Methane in Oort cloud comets

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2003-10, Vol.165 (2), p.391-406
• Main Authors: Gibb, E.L., Mumma, M.J., Dello Russo, N., DiSanti, M.A., Magee-Sauer, K.
• Format: Article
• Language: English
• Subjects: Comets; composition; Infrared observations; Spectroscopy
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/S0019-1035(03)00201-X

We detected CH 4 in eight Oort cloud comets using high-dispersion ( λ/Δ λ∼2×10 4) infrared spectra acquired with CSHELL at NASA's IRTF and NIRSPEC at the W.M. Keck Observatory. The observed comets were C/1995 O1 (Hale–Bopp), C/1996 B2 (Hyakutake), C/1999 H1 (Lee), C/1999 T1 (McNaught–Hartley), C/1999 S4 (LINEAR), C/2000 WM 1 (LINEAR), C/2001 A2 (LINEAR), and 153/P Ikeya–Zhang (C/2002 C1). We detected the R0 and R1 lines of the ν 3 vibrational band of CH 4 near 3.3 μm in each comet, with the exception of McNaught–Hartley where only the R0 line was measured. In order to obtain production rates, a fluorescence model has been developed for this band of CH 4. We report g-factors for the R0 and R1 transitions at several rotational temperatures typically found in comet comae and relevant to our observations. Using g-factors appropriate to T rot as determined from HCN, CO and/or H 2O and C 2H 6, CH 4 production rates and mixing ratios are presented. Abundances of CH 4/H 2O are compared among our existing sample of comets, in the context of establishing their place of origin. In addition, CH 4 is compared to native CO, another hypervolatile species, and no correlation is found among the comets observed.