Detection of Abundant Ethane in Comet C/1996 B2 (Hyakutake) and Comet C/1995 01 (Hale-Bopp): Evidence for Modified Interstellar Ice

M. J. Mumma, M. A. DiSanti, N. Dello Russo (NASA-GSFC), K. Magee-Sauer (Rowan College), R. Novak (Iona College), M. Fomenkova (University of California at San Diego)

Abundant ethane (C tex2html_wrap_inline11 H tex2html_wrap_inline13 ) was detected in Comet C/1996 B2 and in Comet C/1995 01 with the use of high-resolution infrared spectroscopy at the NASA Infrared Telescope Facility on Mauna Kea. Ro-vibrational lines of the tex2html_wrap_inline15 band were used to derive production rates and rotational temperatures for ethane in these two comets. It was found that the ratio of C tex2html_wrap_inline11 H tex2html_wrap_inline13 /H tex2html_wrap_inline11 O was higher in Comet C/1996 B2 by about a factor of two. A high abundance of ethane is consistent with production of C tex2html_wrap_inline11 H tex2html_wrap_inline13 in modified icy grain mantles in the natal cloud. Ethane could form either by photolysis of methane rich ice or by hydrogen- addition reactions to condensed acetylene. The lower abundance of ethane relative to water in C/1995 01 suggests that processing of pre-cometary ices varied for these two comets.