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 (C₂H₆) and methane (CH₄) along with carbon monoxide (CO) and water (H₂O) 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 × 10²⁶ for C₂H₆, 1.2 × 10²⁷ for CH4, 9.8 × 10²⁷ for CO, and 1.7 × 10²⁹ for H₂O. An abundance of C₂H₆ comparable to that of CH₄ 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 C₂H₆ by gas-phase ion molecule reactions in the natal cloud core is energetically forbidden. The high C₂H₆/CH₄ ratio is consistent with production of C₂H₆ in icy grain mantles in the natal cloud, either by photolysis of CH₄-rich ice or by hydrogen-addition reactions to acetylene condensed from the gas phase.