Detection of the 3.4 micron emission feature in comets P/Brorsen-Metcalf and Okazaki-Levy-Rudenko (1989r) and an observational summary

A survey was performed of the 3 micron spectral region of comets, including the broad 3.4 micron emission feature due to C-N bonds in organic molecules. One goal is to determine how the strength of the emission band varies with heliocentric distance r in comets. This depends on both the production rate of the organic molecules and the emission mechanism, neither of which is well determined at present. The observations to date are consistent with cometary organics being present in all comets at comparable abundances relative to water. Loss of contrast in the 3.4 micron feature as comets get closer to the sun is then easily explained by dilution by thermal emission from the continuum grains, whose flux rises more steeply with heliocentric distance than that of organics.