Planetary scientists continue to debate the fundamental character of Comet Shoemaker-Levy 9 and its collision with Jupiter last July. How big were the comet fragments? How far did they penetrate into Jupiter's atmosphere? How much of the exhumed material came from the comet rather than from Jupiter?
Because the fragments struck the back of Jupiter, Earthbound telescopes could not view them directly (SN: 12/17/94, p.412). Now, the Galileo spacecraft, the only eyewitness to the impacts, weighs in with some tentative, but surprising, answers to these questions.
Researchers reported this week that infrared spectra from Galileo suggest that comet fragment G exploded high in Jupiter's atmosphere, never penetrating the uppermost cloud layer. And a preliminary analysis of much weaker spectra from the R fragment, which Galileo radioed back to Earth in January, supports this conclusion.
Fragments G and R may only have breached Jupiter's upper troposphere or lower stratosphere before exploding, says Robert W. Carlson of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. This would place the explosions just above the layer of condensed ammonia that constitutes Jupiter's uppermost cloud deck. He and his colleagues base their analysis on data recorded by the craft's near-infrared mapping spectrometer (NIMS).
According to Carlson, the spectra suggest that even the G fragment, thought to be the largest, had a diameter smaller than half a kilometer and a composition as fragile as a loosely packed snowball. He adds that if other studies confirm that the fragments exploded high in the atmosphere, it would mean that much of the debris generated by the explosions came from the fragments themselves, not from Jupiter. Paul R. Weissman of Carlson's …