12. SpeciesT_s * ISM-GasISM-DustMost CometsP/HalleyHyakutakeHale-BoppImplications H2OH2O152100 - CH 3 OH994-511-51.50.0230.049 Consistent with grains in ISM cloud cores HCN952-0.020.1--- NH 3 781100.1-2 0.25- Consistent with molecular clouds CO 2 72100-3-2-45-- H 2 CO640.2-1-0.50-50.002- Variable in P/Halley CH 4 310.2-70-50.02-0.7- First good detection in Hyakutake CO251000131-775-5.86.4 Distributed source and nuclear source in Halley N2N2 2210-1000-0.15--- S2S2 20--0.03-0.01- Seen in only 1 comet before C2H6C2H6 -----0.40.13 Could be a lower limit CH 4 :CO-0.001-0.0030.13-2.4--0.12-- Comet and ISM Composition Comparison