In 1994, one year after its discovery, the fragmented remains of Comet Shoemaker-Levy 9 crashed into Jupiter’s upper atmosphere in a sequence of 23 large impacts, each releasing the energy equivalent of 25,000 megatons of TNT, more than one million times as much energy as released by the nuclear bomb dropped on Hiroshima. Orbital analysis between its discovery in 1993 and its demise in 1994 indicates that Shoemaker-Levy 9 experienced a close encounter with Jupiter in 1992, and it is thought that tidal forces due to this close encounter broke apart the loose mass of ice and dust.
The nucleus of a comet is a dirty snowball formed by accretion of hydrogen compounds in the outer solar system early in its formation. These comets are primarily composed of water ice and carbon dioxide ice, and generally are less dense than astronomers would expect, at about 0.6 g/mL. Scientists think that this lower density may be due to comets’ outgassing of material as they partially sublimate during their passes at the inner solar system. The selective melting of material may leave holes and crevices that lower the density of the comet below what we would expect. It is not known how the tidal forces of Jupiter were able to so completely break apart comet Shoemaker-Levy 9, but it is possible that such crevices weakened the internal structure of the comet nucleus.
Regardless of its structure, the energy released by the ice fragments as they plunged into Jupiter’s upper atmosphere at 63 kilometers-per-second was formidable, and the Earth size purple bruises left on Jupiter’s surface served as a reminder of the dangers of solar system objects to any inhabitants of its larger worlds.
Much of the information in this blog post came from Cosmic Pinball by Carolyn Somners and Carlton Allen and I would highly recommend reading it for more information about solar system collisions.
Find more information about comet structure and discovery here.