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T. Rettig (Univ. Notre Dame), M. J. Mumma (NASA GSFC), M. A. DiSanti, N. Dello Russo (Catholic Univ. and NASA GSFC), K. Magee-Sauer (Rowan Univ.)
Ethane and carbon monoxide were detected in a short-period comet of probable Kuiper belt origin. Ethane is substantially less abundant in Giacobini-Zinner compared with Hyakutake and Hale-Bopp, two comets of probable origin in the giant planet's nebular region. This difference suggests the presence of a heliocentric gradient in ethane in pre-cometary ices of the solar nebula. It is argued that processing by X-rays from the young sun may be responsible. The detection of methanol within the normal range of cometary abundances (Biver et al. 1999) argues against a general depletion of organic volatiles in G-Z. The depletion of ethane in this quintessential C2-depleted comet suggests that ethane will be depleted in the 1/3 Jupiter-family comets identified as carbon-chain depleted by A'Hearn et al. 1995.