Temporal evolution of DI ejecta based on NIRSPEC observations at keck 2: parent volatiles and dust

Abstract

The Deep Impact encounter with 9P/Tempel 1, a Jupiter Family Comet (JFC), on UT 2005 July 04 was observed at high spectral resolution (λ/δλ ≈ 25,000) using the cross-dispersed near-infrared echelle spectrometer (NIRSPEC) at Keck-2. This permitted a simultaneous measure of individual line intensities for several parent molecules in addition to dust continuum emission. Column abundances are presented for H2O and C2H6 beginning 30 minutes prior to impact (T-30) and ending 50 minutes following impact (T+50), and for H2O and HCN from T+50 until T+96, in time steps of approximately six minutes 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 C2H6 and HCN in the ejecta agree with those found in the majority of Oort cloud comets, perhaps indicating a common region of formation. This stands in contrast with the recently observed split JFC 73P/Schwassmann-Wachmann 3 and the disintegrated Oort cloud comet D/1999 S4 (LINEAR), both of which were depleted in most organic volatiles, but not HCN. The expanding dust plume was tracked through the 3.5-μm spectral continuum and through 2-μm images acquired with the slit-viewing camera (SCAM). These showed a monotonic increase in continuum intensity following impact. The relatively sudden appearance of the volatile ejecta signature is attributed to heating of icy grains (perhaps to a threshold temperature) that were decreasingly shadowed by intervening (sunward) particles. © 2009 Springer-Verlag Berlin Heidelberg.