Species T_s* ISM-Gas ISM-Dust Most Comets P/Halley Hyakutake Hale-Bopp Implications

H2O 152 100 100 100 100 100 100 -

CH3OH 99 4-5 1 1-5 1.5 0.023 0.049Consistent with grains in ISM cloud cores

HCN 95 2 - 0.02 0.1 - - -

NH3 78 1 10 0.1-2 0.1-2 0.25 - Consistent with molecular clouds

CO2 72 10 0-3 - 2-4 5 - -

H2CO 64 0.2-1 - 0.5 0-5 0.002 - Variable in P/Halley

CH4 31 0.2-7 0-5 0.02 - 0.7 -First good detection in Hyakutake

CO 25 1000 13 1-7 7 5-5.8 6.4Distributed source and nuclear source in Halley

N2 22 10-1000 - 0.1 5 - - -

S2 20 - - 0.03 - 0.01 - Seen in only 1 comet before

C2H6 - - - - - 0.4 0.13 Could be a lower limit

CH4:CO - 0.001-0.003 0.13-2.4 - - 0.12 - -

Comet and ISM Composition Comparison