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A Comet's Fiery Dance at Jupiter

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From The Galileo Messenger
May 1995
By Scott Bowdan

A Comet's Fiery Dance at Jupiter

Scientists are still piecing together the details of what happened when comet Shoemaker-Levy 9 collided with Jupiter in July 1994. The final data playback from Galileo, covering onboard observations of comet fragments R and W, was completed in late January of this year.

Collision events have thus far been divided into three phases: The incoming comet fragment first hits the Jovian atmosphere and heats up (the "meteor" phase), then explodes into a fireball of extremely hot gas (the "fireball" phase, see figure). The gas backflushes out the tunnel cleared by the incoming fragments, and the gas and debris expand, rise, and cool, forming the plumes seen above the Jupiter limb by the Hubble Space Telescope and ground-based observers. Then the plume ejecta--a mix of cometary and atmospheric material--fall back toward the planet, heating the atmosphere and producing intense thermal emissions (the "splash" phase).

Images from Galileo's Solid-State Imaging (SSI) camera show both the flash of the comet fragments colliding with the Jovian atmosphere and the initial hottest phase of the resultant explosions and fireballs. The latest SSI returns confirmed a characteristic brightness profile in the meteor and early fireball phases for fragments K, N, and W: peak brightening at 0.56-Ám (green) and 0.889-Ám wavelengths was observed less than 10 seconds after initial detection, then the object faded for some 20 seconds, until contact was lost. In addition, initial peak brightness pulses for K, N, and W were within a factor of 2 of each other. Given such a similarity in brightness profiles among these events, it is still a mystery as to why there are significant differences in the longer lasting effects observed from Earth.

Based on data from Galileo's Near-Infrared Mapping Spectrometer (NIMS) (see figure), we know that the super hot fireballs associated with fragments G and R lasted about 1 minute (before cooling sufficiently to be invisible to NIMS). We also know (from NIMS) that the plume ejecta began falling back into the atmosphere about 6 minutes later, getting brighter and brighter for the 3 minutes observed. Ground-based data indicate that the total splash phase for each of the two fragments lasted about 10 minutes, with peak brightness at a wavelength of 4.38 Ám reached at about 5 minutes after initial detection. The 6-minute flight time implies that the ejecta exploded out of the atmosphere at a minimum vertical velocity of 4.3 km/s, with particles reaching at least a 380-km vertical height.

The similarity in the G and R event timelines--confirmed by ground-based observations--is surprising since the G fireball was four times brighter than the R fireball, and the G splash was about twice as intense as R's. (Based on an assumed comet density of 1 g/cm, fragment G must have been at least 150 meters in diameter prior to impact.) The time similarity suggests that the ejecta flight time was determined by fireball and plume physics, not by observational geometry or the mass of the incoming comet fragments.

Preliminary analysis of NIMS spectra suggests that the splash material contains OH, which could be the hot remnants of water (HO) molecules from the vaporized comet or from Jupiter's presumed water clouds, as well as CH, which may indicate atmospheric methane or comet-derived hydrocarbons as the emitting source. These preliminary findings may also offer an explanation as to why the immense black patches are persisting in Jupiter's high atmosphere. The black patches may be composed of micrometer-sized carbon particles derived from comet or atmospheric materials, which would linger for about 1 year from the beginning of the splash period--acting much like volcanic dust in Earth's stratosphere. Another possibility is that the patches contain sulfur that erupted from a lower cloud layer of condensed ammonium hydrosulfide.

Further analysis of the available data--perhaps supplemented by new information from Galileo as it nears the Jovian system--should help answer many of the questions that remain about comet Shoemaker-Levy 9 and its fiery destruction at Jupiter.

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