Galileo: Jovian crash didn't breach clouds.

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 team reported the findings at the annual Lunar and Planetary Science Conference in Houston.

Each explosion's fireball radiated much of its energy in the infrared; NIMS recorded the light show from four impacts, including the G and R fragments.

Carlson and his collaborators find that 5 seconds after the G fragment exploded, its fireball had reached a diameter of 20 km and a temperature of 4,000 kelvins. After 40 seconds, the fireball, expanding at the rate of about 2 km per second, had ballooned to 80 km across and cooled to 1,300 kelvins.

NIMS also provided researchers with another key measure. To reach Galileo, infrared light had to pass through whatever depth of Jovian atmosphere lay above the fireball. Atmospheric methane and molecular hydrogen absorb some of the light, leaving dark absorption lines in the spectra. From the intensity of the absorption, Carlson's team deduced that relatively little atmosphere resided above the G fireball and that the fragment probably hadn't plowed deeply into Jupiter.

Carlson cautions that Galileo could only see the tops of the fireballs. And though he deems it unlikely, he notes that the fragments might have plowed deeper, their radiation hidden by clouds.

Because spectra taken by the Hubble Space Telescope appeared to betray the presence of large amounts of sulfur, some researchers have argued that the fragments did burrow deeper--breaching a layer of ammonium hydrosulfide believed to lie beneath the ammonia clouds (SN: 7/30/94, p.68).

But Keith S. Noll of the Space Telescope Science Institute in Baltimore told Science News that new calculations show that Hubble had detected much less sulfur. The revised estimate may strengthen the case for a high-altitude crash, Carlson says.