A Small Probe Unlocks Startling Secrets of Red Giant

AFTER centuries of arguing about the composition of the giant planet Jupiter, scientists have sampled the object of their debate. Those samples are now rattling theories about planetary formation in this solar system - and beyond.

The data the Galileo probe obtained before being crushed by Jupiter's atmosphere last month should enable scientists to decide between the two major competing theories as to how Jupiter formed, says Tobias Owen of the State University of New York.

The new Jovian portrait comes on the heels of a historic week of stunning revelations about the nature of our universe, ranging from an abundance of mysterious ''dark matter'' to a quintupling of the estimated number of galaxies to the discovery of two planets outside of our solar system that may support organic life.

''Things we are now learning about Jupiter ... are applicable to understanding these other worlds,'' says Richard Young, the Galileo project scientist at the NASA/Ames Research Center at Moffett Field, Calif.

While the probe found less water than expected, Jupiter does have the possibility of having liquid water. It has that in common with the newly discovered ''Jupiter-like'' planets, says Ames researcher Alvin Seiff.

Presenting their findings during a press conference at Ames on Jan. 22, Galileo probe scientists emphasized that they have only begun to mine the hoard of new information. Dr. Young warned that some preliminary numbers will change. But he and other investigators say it is already clear that they will have to rethink their ideas about Jupiter's formation and evolution.

Among the surprises:

* Less Water. The probe found less water than expected. Data from the preceding Voyager mission indicated Jupiter's atmosphere should have twice as much water as does the sun's atmosphere as judged by the latter's oxygen content. The way waves raced across Jupiter's cloud tops when the Shoemaker-Levy comet fragments struck last year suggested water could be 10 times more abundant than in the sun. Yet probe data indicate the Jovian atmosphere has only about the same water content as the sun.

* Fewer bolts. The probe found less lightning than expected. It seems to occur at only a tenth of Earth's average lightning rate.

* Light on helium. The atmosphere has much less helium and neon than thought. The ratio of helium to hydrogen is 26 on the sun. Scientists believe this should be typical of the primordial nebula from which the sun and planets formed. On Jupiter, however, the ratio is only 13. Young noted this suggests helium and neon are moving deeper into the planet. That, in turn, suggests that ''we have to rethink how the planet evolved,'' Young said.

* Clear skies: Scientists think the planet's atmosphere has a three-cloud-layer structure. They expected to find an upper layer of ammonia crystals, a middle layer of ammonium hydrosulfide, and a thick bottom layer of water and ice crystals. Instead, the probe entered a rare clear region and ''saw'' few indications of clouds. This does not mean Jupiter lacks cloud structure. The probe was not in a position to detect them.

The findings and those to come in the next two years from Galileo may not cause a ''major revision'' of planetary theory, says Dr. Owen, but will mean ''modification'' of the two prevalent models. The probe data should help answer whether Jupiter condensed from the primordial nebula or first formed a rocky core that became large enough to draw in gas and other material from the nebula. Scientists may also posit how much of its material - especially water - came from impacting bodies such as comets.