First High-Resolution Infrared Spectroscopic Measurements of Comet 2P/Encke: Unusual Organic Composition and Low Rotational Temperatures

We present the first high-resolution infrared spectra of the ecliptic comet 2P/Encke, acquired on UT 4 - 6 Nov. 2003, with the Near Infrared Echelle Spectrograph (NIRSPEC) on the Keck II telescope. 2P/Encke is a dynamical end-member among comets. Its very short period of 3.3 years (with perihelion at 0.34 AU and aphelion at 4.09 AU) exposes the nucleus to unusually high insolation throughout its orbit, raising the prospect that native ices may have experienced significant fractionation over time. Here, we present flux-calibrated spectra, production rates, and mixing ratios for H₂O, CH₃OH, HCN, H₂CO, C₂H₂, C₂H₆, CH₄ and CO, and compare the abundance ratios with the “organics-normal” population. We also extracted very low rotational temperatures (20 - 30 K) for H₂O, HCN, and CH₃OH in the near-nucleus coma, which correlate with one of the lowest cometary gas production rates ( 10²⁷ molecules s^-1) measured thus far in the infrared. We determined that 2P/Encke is enriched in CH₃OH, but depleted in C₂H₆, C₂H₂, HCN, CH₄, H₂CO and CO. We compared mixing ratios of these organic species measured on separate dates, and found no evidence of macroscopic chemical heterogeneity in this cometary nucleus, however, we are limited by sparse temporal sampling of our observations. The depleted abundances of most measured species but retention of the high temperature volatiles (H₂O, CH₃OH) are consistent with fractionation of 2P/Encke’s native ices by thermal processing while in its current orbit. 2P/Encke is unique in terms of its short period, unusual organic composition, low rotational temperatures and low production rates. The discovery of its unusual organic composition is an important contribution to the emerging chemical taxonomy of comets.