Pasadena, Calif. — Although the Voyager 1 spacecraft has finished its Saturn mission, Voyager scientists have scarcely begun their work. "It's nontrivial analysis task ahead of us," said project scientist Edward Stone with calculated understatement.
As an example of what he meant, he noted that project scientists do not yet know the diameter of the moon Titan. "It's buried in those data that were coming in one image every second," he said, adding that it will take weeks to break out the right number, if indeed it an be found at all.
And the size of Titan -- that is, size of the solid body underlying the thick atmosphere -- is only one of hundreds of facts about the Saturn system that scientists are trying to pin down.
Already, it is obvious that planetary science has leaped ahead. More knowledge has been gained about the Saturnian system during Voyager 1's flyby than was learned throughout all previous human history.
Consider Titan itself, one of the largest moons in the solar system and the only one known to have a substantial atmosphere. Although its surface could not be seen and its exact size is still in doubt, Dr. Stone says scientists "still have learned a great deal about Titan in the sense that this is a big first step."
They have data showing how temperature varies with height throughout Titan's atmosphere. They now know Titan has a thick atmosphere with a surface pressure at least 1.5 times the sea-level pressure on Earth. And they may be able to define that pressure more exactly. They have know the basic composition of that atmosphere, which consists largely of hydrogen with an intriguing mixture of organic chemicals.
The theorists can go on to calculate the characteristics of such an atmosphere and more definitively analyze the kind of chemistry that has gone on and is going on there. It is an exciting prospect because scientists now realize that Titan may provide an example of the kind of atmosphere Earth once had before the rise of organic life -- an example frozen in time because of Titan's extreme cold.
There is the puzzle of Saturn's rings, which have been the centerpiece of the Voyager observation. Discovering that what had appeared to be a few broad bands of orbiting particles really contains hundreds of individual rings was a stunning surprise. It told planetary scientists that they have to rethink their notion of what gives stability to the planet's ring system. The old concept of a simple gravitational interaction with some of Saturn's larger moons fell apart.
"What we've got is an entirely new observation," Stone said. And, while some tentative ideas of how more complex gravitational interaction might be at work have been tossed around, scientists really have to think through the problem along entirely new lines, he explained.
For example, Stone noted, features such as some of the outer rings or the mysterious spokelike markings seen across the "B" rings in the middle of the ring system may be electrically charged dust particles. Saturn's electric and magnetic fields would influence such particles strongly. This broadens the problem beyond purely gravitational considerations.
"With the new observations we have resort to a larger repertory of physics," says Dr. Bradford A. Smith of the University of Arizona, and leader of Voyager 1 's imaging team, clearly implied that planetary scientists would find this difficult exercise good for them.
Scientists interested in Saturn's weather are similarly challenged. The two giant planets, Jupiter and Saturn, with their circling bands of clouds and high-speed jet streams, are forcing scientists to stretch their understanding of atmospheric dynamics, which was developed for Earth, to explain quite different circulation systems elsewhere.
Jupiter and Saturn have no solid surfaces to set boundaries for weather systems. Their atmospheres are deep while Earth's is shallow. their gravity is far stronger than Earth's. With a roughly 10-hour day, they rotate much faster than does Earth. And -- one of the most significant points of all -- unlike Earth, they have internal heating that feeds more energy into their atmospheres than does the Sun.
Theorists have gone a long way to explain what they can see of Jupiter's circulation. Computer models designed to simulate Earth's atmosphere have been made to simulate many of Jupiter's features simply by changing the numerical values for gravity and planetary rotation and by specifying an internal heat source.
Voyager scientists hope to see enough of Saturn's circulation to compare it with Jupiter's weather and to have yet another example with which to test their theories.
Because of Saturn's high-level haze, Stone says the details needed to study Saturn's weather lie in the images to be brought out by computer enhancement. Already, he added, some differences with Jupiter's atmosphere have been noted. What might cause such differences? Stone says, "I don't know. I'm just telling you what we have observed. What you're asking for is understanding. The problem is we've just gone through a period of observation and we're only now beginning to enter a period of understanding."