Temporal evolution of parent volatiles and dust in Comet 9P/Tempel 1 resulting from the Deep Impact experiment

Byline: Michael A. DiSanti (a), Geronimo L. Villanueva (a), Boncho P. Bonev (a)(b), Karen Magee-Sauer (c), James E. Lyke (d), Michael J. Mumma (e) Keywords: Comets; Spectroscopy; Infrared observations Abstract: The Deep Impact encounter with the Jupiter family Comet 9P/Tempel 1 on UT 2005 July 4 was observed at high spectral resolving power (I'/[delta]I'[approximately equal to]25,000) using the cross-dispersed near-infrared echelle spectrometer (NIRSPEC) at Keck-2. We report the temporal evolution of parent volatiles and dust (simultaneously measured) resulting from the event. Column abundances are presented for H.sub.2O and C.sub.2H.sub.6 beginning 30 min prior to impact (T-30) and ending 50 min following impact (T+50), and for H.sub.2O and HCN from T+50 until T+96, in time steps of approximately 6 min post-impact. The ejecta composition was revealed by an abrupt increase in H.sub.2O and C.sub.2H.sub.6 near T+25. This showed C.sub.2H.sub.6/H.sub.2O to be higher than its pre-impact value by a factor 2.4[+ or -]0.5, while HCN/H.sub.2O was unchanged within the uncertainty of the measurements. The mixing ratios for C.sub.2H.sub.6 and HCN in the ejecta agree with those found in the majority of Oort cloud comets, perhaps indicating a common region of formation. The expanding dust plume was tracked by continuum measurements, both through the 3.5-[mu]m spectral continuum and through 2-[mu]m images acquired with the SCAM slit-viewing camera, and each showed a monotonic increase in continuum intensity following impact. A Monte Carlo model that included dust opacity was applied to the dust coma, and its parameters were constrained by observations; the simulated continuum intensities reproduced both spectral and SCAM data. The relatively sudden appearance of the volatile ejecta signature is attributed to heating of icy grains (perhaps to a threshold temperature) that are decreasingly shadowed by intervening (sunward) dust particles in an optically thick ejecta plume, perhaps coupled with an accelerated decrease in dust optical depth near T+25. Author Affiliation: (a) Solar System Exploration Division, NASA Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA (b) Department of Physics, The Catholic University of America, Washington, DC 20064, USA (c) Department of Physics and Astronomy, Rowan University, Glassboro, NJ 08028-1701, USA (d) W.M. Keck Observatory, 65-1120 Mamalahoa Hwy, Kamuela, HI 96743, USA (e) Solar System Exploration Division, NASA Goddard Space Flight Center, Code 690, Greenbelt, MD 20771, USA Article History: Received 8 May 2006; Revised 13 September 2006