Outbursts of H 2O in Comet P/Halley

Abstract

We detected rapid temporal variations in H 2O emission from Comet P/Halley while monitoring its gas production from the NASA Kuiper Airborne Observatory with a Fourier transform spectrometer. A continuous record of an H 2O outburst was fortuitously captured in one interferogram on UT 1986 March 20.7. According to the light curve derived from this scan the comet's H 2O brightness increased by a factor of 2.2 in less than 10 min. We attribute this outburst to some energetic process in the nucleus that dispersed approximately 1.0 × 10 −3 km 3 of nuclear material into the coma as gas and icy grains. A similar event apparently occured on UT 1986 March 24.7 when other observers also noted large changes in Comet P/Halley's activity. Conditions in the nucleus that could have powered the H 2O outbursts include crystallization of amorphous H 2O ice, chemical explosions, thermal stress, and pockets of compressed gas. The first two processes require sources of latent chemical energy in cometary matter, while the second two result from the deposition of solar energy in the nuclear surface. The timing and energy budget of the H 2O outbursts seem to require an internal source of energy. Crystallization of amorphous ice is most consistent with the observations and with compositional and thermal models of cometary nuclei, although exothermic chemical reactions cannot be excluded. Either process means that minimal thermal processing of Halley's volatile solids occurred in the solar nebula. This possibility reinforces other recent evidence, such as the detection of H 2CO and the low nuclear spin temperature deduced from the ortho/para H 2O ratio, that Comet P/Halley did not form in the planetary region of the solar nebula.