Light Scattering Properties of Asteroids and Cometary Nuclei

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Li, Jian-Yang
A'Hearn, Michael F.
McFadden, Lucy A.
The photometric properties of asteroids and cometary nuclei, bodies important for understanding the origin of the Solar System, are controlled by the physical properties of their surfaces. Hapke's theory is the most widely used theoretical model to describe the reflectance of particulate surfaces, and has been applied to the disk-resolved photometric analyses of asteroid 433 Eros, comet 19P/Borrelly, and asteroid 1 Ceres, in this dissertation. Near Earth Asteroid Rendezvous returned disk-resolved images of Eros at seven wavelengths from 450nm to 1050nm. The bidirectional reflectance of Eros's surface was measured from those images with its shape model and geometric data. Its single-scattering albedo, w, was found to mimic its spectrum, with a value of 0.33+/-0.03 at 550nm. The asymmetry factor of the single-particle phase function, g, is -0.25+/-0.02, and the roughness parameter, theta_bar, is 28+/-3 deg, both of which are independent of wavelength. The V-band geometric albedo of Eros is 0.23, typical for an S-type asteroid. From the disk-resolved images of Borrelly obtained by Deep Space 1 (DS1), the maps of its w, g, and theta_bar were constructed by modeling the reflectance of Borrelly terrain by terrain. w varies by a factor of 2.5, with an average of 0.057+/-0.009. g changes from -0.1 to -0.7, averaging -0.43+/-0.07. theta_bar is <=35 deg for most of the surface, but up to 55 deg for some areas, with an average of 22+/-5 deg. The 1-D temperature measurement from DS1 can be well described by the standard thermal model assuming a dry surface, except for one area, where the discrepancy can be explained by a sublimation rate that is consistent with the observed water production rate. HST images through three filters, covering more than one rotation of Ceres, were acquired. Its V-band lightcurve agrees with earlier observations very well. A strong absorption band centered at about 280nm is noticed, but cannot be identified. w of Ceres was modeled to be 0.073+/-0.002, 0.046+/-0.002, and 0.032+/-0.003 at 555nm, 330nm, and 220nm, respectively. The maps of w for Ceres at three wavelengths were constructed, with eleven albedo features identified. Ceres' surface was found to be very uniform.