A SENSITIVE UPPER LIMIT ON THE METHANE ABUNDANCE IN COMET LEVY (1990C)

We conducted a sensitive search for infrared fluorescent emission from CH4 at lambda approximately 3.3-mu-m in comet Levy (1990c). If CH4 is a parent molecule released directly from the nucleus, then the 3 sigma limit on its abundance is CH4/H2O < 0.0031, assuming that the kinetic temperature of the inner coma is approximately 50 K and that the CH4 spin species are equilibrated at a temperature of 50 K or above. This upper limit is similar to the abundance of CH4 in comet Halley estimated from lower resolution infrared spectra (Kawara et al.) and is significantly lower than the abundance derived from a possible detection of CH4 in comet Wilson (Larson et al.). Since CO/H2O approximately 0.03 in comet Levy, we find that CH4/CO less-than-or-similar-to 0.1. This upper limit is not sufficient to discriminate between a solar nebula and an interstellar origin for cometary volatiles.