Discovery of Abundant Ethane and Methane in C/1996 B2 Hyakutake; Implications for Cometary and Interstellar Organic Chemistry

Details Discovery of Abundant Ethane and Methane in C/1996 B2 Hyakutake; Implications for Cometary and Interstellar Organic Chemistry Discovery of Abundant Ethane and Methane in C/1996 B2 Hyakutake; Implications for Cometary and Interstellar Organic Chemistry

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0914m&link_type=abstract

American Astronomical Society, DPS meeting #28, #09.14; Bulletin of the American Astronomical Society, Vol. 28, p.1090

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We used high resolution infrared spectroscopy at the NASA IRTF to detect, for the first time, cometary emission from ethane (C_2H_6) and methane (CH_4), in C/1996 B2 Hyakutake on UT 1996 March 23.5 and 24.5 (Mumma et al. Science 272:1310-1314, 1996). We also detected CO and H_2O, and other forms of volatile carbon (e.g. HCN, C_2H_2, H_2CO, CH_3OH), and searched several other hydrocarbons. The emissions exhibit spatial profiles expected for parent volatiles, except for CO which shows evidence for both direct nuclear and distributed sources (DiSanti et al., this meeting). The production rates (in molecules/s) on UT March 24.5 were: C_2H_6 6.4 x 10(26) , CH_4 1.2 x 10(27) , CO 9.8 x 10(27) , and H_2O 1.7 x 10(29) . The CO production rate refers mainly to the direct nuclear source. The high ethane/methane ratio is consistent with production of ethane in icy grain mantles in the natal cloud core or in the solar nebula, either by hydrogen-addition reactions to acetylene condensed from the gas phase or by photolysis of methane-rich ice. This observed ratio has been reproduced by condensation fractionation at 66K in the laboratory (Notesco et al. 1996, Icarus, submitted); however, it is not clear whether the observed abundances of other volatile organics are also consistent with that model. The presence of abundant CH_4 and C_2H_6, the relatively low abundance of CH_3OH, and the high abundance of CO and C_2H_2 (Brooke et al. 1996, Nature, submitted) provide a stringent test of models for the origin of cometary ices. These suggest that comet Hyakutake's ices did not originate in a thermochemically equilibrated region of the solar nebula. The ethane/methane abundance ratio is consistent with a kinetically controlled production process, but production of ethane by gas-phase ion molecule reactions in the natal cloud core is energetically forbidden.

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