Detection of Abundant Ethane and Methane, Along with Carbon Monoxide and Water, in Comet C/1996 B2 Hyakutake: Evidence for Interstellar Origin

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Abstract
The saturated hydrocarbons ethane (C 2 H 6 ) and methane (CH 4 ) along with carbon monoxide (CO) and water (H 2 O) were detected in comet C/1996 B2 Hyakutake with the use of high-resolution infrared spectroscopy at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. The inferred production rates of molecular gases from the icy, cometary nucleus (in molecules per second) are 6.4 × 10 26 for C 2 H 6 , 1.2 × 10 27 for CH 4 , 9.8 × 10 27 for CO, and 1.7 × 10 29 for H 2 O. An abundance of C 2 H 6 comparable to that of CH 4 implies that ices in C/1996 B2 Hyakutake did not originate in a thermochemically equilibrated region of the solar nebula. The abundances are consistent with a kinetically controlled production process, but production of C 2 H 6 by gas-phase ion molecule reactions in the natal cloud core is energetically forbidden. The high C 2 H 6 /CH 4 ratio is consistent with production of C 2 H 6 in icy grain mantles in the natal cloud, either by photolysis of CH 4 -rich ice or by hydrogen-addition reactions to acetylene condensed from the gas phase.