Even as the first pictures of the planet Saturn and its moon were being received here today from the Pioneer 11 spacecraft, scientists at this space agency center were speculating about the origins and early evolutionary stages of that distant system.

"Saturn is mostly a gaseous object," said Dr. James Pollack, a scientist with the agency's Ames Reasearch Center here, about 40 miles south of San Francisco. "Unlike the Earth, which is mostly a rocky object, and so it [Saturn's] evolution is more likely to be that of the sun than our own."

Because Saturn radiates two to three times more energy than it receives from the sun, Pollack and other scientists believe that it was anywhere from 100 to 1,000 times bigger whn it first began to form 4 billion to 4 1/2 billion years ago.

For about 1 million years after that, Pollack said, it slowly contracted until it was 10 to 100 times it's present size, which is approximately 70,000 miles in diameter.

Then, because of major changes in the hydrogen making up the bulk of the planet, Saturn underwent a drastic shrinkage in the span of just a few years, collapsing to a sphere 2 to 3 times bigger than it is now.

For the last few billion years Saturn has been slowly and steadily shrinking to its present configuration. All of these various contractions have caused the planet to be compressed, and compression releases heat -- more heat, in fact, than Saturn gets from the sun.

Still, parts of the Saturnian system have cooled quite a bit over the last 4 1/2 billion years, and the scientists said here today that the planet's rings and some of its moons appeared to be evidence of this

Dr. David Morrison, a University of Hawaii scientist, said the six inner moons of the planet and the rings around Saturn may all be icy leftovers from water in the original planetary nebula.