Earth-based observations at near- and mid-infrared wavelengths were obtained for at least 15 of the SL9 impacts, ranging from the spectacular G, K and L events to the barely-detected N and V impacts. Although there were a few exceptions, most of the IR lightcurves fit a common pattern of one or two relatively faint precursor flashes, followed several minutes later by the main infrared event as the explosively-ejected plume crashed down onto the jovian atmosphere. Correlations with the impact times recorded by the Galileo spacecraft and plumes imaged by the Hubble Space Telescope lead to an interpretation of the twin precursors in terms of (i) the entry of the bolide into the upper atmosphere, and (ii) the re-appearance of the rising fireball above Jupiter's limb. Positive correlations are observed between the peak IR flux observed during the splashback phase and both pre-impact size estimates for the individual SL9 fragments and the scale of the resulting ejecta deposits. None of the fragments observed to have moved off the main train of the comet by May 1994 produced a significant impact signature. Earth-based fireball temperature estimates are on the order of 750 K, 30-60 sec after impact. For the larger impacts, the unexpectedly protracted fireball emission at 2.3 μm remains unexplained. A wide range of temperatures has been inferred for the splashback phase, where shocks are expected to have heated the re-entering plume material at least briefly to several thousand K, and further modelling is required to reconcile these data.

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Earth-Based observations of impact phenomena

Long-term response of Jupiter's thermal structure to the SL9 impacts

Galileo observations in the UV, visible, and infrared uniquely characterize the luminous phenomena associated primarily with the early stages of the impacts of SL9 fragments—the bolide and fireball phases—because of the spacecraft's direct view of the impact sites. The single luminous events, typically 1 min in duration at near-IR wavelengths, are interpreted as initial bolide flashes in the stratosphere followed immediately by development of a fireball above the ammonia clouds, which subsequently rises, expands, and cools from ∼ 8000 K to ∼ 1000 K over the first minute. The brightnesses of the bolide phases were remarkably similar for disparate events, including L and N, which were among the biggest and smallest of the impacts as classified by Earth-based phenomena. Subsequent fireball brightnesses differ much more, suggesting that the similar-sized fragments were near the threshold for creating fireballs and large dark features on Jupiter's face. Both bolides and fireballs were much dimmer than had been predicted before the impacts, implying that impactor masses were small (∼0.5km diameter). Galileo data clarify the physical interpretation of the “first precursor,” as observed from Earth: it probably represents a massive meteor storm accompanying the main fragment, peaking ∼10s before the fragment penetrates to the tropopause; hints of behind-the-limb luminous phenomena, recorded from Earth immediately following the peak of the first precursor, may be due to reflection of the late bolide/early fireball stages from comet debris very high in Jupiter's atmosphere.

Galileo observations of the impacts

Galileo infrared observations of Jupiter have been performed with two instruments, the Near Infrared Mapping Spectrometer (NIMS) and the Photo-Polarimeter Radiometer (PPR). A first data set was obtained at the time of the collision of comet Shoemaker—Levy 9 with Jupiter in July 1994. Information was retrieved about the energy budget of the event, the penetration level of the explosion, the temperature and diameter of the fireball and its evolution with time, the temperature of the atmosphere heated by the infalling debris, and the amount of water formed after the explosion. The second set of data was recorded in 1996 after the Galileo probe entry. Evidence was found for a very low water abundance in the hot spots; this result is in full agreement with the conclusions derived from the Galileo probe Infrared observations of the Great Red Spot allow us to derive its 3-D structure, which appears compatible with the model of an anticyclonic vortex.

Galileo Infrared Observations of Jupiter

Detection of h2o in the splash phase of g- and r- impacts from nims-galileo

Some timing and spectral aspects of the G and R collision events as observed by the Galileo near infrared mapping spectrometer.

The Galileo spacecraft was fortuitously situated for a direct view of the impacts of comet Shoemaker(ka)evy 9 in Jupiter's atmosphere and measurements were recorded by the Near Infrared Mapping Spectrometer (NIMS) instrument for several of the impact events. Seventeen discrete wavelength channels were used between 0.7 to 5.0 microns, obtained with a time resolution of 5 seconds. Two phases of the impact phenomena are found in the data: the initial fireball, which was evident for one minute, and subsequent fallback of impact ejecta onto the atmosphere, starting six minutes after fireball initiation.

Galileo Infrared Observations of the Shoemaker-Levy 9 G and R Fireballs and Splash

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Some timing and spectral aspects of the G and R collision events as observed by the Galileo near infrared mapping spectrometer.

Galileo Infrared Observations of the Shoemaker-Levy 9 G and R Fireballs and Splash