36th DPS Meeting, 8-12 November 2004
Session 34 Comets: Nuclei, Tails, Solar Wind
Poster II, Thursday, November 11, 2004, 4:15-7:00pm, Exhibition Hall 1A

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[34.04] Surface Photometric Variation of Comet Borrelly's Nucleus

J. Li, M. F. A'Hearn, L. A. McFadden (Department of Astronomy, University of Maryland College Park)

Comet Borrelly was visited by Deep Space 1 in Sept. 2001 (Soderblom et al. 2004) The images of comet Borrelly's nucleus show large brightness variation over the surface even after the effect of shape is taken into account (Oberst et al. 2004, Kirk et al. 2004). It is not yet known whether this variation is caused by albedo variation (Oberst et al. 2004, Buratti et al. 2004) or the variation of other physical properties such as surface roughness (Kirk et al. 2004) or solar phase function.

In our analysis, the disk-resolved images from the DS1 spacecraft (Soderblom et al. 2004) were used, coupled with the shape model of Borrelly's nucleus developed from stereo imaging (Oberst et al. 2004, Kirk et al. 2004), to fit the bidirectional reflectance as a function of local illumination and viewing geometry for individual terrains as defined by Britt et al. (2004). Results show that the surface reflectance variation is, contrary to previous interpretations, most likely due to the combination of albedo variation (a factor of 1.5) and the variation of the asymmetry factor (g) of the single-particle phase function. We find the roughness parameter (theta_bar) is <25o over the surface. The surface on Borrelly's nucleus can be highly back-scattering (g <= -0.7) for mottled terrain, and close to isotropic scattering ( g ~ -0.15) for smooth terrain, with single scattering albedo ranging from 0.05 to 0.07. This work is supported by NASA grant NNG04GA92G.

The author(s) of this abstract have provided an email address for comments about the abstract: jyli@astro.umd.edu

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.