GREENBELT, Maryland -- Comet fragments completed their bombardment of Jupiter on Friday, but the scorched marks may remain for weeks above the cloudtops of the solar system's largest planet.


To astronomers, the comet shower on Jupiter has been more than just a light show.


The comet fragments provided a natural probe of Jupiter's atmosphere. By penetrating deeply and then exploding, the space rocks performed a sort of chemical analysis. Astronomers can use measurements of the light spectrum to identify molecules in the black patches left by the impacts.


Already they have found sulphur, ammonia and hydrogen sulfide. They are still looking for water.


The planetary pummeling has also allowed experts to verify their ability to gauge the size of comet fragments based on the apparent brightness. They used this scale to judge which pieces would be the largest and the impacts have proved their calculations correct.


By watching how the black impact marks eventually dissipate, planetary scientists could also learn more about the weather and wind currents of Jupiter.


Astronomers worldwide also have honed skills of tracking a comet and predicting its ultimate path. Calculations, some made months before, have proven accurate to within minutes. This could come in handy if ever a comet or asteroid is detected speeding through space toward an ultimate impact with Earth.


Fragment W, the last piece of comet Shoemaker-Levy 9, collided with Jupiter a little after 8 A.M. GMT Friday, according to Hien Nguyen and Joe Spang, who observed the impact through the University of Chicago's infrared telescope at the South Pole.


Comet Shoemaker-Levy 9 was discovered in March 1993. Researchers backtracking its orbital path learned that it had passed very close to Jupiter the previous year. It is believed the immense tidal forces of the planet caused the comet to break apart. The pieces lined up like a series of freight cars, looped far back out into space and then raced back toward collision with Jupiter.


Shoemaker said the original comet was about six miles (10 kilometers) across and probably weighed about 500 billion tons. The pieces all smacked into Jupiter at about 130,000 mph (210,000 kph), creating fireballs and pocking the southern area of the planet with patches so big that they have become the most easily seen feature on the planet's face.


Fragment G, which hit Monday, appeared to be the largest. Shoemaker said its impact released an explosive energy equal to about 6 million megatons of TNT. All of the pieces together, he said, released energy equivalent of 40 million megatons of TNT. This is far more powerful than all of the Earth's nuclear weapons.