The Volatile Composition of Comet 17P/Holmes after Its Extraordinary Outburst

The volatile abundances in comet 17P/Holmes were measured on three dates (UT 2007 October 27.6 and 31.3 and November 2.3) using high-dispersion (λ/Δ λ ~ 2.5 × 10⁴) infrared spectroscopy with NIRSPEC at the W. M. Keck Observatory and CSHELL at the NASA Infrared Telescope Facility. Compared to other comets, the relative gas production rates in the coma show an enhancement of C₂H₆, HCN, and C₂H₂ with respect to H₂O, by factors of ~2-3. CH₃OH was also detected with an abundance relative to H₂O that is similar to or perhaps slightly enhanced compared to the values observed in other comets. The apparent enrichment of some volatiles in the coma of 17P/Holmes does not necessarily imply an unusual composition for its nucleus because fractionation effects may be important at the relatively large heliocentric distance (R_h = 2.45 AU) at which our observations were performed. Rotational temperatures were determined for H₂O, HCN, C₂H₆, and C₂H₂ in the coma on UT October 27.6 and found to be between 60 and 80 K. We used lines in both the ν₅ and ν₇ bands to obtain the best constraints yet achieved for the rotational temperature of C₂H₆. The spatial distributions of all measured volatiles in the coma are consistent with each other and suggest at most only a minor contribution from sublimating icy grains within our aperture. The overall gas production rate declined by approximately a factor of 7 between UT October 27.6 and November 2.3 with no significant change measured in the relative production rates of C₂H₆ and H₂O during this time.