Parent volatiles in Comet 103P/Hartley 2 observed by Keck II with NIRSPEC during the 2010 apparition

Abstract: We conducted high-dispersion spectroscopic observations of Comet 103P/Hartley 2 in the near-infrared wavelength region using the Keck II telescope with NIRSPEC. We obtained observations on four dates, with the last observations performed during the EPOXI closest approach. For this work we focus on the observations performed on UT 2010 October 17 and 21, while observations carried out on UT 2010 September 16 and November 4 have been reported elsewhere. On all dates the spatial distributions in the coma of C2H6, HCN and C2H2 were similar to each other and consistent with the CN-jet morphology observed from optical observations. The spatial distributions in the coma of H2O and CH3OH were also similar to each other, but were generally different from C2H6, HCN and C2H2. There might be two distinct phases of ice in Comet Hartley 2; one is enriched in H2O and CH3OH, and another is enriched in more volatile species (C2H6, C2H2, and HCN). It is possible that highly volatile species like C2H6 were segregated from the H2O matrix when warmer conditions were prevalent. We summarize our spectroscopic observations and report absolute production rates and mixing ratios of parent volatiles. There was no evidence of any significant diversity in the mixing ratios of parent volatiles on different dates. Comet Hartley 2 was normal in CH3OH/H2O, C2H6/H2O, C2H2/H2O, NH3/H2O but depleted in H2CO/H2O and CH4/H2O. OPRs of H2O in Comet Hartley 2 measured in its 2010 apparition were consistent with those observed by the Infrared Satellite Observatory in 1997. [Copyright &y& Elsevier]