles ranged from larger to smaller than the wavelength. The particle compositions were Al2O3, Fe2O3 and CaCO3 and the wavelength was 633 nm. We measured the variation of reflectance as the phase angle varied from 0.05 to 140 degrees. From these datasets the following scattering parameters were deduced: the single scattering albedo, single scattering phase function, transport mean free path, and scattering, absorption and extinction coefficients. The results and conclusions of this study are as follows.
(1) A scattering model based on the equation of radiative transfer was empirically able to quantitatively describe the variation of intensity with angle for each sample. Thus, such models can be used to characterize scattering from regoliths even when the particles are smaller than the wavelength.
(2) The scattering parameters were remarkably insensitive to particle size (with the exception of the single scattering albedo, which for absorbing particles decreased as particle size increased). These results are consistent with previous laboratory studies [1,2], but show in addition that the lack of large size effects are not confined to the opposition effect. These findings are contrary to current theoretical models, which predict large size effects.
(3) These results imply that the particles composing the medium are NOT the fundamental scattering units of the medium. Thus, a basic assumption made by virtually all regolith scattering models appears to be incorrect. Our understanding of scattering by regoliths appears to be incomplete even when the particles are larger than the wavelength.
This research was performed under a grant from NASA's PGG program.
References cited: [1.] Nelson et al, Icarus, 147, 545-558 (2002). [2.] Nelson et al, Planet. Space Sci., 50, 849-856 (2002).
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.