Infrared investigation of water in comet P/Halley

Summary

Gaseous, neutral water was detected in the coma of comet P/Halley during infrared (IR) observations from the NASA Kuiper Airborne Observatory (KAO). High resolution spectra revealed intense, solar-pumped IR fluorescent emission in the (001–000) band of H₂O near λ ~ 2.65 µm. Three lines of the (011–010) “hot” band were also observed. In this paper, we summarize the results of our KAO H₂O investigation. Besides detecting H₂O and measuring its absolute production rate, we measured its spatial distribution, temporal variability, outflow velocity, and have determined the H₂O ortho/para ratio. The measured line intensities imply that water in the coma is rotationally relaxed, as predicted by recent nonthermal equilibrium models. However, significant differences in excitation are detected between a fluorescence equilibrium model and the observed spectra, and these differences may yield important information on the coma kinetic temperature. We find that the aperture-averaged excitation temperatures for the low-lying rotational transitions are ~ 40 K, indicating that the coma kinetic temperature is extremely cold. Excitation differences are also detected between the pre- and post-perihelion data, and between nucleus-centered and offset exposures. Swings effects may explain some of the differences. We have detected significant short-term temporal variations in the H₂O line intensities, and find a strong pre- to post-perihelion asymmetry (a factor of ~ 7) in the water production rate at a heliocentric distance of ~ 1 AU. Finally, we comment on possible future directions for high resolution IR spectroscopy of comets.