Next: from 948 million miles, a clear look at cloudy Titan
Pasadena, Calif. — A page copied from an old Buck rogers novel and passed around among scientists here captures the atmosphere that pervades the headquarters of the Voyager Saturn mission.
"The spectacle was beautiful and awe- inspiring to behold," it reads, "one of the strangest to be seen in the entire solar system. 'To think that no other person from Earth has ever seen Saturn and its rings so close! . . .' Wilma said. 'I wonder what Titan will be like.' 'Well . . .,' Buck replied, 'It won't be long now.'"
With the "awe-inspiring" spectacle of Saturn's rings growing ever larger on the ubiquitous TV monitors and with the scientists speculating excitedly as to the nature of the great moon Titan, those words written decades go seem prophetic. Fiction has become a reality. A spaceship from Earth has entered the Saturn system. But, in this case, hundreds of millions of people back on earth are sharing the wonder of its discoveries with the mission team at the NASA Jet Propulsion Laboratory here.
Looking forward to Wednesday, when Voyager I will pass within 4,100 kilometers (2,500 miles) of Titan's cloud tops, Dr. Edward C. Stone of the California Institute of Technology explained some of the puzzles he hopes will be cleared up. Titan has an atmosphere rich in methane. Estimates of its thickness are very uncertain. The surface pressure may be anywhere from 2 percent to 200 percent of that of the atmosphere on Earth.
Surface temperature also isn't certain. Measurements of infrared (heat) radiation made by the Pioneer Saturn spacecraft some 14 months ago indicate a temperature of minus 198 degress Celsius with an uncertainty of three degrees. It is unclear whether this represents the surface or some intermediate level of the atmosphere. Measurements by Earth-based radiotelescopes suggest a surface temperature of minus 186 degrees c. with an uncertainty of 9 degrees.
Pioneer Saturn measured the radius of Titan including its atmosphere as 2,880 kilometers in red light and at 2,895 kilometers in blue light, with an uncertainty of 22 kilometers. However, since no one knows how thick the atmosphere is, they don't know the size of the underlying solid. Measurements by Earth-based radiotelescopes suggest 2,400 kilometers with an uncertainty of 250 kilometers.
If the actual radius of the solid body is 2,200 kilometers, then the moon has an average density three times that of water and may be like Jupiter's rocky Europa, Dr. Stone said. If the radius is 2,600 kilometers, then the density would be 1.8 like the icy Jovian satellite Ganymede. That is a substantial uncertainty to be cleared up.
Likewise, Titan's surface is a matter of speculation. With a thin atmosphere it may be covered with liquid methane. If the atmosphere is thick, there might be liquid nitrogen.
Then there are tantalizing hints of some kind of organic chemistry driven by sunlight, Stone noted. Ethane, acetylene, and ethylene have already been identified in Titan's atmosphere. Are there other organic identified in Titan's atmosphere. Are there other organic chemicals being formed? Are they precipitating out on the surface? Specifically, are prebiotic chemicals -- precursors of organic life -- forming and collecting there? Does Titan provide clues to how chemicals have formed in some meteorites?
Stone said, hopefully, that Voyager will answer many such questions. It should measure Titan's surface temperature directly and may give a profile of the temperature throughout the height of the atmosphere. Its infrared sensors should also be able to look at the photochemistry that may be going on. Radio data will reveal other characteristics of the atmosphere, including density variations with height. And most importantly, they will give an accurate measurement of Titan's diameter and determine once and for all what kind of a body it is, Stone said.
Meanwhile, Saturn itself continues to reveal intriguing details. A red spot, first noticed in August, now can be seen clearly at 55 degrees south latitude and has been joined by a comparable red spot at roughly the same position in the comparable red spot at roughtly the same position in the northern hemisphere. They measure on the other of 12,000 kilometers across with an uncertainty of 1, 000 km. Thus they are smaller than Jupiter's famous red spot and about the size of the white ovals seen on that planet. These are believed to be great storms. Drawing this analogy, Bradford A. Smith of the University of Arizona said we may be seeing the top of a convective cell with material welling up from below and diverging outward on top. The force of Saturn's rotation would cause a spiraling motion.
The banded structure of Saturn's atmosphere also is becoming clearer and subtle effects are beginning to appear. The southern hemisphere, for example, seems a little brighter than the northern, with the change in brightness occurring more or less along the equator. Smight pointed out that these are very subtle features. It takes considerable computer enhancement of the image contrasts to see them at all. Trying to figure out what they imply for Saturn's "weather" must await clearer views.
However, it is the spectacular ring system that has become the central puzzle. The old theory that nearly accounted for the forces that shaped the rings as they used to be known has been shattered. With only three of four rings as seen from Earth to consider, it seemed sensible to think that some of Saturn's 9 or 10 moons then-known were stabilizing the rings by gravitiational interaction with the ring's particles. These are believed to be small bodies a few centimeters to a few meters in size.
Now Voyager is showing something like 100 ringlets within the main rings, with more structure showing up all the time. Moreover, small moons on the order of 100 kilometers in diameter keep showing up. Several have been found since the Pioneer Saturn mission. The latest was announced Nov. 8, bringing the number to 15.
Sometimes intimately associated with the ring material, these small moons may help control ring structure. For example, two small moons orbit on either side of the "F" ring -- a ring just beyond the "A" ring, which is the outermost of the main rings seen from Earth. Like dogs herding sheep, they may constrain the F ring particles to orbit where they do, Smith said. Likewise, the newly found 15th moon orbits at the outer edge of the A ring, about 136,000 kilometers from the center of Saturn. It may be "herding" the A-ring particles.
Smith said he wouldn't be surprised by another moon at the A ring's inner edge. Indeed, many moons may turn up within the ring system.
While this "sheep dog" concept is intriguing, it doesn't begin to explain how the rings form and how they are maintained, or, indeed, where the moons themselves come from.