Jovian comet crash: data trickles in still.

Among all the instruments that stared at Jupiter during last month's comet crash, just one had the right equipment and the right location to directly view the initial fireworks. Because each fragment of Comet Shoemaker-Levy 9 hit the far side of Jupiter, just out of view of Earth, only the Galileo spacecraft could view the pieces as they entered the Jovian atmosphere, flashing like shooting stars.

Last week, astronomers finally received the first trickle of imaging data from the craft, which has a broken communications antenna. One image sequence, taken as the last large fragment, known as W, struck Jupiter, shows a point of light brightening and fading. But scientists aren't sure if Galileo caught the impact coming or going: The 7-second flash could have accompanied the fragment's entry into Jupiter, or it could represent part of the fireball that rocketed from Jupiter just after W exploded.

Determining the correct explanation may help pinpoint the energy of the impacts and might indicate how high in the upper atmosphere the fragments exploded. Further images in the same sequence, expected to arrive at Earth in September, may solve the puzzle, says Clark R. Chapman of the Planetary Science Institute in Tucson. For instance, if the later images show a dark interval followed by another, longer period of brightening, scientists may attribute the first flash to the fragment's entry and the second to the fireball.

In another development, scientists now report the presence of water in Jupiter's upper atmosphere within minutes of several of the explosions. Some researchers assert that the water most likely comes from the fragments rather than material exhumed from the planet.

Using an infrared spectrometer aboard the Kuiper Airborne Observatory Gordon Bjoraker of NASA's Goddard Space Flight Center in Greenbelt, Md., and his colleagues identified emissions from water vapor immediately after the fireballs from the G and K impacts became visible. The emissions all but disappeared within 15 minutes. The team announces its results in an Aug. 13 circular of the International Astronomical Union.

Using the same flying observatory, Ann L. Sprague of the University of Arizona in Tucson and her coworkers found water vapor 15 minutes after the R impact; the emissions lasted for about 10 minutes, Sprague told Science News. In contrast, she notes, enhanced emissions from ammonia, thought to come from Jupiter's upper cloud layers, didn't appear for another hour.

In interpreting their results, Sprague and Bjoraker assume that the fragments exploded well above the planet's proposed water cloud layer. If so, then the immediate presence and short duration of the water vapor suggests that it originates from the exploded fragments, they assert. Had the water instead come from Jupiter, it should have appeared later, along with ammonia and other hydrocarbons that rise like a hot bubble about 90 minutes after each explosion.

Bjoraker says that the water is evidence that Shoemaker-Levy 9 was indeed an icy comet, rather than an asteroid. But Harold A. Weaver of the Space Telescope Science Institute in Baltimore notes that the heat generated by each blast vaporized all material in the fragments, breaking molecules into their atomic constituents. As some of the atoms cool, they might recombine into water. Alas, says Weaver, asteroids and comets have roughly similar abundances at the atomic level, so that the presence of small amounts of water can't offer a distinction between the two.