The organic composition of Comet C/2000 WM 1 (LINEAR) revealed through infrared spectroscopy

Abstract

We investigated the parent volatile composition of the Oort cloud Comet C/2000 WM 1 (LINEAR) on 23–25 November 2001, using the Near Infrared Echelle Spectrograph on the Keck II telescope. Flux-calibrated spectra, absolute production rates, and mixing ratios are presented for H 2O, HCN, CH 4, C 2H 2, C 2H 6, H 2CO, CH 3OH and CO. Compared with “organics-normal” comets, WM 1 is moderately depleted in HCN, CH 4 and CH 3OH, and is even more depleted in C 2H 2 and CO. Its composition is thus intermediate to comets that are severely depleted in their organic volatile composition and those that exhibit “normal” organic volatile abundances. We argue that WM 1 may have formed closer to the young Sun than “organics-normal” comets, but at greater distance than the severely depleted comets, before its ejection to the Oort cloud. The mixing ratios of the above-listed organic volatiles agree day-by-day for 23–25 November 2001. Thus, there is no evidence of macroscopic heterogeneity in chemistry of this comet’s nucleus at the achieved measurement accuracy. As the first comet to show moderate organic depletion in parent volatiles, WM 1 represents an important addition to the emerging taxonomic classification based on chemical composition.