A TENTATIVE IDENTIFICATION OF METHANOL AS THE PROGENITOR OF THE 3.52-MU-M EMISSION FEATURE IN SEVERAL COMETS

We observed Comets P/Brorsen-Metcalf (1989 X), Okazaki-Levy-Rudenko (1989 XIX), Austin (1989c1), and Levy (1990c) in the 3.44 - 3.64 μm spectral region. Continuum emission was detected in all four comets. The organic feature at 3.4 μm and an emission feature at 3.52 μm were detected in all comets except Okazaki-Levy-Rudenko. The spatial brightness profile of the 3.52-μm emission feature was measured for the first time. The observed spatial profile is consistent with that expected for uniform outflow from a central unresolved source and exhibits an optocenter which is displaced from that of the dust emission by 630 ± 90 km in the antisunward direction. We conclude that the molecule which gives rise to the emission is either directly sublimated from the cometary surface or is the dissociation product of a very short-lived parent species. The mixing ratio of the progenitor of the 3.52-μm feature relative to water varies by an order of magnitude in our sample of comets. The modeled infrared spectrum of CH3OH indicates that the observed spectra are consistent with the emission at 3.52 μm being due to methanol. Production rates of methanol are retrieved from a new fluorescence model, and the possible significance of variations in the mixing ratio is discussed in terms of cometary processing in the Oort cloud. © 1991.