ABSTRACT

The images returned by Deep Space 1 (DS1) during its encounter with comet 19P/Borrelly were used to study both disk-integrated and disk-resolved photometry, as well as the thermal properties, of this comet. The disk-integrated phase function has been constructed from both DS1 images and ground-based observations, showing geometric albedo of 0.072±0.020, and a phase slope of 0.043 mag/deg. With the help of the shape model of Borrelly [Kirk et al. 2004, Icarus 167, 54-69], and the ephemerides of DS1 and the comet, the disk-resolved photometry was analyzed with Hapke theoretical model. It has been found that the surface of Borrelly displays large photometric heterogeneities in all three modeled photometric parameters. The singlescattering albedo, w, varies by a factor of 2.5, with an average of 0.057±0.009; the asymmetry factor of single-particle phase function, g, ranges from almost isotropic (- 0.1) to strongly backscattering (-0.7), averaging -0.43±0.07; the roughness parameter, ¯θ, is less than 35◦ for most part of surface, but ranges up to 55◦ for some areas, and averaging 22◦±5◦. The observed 1-D temperature profile from DS1 can be modeled well by the standard thermal model (STM) for inactive regions, consistent with a very low thermal inertia. Water sublimation in the source region of the fan jet is observed to decrease the surface temperature from the STM predicted by 20-40 K. The source areas of two collimated jets could not be determined from either photometric model or thermal model. It is evident that the fan jet activity occurring on Borrelly’s surface can be correlated to areas of relative high albedo, weak backscattering, and high roughness.