Infrared Observations of an Outburst of Small Dust Grains from the Nucleus of Comet P/Halley 1986 III at Perihelion

A close examination of the 0.7- to 23-μm infrared data base acquired by Gehrz and Ney (1992, Icarus 100, 162-186) suggests that the nucleus of Comet P/Halley 1986 III emitted a burst of small dust grains during a 3-day period commencing within hours of perihelion passage on 1986 February 9.46 UT. The outburst was characterized by significant increases in the coma's grain color temperature T_obs, temperature excess (superheat: S = T_obs/T_BB), infrared luminosity, albedo, and 10-μm silicate emission feature strength. These changes are all consistent with the sudden ejection from the nucleus of a cloud of grains with radii of approximately 0.5 μm. This outburst may have produced the dust that was responsible for some of the tail streamers photographed on 1986 February 22 UT. The peak of the dust outburst occurred about 3 days before a pronounced increase in the water production rate measured by the Pioneer Venus Orbiter Ultraviolet Spectrometer. We suggest that jets that release large quantities of small particles may be largely responsible for some of the variable infrared behavior that has been reported for P/Halley and other comets during the past two decades. Such jets may also account for some of the differences between IR Type I and IR Type II comets.