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M. K. Shepard (Bloomsburg University), P. Helfenstein (Cornell University)
We conducted a blind-test of the Hapke photometric function (Hapke, Theory of Reflectance and Emittance Spectroscopy, 1993) using data obtained with the Bloomsburg University Goniometer Laboratory (BUG Lab) (Shepard, LPSC abstract 1015, 2001). One goal of this work was to test the common assumption that photometric roughness is independent of wavelength. Six different materials with strong color contrasts were measured at approximately 700 different viewing geometries in multiple wavelengths and inverted using the Hapke (1993) model. In all cases, the optimal model fits imply surface roughness (theta-bar) of 5-16 degrees even though the samples appear macroscopically smooth to visual inspection. Additionally, the surface roughness shows a negative correlation with albedo. These observations are consistent with the hypotheses that photometric roughness (1) is dominated by micro-scale roughness and (2) is sensitive to the smallest scales at which shadows exist (Shepard and Campbell, Icarus, 134, pp 279-291,1998).
This work was supported by NASA's Planetary Geology and Geophysics program.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.