Detection of Abundant Ethane and Methane, Along with Carbon Monoxide and Water, in Comet C/1996 B2 Hyakutake: Evidence for Interstellar Origin
The saturated hydrocarbons ethane (C_2H_6) and methane (CH_4) along with carbon monoxide (CO) and water (H_2O) 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 x 1026 for C_2H_6, 1.2 x 1027 for CH_4, 9.8 x 1027 for CO, and 1.7 x 1029 for H_2O. An abundance of C_2H_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_2H_6 by gas-phase ion molecule reactions in the natal cloud core is energetically forbidden. The high C_2H_6/CH_4 ratio is consistent with production of C_2H_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.