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

The Deep Impact encounter with the Jupiter family Comet 9P/Tempel 1 on UT 2005 July 4 was observed at high spectral resolving power (lambda/delta lambda similar 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 H2O and C2H6 beginning 30 min prior to impact (T - 30) and ending 50 min following impact (T + 50), and for H2O 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 H2O and C2H6 near,T + 25. This showed C2H6/H2O to be higher than its pre-impact value by a factor 2.4 +/- 0.5, while HCN/H2O was unchanged within the uncertainty of the measurements. The mixing ratios for C21-16 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. (c) 2006 Elsevier Inc. All rights reserved.