Voyager's message to Earth: 'There's more up here than meets the eye'
Voyager 1, Earth's ambassador to the solar system's "Lord of the Rings," is sending home dispatches that show how parochial a view we have had of our planetary system.
As is always the case with reports from an emissary to an exotic foreigh land , the folks at home -- in this case, Voyager team scientists -- are having to shake off preconceptions to try to understand what the reports are describing.
Laurence Soderblom of the US Geological Survey has been stressing this point, especially in connection with Saturn's moons. Earth-centered thinking, he explained, had conditioned us to consider a moon to be a dry, relatively inactive rocky body. Thus, it came as a shock in March 1979, when Voyager 1 descovered that Jupiter had such exotic moons as Io, with its sulfurious volcanoes, or icy Ganymede.
Now Voyager 1 may be about to deliver another such shock.
Dr. Soderblom notes that Jupiter's moons soon became part of the preconception that conditioned scientists' expectations. As Voyager 1 closes in on Saturn's moons, geologists here at the NASA Jet Propulsion Laboratory have been trying to interpret the details they are beginning to see in terms of what they already know about, say, Ganymede.
This limited view should be dropped, Soderblom warned, for it blinds one to the true implication of Voyager's messages -- namely, that the solar system is a far more varied place than we have imagined.
The giant outer planets -- rich in hydrogen and helium gases and accompanied by a host of moons and encircling rings -- formed under different conditions than did the rocky inner planets. They formed farther from the sun under colder temperatures and now contain most of the solar system's mass outside of the sun itself.
The solar system and its evolution cannot be understood without knowing the differences between the inner and outer planets and how they came into being. More and more, our own earth and moon are being seen as just one of a variety of planetary systems. Our own system can never be adequately understood simply by studying it in isolation, as has been the case until space probes began to visit other planets.
Thus, Soderblom said, Voyager scientists are trying to look at the images coming across 948 million miles of interplanetary space with fresh eyes.
At this writing, Voyager 1 had already passed within 4,000 kilometers of the cloud tops of Titan, the second largest moon known in our solar system and the only one known to have an appreciable atmosphere. It was too early to know just what Voyager had found, but scientists expected data that would help them understand Titan's nature and how such a small body, only 87 percent more massive than our own, has managed to hold onto its atmosphere in Saturn's massive presence.
One theory suggests that hydrogen boils off rapidly from Titan's atmosphere but is trapped in a torus around Saturn. As Titan sweeps through this donut-shaped zone, it recaptures much of that hydrogen. Thus there is little net loss. If this turns out to be the case, it will be an interesting example of how a moon orbiting a massive body at just the right place can retain an atmosphere.
Meanwhile, other moons such as Dione and Tethys have begun to show vague details. Soderblom said this indicates that the sharper images expected as Voyager moves among them will reveal many interesting, perhaps dramatic, features that will add to the variety of moons that can be studied.
"We had thought these moons might be only dull gray snowballs," Soderblom observed, "[it's] not true."
Saturn's magnificent rings also emphasize the need for a multiple-planet perspective. Scientists once thought of Saturn's rings as an exotic eccentricity. Now they are having to ask why at least three outer planets -- Jupiter, Saturn, and Uranus -- have rings, while the inner planets do not. Also , why are Saturn's rings so much more massive and broad than those of Jupiter and Uranus.
Raising the latter question rhetorically, Dr. James B Pollack of the NASA Ames Research Laboratory noted that the reason probably lies in the conditions of the early solar system.
Saturn's ring material seems to be rich in water ice. Perhaps the temperature near the young planet was cold enough for ice to form at a distance where it can be retained in a highly stable orbiting system. Thus Saturn's rings, made of particles a few centimeters to a few meters across and interspersed withsmall moons, are probably primordial.
In Jupiter's case, early conditions would have been too warm for water ice to form. Jupiter's rings appear to be made up of fine dust, perhaps from an unseen satellite. This dust does not stay in orbit for more than a few thousand years. Thus the ring must be constantly renewed.
Uranus is a planet whose evolution proceeds, more slowly than does that of Jupiter of Saturn because it is a less massive object. It may not have shrunk to the point where Saturn-like rings can form. In other words, the distance from the planet's center where icy rings would be stable is still inside the planet itself. So Uranus has only very narrow rings like Saturn's outer f ring.
All of this is just speculation, Dr. Pollack warned. But it does show how the rings pose fundamental questions.
This is a study that will occupy scientists for years as they sift the volumnious data Voyager 1 is now sending and which Voyager 2 is expected to send next in August. Bradford A. Smith, leader of the Voyager "imaging" team, summed it up when he said: "The mystery of the rings -- their structure and what governs the structure -- keeps getting deeper and deeper until we begin to feel it's a bottomless pit."