Photometry and Imaging Results for Comet 9P/Tempel 1 andDeep Impact: Gas Production Rates, Postimpact Light Curves, and Ejecta Plume Morphology

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Abstract
We present Earth-based imaging and photometry results of Deep Impact target comet 9P/Tempel 1 obtained at and in the nights surrounding the time of the spacecraft's impact. These observations establish the baseline behavior of Tempel 1 prior to impact, including a water vaporization rate of 6 × 1027 molecules s-1, thereby enabling us to determine the effects directly caused by this explosive event. The instantaneous fireball was not detected, but a postimpact brightening from ejecta material was prominent and shows evidence of a slowly decreasing optical depth over more than an hour of time. We have successfully reproduced both the general morphology of the ejecta plume and many details of the shape and brightness distribution on successive nights following the impact using a modified Monte Carlo jet model, with an initial impulse event in the shape of an open, thick-walled cone. As seen from Earth, the center of the plume is derived to be at a position angle of about 255° and about 20° this side of the plane of the sky, i.e., near the limb of the nucleus. Most of the observed ejecta material had an initial outflow velocity of less than 0.23 km s-1 and particle sizes of less than 2.5 μm (assuming compact grains). This resulted in the rapid development of a dust tail from ejecta particles as they are pushed away from the Sun by radiation pressure. Each daughter gas species exhibited an increase in production, with peaks occurring 1-2 days after impact, followed by a gradual decay back to baseline values; the shapes of these light curves suggest that the postimpact excess is due to the vaporization of ices within the ejecta over the course of a few days. In nearly all respects, comet Tempel 1 returned to preimpact conditions only 6 days after the event.