Galileo infrared observations of the Shoemaker‐Levy 9 G Impact Fireball: A Preliminary report

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Abstract
The Galileo spacecraft was fortuitously situated for a direct view of the impacts of the fragments of comet Shoemaker‐Levy 9 in Jupiter's atmosphere. The Galileo Near Infrared Mapping Spectrometer (NIMS) instrument observed several of the impact events in several discrete bands and with a temporal resolution of roughly five seconds. Data have been received for the G impact showing two phases of strong infrared emission. The first phase is approximately one minute in duration and corresponds to the initial fireball and early plume development. This is followed six minutes later by the onset of heating by plume ejecta falling back on the upper atmosphere. This report provides a preliminary description of the fireball phase. The first detection of the G fireball occurred at 07:33:37 UT on July 18, 1994, approximately five seconds after the initial signal recorded by the Galileo Photopolarimeter‐Radiometer (PPR) instrument. The preceding NIMS measurement, occurring approximately one second before the initial PPR signal, showed no evidence of fireball emission. The detected duration of the fireball at 4.38 µm was 70 seconds. Spectra in the first half of this period show blackbody‐like emission, with absorption features from overlying methane and molecular hydrogen. The strengths of these features place the fireball in the upper troposphere and lower stratosphere, above the ammonia cloud layer. The emitting surface rises and accelerates, achieving a velocity of 2–3 km/sec after 25 seconds, in qualitative agreement with that expected for an explosion in an inhomogeneous atmosphere.The Galileo spacecraft, en route to Jupiter, was in a position to obtain a direct view of the impacts of Comet Shoemaker‐Levy 9 fragments on the nightside of Jupiter, providing an opportunity to investigate the early temporal evolution of the impact events. It was predicted that the comet fragments would produce high temperature bolides as they entered the atmosphere and then explode, producing hot fireballs which would rise, expand, and cool (Sekanina, 1993; Zahnle and Mac Low, 1994; Chevalier and Sarazin, 1994; Ahrens et al., 1994; Boslough et al., 1994). Much of the predicted radiation occurs in the infrared region, and time‐resolved infrared spectral observations, obtained over a broad wavelength range, are ideal for studying these phenomena.