Spectroscopy of the 3.4 micron emission feature in comet Halley

Infrared spectra in the 3-5 micron region have been obtained of Comet Halley after perihelion, at heliocentric distances of 1.6 and 2.0 AU. A broad emission feature, peaking near 3.4 microns and containing some spectral substructure, was observed, while at longer wavelengths only a featureless blackbody emission spectrum was seen. The emission feature probably arises from UV-pumped infrared fluorescence of organic molecules which are either in the gas phase or are embedded in very small grains. In the former interpretation the molecules must be quite large. These results lend support to the idea that comets formed out of interstellar grains whose molecular ice mantles largely consist of nonvolatile complex organic molecules. 21 references