Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...314..315s&link_type=abstract
Astronomy and Astrophysics, v.314, p.315-327
Astronomy and Astrophysics
Astrophysics
1
Impact Of Shoemaker-Levy 9 On Jupiter, Precursors, Fragmentation As Dominant Ablation Of Brittle Impactors, Penetration Depth, Terminal Explosion
Scientific paper
The impacts of fragments K and R of comet Shoemaker-Levy 9 are examined with the aims to interpret the timing of the observed precursors to the main thermal emission event and to correlate the results of ground-based infrared observations with a variety of observations made onboard the Galileo spacecraft. Analysis of the phenomena associated with the impact and explosion of fragment K shows that there is no discrepancy in the timing of the Earth- and Galileo-based observations and that the time of 53+/-3 seconds between the emission peak of Precursor 1 and the onset of Precursor 2, as recorded by terrestrial observers, can be interpreted as the interval between the impactor's disappearance behind the Jovian limb and the first appearance of the ejecta's plume over the limb following the explosion of the fragment's residual mass. It is concluded that the impactor exploded at an altitude of 45 to 50km above the pressure level of 1 bar and that the residual mass involved in the explosion, approximately 6 to 7million tons and about 400meters across, represented only a fraction of 1percent of the fragment's preatmospheric mass. The explosion is calculated to have taken place under a dynamic pressure of several hundred bars and the explosion energy is found to have been on the order of 10^26^erg. The results for fragment R show it to be smaller and less massive than fragment K, exploding slightly higher in the Jovian stratosphere, 50-60km above 1bar. Most of the entry kinetic energy of either impactor was rapidly dissipated during atmospheric flight by prolific mass ablation - dominated by the object's fragmentation - before the point of explosion was reached. The rate of ablation of these massive fragments was comparable with that observed in the Earth's atmosphere for category IIIb fireballs, which belong to the population of "soft" cometary material and whose bulk density was determined to be typically 0.2g/cm^3^. Vigorous fragmentation - both continuous and occurring in discrete events of extremely short duration - is known for such impactors to be independent of their mass and to surpass vastly (especially for massive objects) ablation effects due to evaporation and spraying. Evidence from several other observations of comet Shoemaker-Levy 9 appears to corroborate the present conclusions on the bulk properties and the penetration depths of the major fragments of comet Shoemaker-Levy 9.
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