Ground-based near-infrared imaging of Comet P/Halley 1986 III

We present an analysis of 1-to 5-μm, near-infrared broadband images of Comet P/Halley 1986 III covering a 10⁴-km square region of the inner coma obtained on three consecutive nights in 1986 March during post-perihelion passage. In all images, the coma is extended in the sunward direction and appears distinctly non-spherical, similar to morphology in the 10-μm made by Hayward et al. (1987) 3.0 days later during the Giotto encounter. Marked variation in the coma's overall structure and brightness also was apparent, including the presence of a jet feature which we associate with a short-term outburst of material from the comet nucleus. The observed coma surface brightness dependence upon nucleocentric distance (r) at all wavelengths in both the comet dust tail and in the jet deviates from the dependence predicted by the "steady state" model for comet nucleus ablation, with the radial decrease in surface brightness being slower than r^-1 on the jet side of the nucleus and faster that r^-1 on the tail side. The near-infrared colors of the coma are not constant as a function of nucleocentric distance, suggesting that the grain properties are not uniform across the coma. Based on an elementary dynamical analysis of the trajectories of dust particles ablated from the nucleus we argue that these observations may be consistent with the hypothesis that particles emitted in jets fragments in the outflow on time scales of a few hours.