DPS 2001 meeting, November 2001
Session 62. Laboratory Studies
Oral, Chairs: R. Wu, R. Hudson, Saturday, December 1, 2001, 4:40-5:50pm, Regency GH

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[62.03] Scattering Properties of Planetary Regolith Analogs with Particle Sizes Larger and Smaller than the Wavelength

J.L. Piatek, B. Hapke (University of Pittsburgh), R.M. Nelson, W.D. Smythe, A. Snyder Hale (Jet Propulsion Laboratory)

To a good approximation, the particles in a planetary regolith can be treated as independent scatterers when they are large compared to the wavelength. However, the nature of the scattering when the particles are comparable to or smaller than the wavelength is poorly understood. The reflectances of well-sorted abrasive powders and pigments were measured in linearly and circularly polarized light in order to determine how their scattering properties vary with particle size. Thirteen aluminum oxide powders with sizes between 0.1 and 30 micrometers and six iron oxide powders with sizes between 0.15 and 2.5 micrometers were measured at a wavelength of 0.635 micrometers over a range of phase angles from 0.5 to 140 degrees. Data at smaller angles (0.5 - 5 degrees) for the same aluminum oxide samples have previously been discussed by Nelson et al., 2000 (Icarus 147, 545-558).

Initial results are as follows. Contrary to theoretical expectations, the aluminum oxide powders all exhibit coherent backscatter peaks with HWHM's of about 2 degrees, regardless of particle size. The iron oxide powders all exhibit coherent backscatter peaks with HWHM's of about 20 degrees, again showing no dependence on particle size. The single particle phase functions of all the powders exhibit well-developed backscatter lobes. Over the range of phase angles studied, none of the powders had the strong forward scattering lobes predicted by theoretical models based on scattering by spheroids. As the particle size decreases there is no indication of the change from Mie-type forward scattering to Rayleigh scattering predicted by models that assume the particles scatter independently.

This work was funded by grants from NASA's Planetary Geology and Geophysics program and the Pennsylvania Space Grant Consortium.

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