VOYAGE TO THE GIANT PLANETS
For earthly sky-watchers, Jupiter and Saturn are putting on a special display in 1981 -- a rare triple conjunction. On News Year's Eve and again on March 4 and July 23, they will pass close by one another, as seen from Earth.
It's as though the planets themselves were celebrating the highly successful Voyager explorations of the Jupiter-Saturn systems -- one of the greatest scientific adventures of all time.
Next August, the exploratory phase of that adventure will diminish as Voyager 2, now approaching Saturn, follows up the survey made by Voyager 1 in November. Voyager 2 is scheduled to go on to Uranus. So there is more planetary exploration to come. But the spacecraft won't arrive at Uranus until January 1986.
Meanwhile, the less dramatic but all-important study of the Voyager data will continue. For scientists, this is where the real adventure lies.
Those spectacular images of Saturn's rings and Jupiter's multicolored atmosphere that thrilled the world and awed even seasoned planetologists typify the new perspective planetary science has gained. Giant planets that were only seen with sparse detail through telescopes, and major moons that, to terrestrial observers, were mere pinpoints of light, provide new examples of planetary development under conditions different from those experienced by Earth but governed by common principles.
Those famous Jovian moons discovered in 1610 by Galileo also illustrate this point. Although formed from remnants of the great gas ball that became Jupiter, Io, Europa, Ganymede, and Callisto belong to what has parochially been called the "terrestrial" family of planets. Together with Saturn's largest moon, Titan (one of the largest moons in the solar system), they join Mercury, Venus, Earth and its Moon, and Mars as a distinctive group of objects similar in size and composition -- objects that should have evolved by comparable processes and on similar time scales.
As Lawrence A. Soderblom of the US Geological Survey (USGS) says in pointing out this kinship: ". . . The number of earthlike objects with which one can test theoretical models of planetary evolution has doubled. One can now appreciate that the planets of the inner solar system occupy only a small part of the spectrum of characteristics and evolutionary possibilities open to such objects."
Giant planets such as Jupiter and Saturn are believed to be large enough and to have formed far enough from the Sun to have retained the primordial material with which they condensed from the solar nebula. That would include such volatiles as hydrogen and helium, which the activity of the young Sun would have driven out of the inner solar system. Thus Jupiter and Saturn, with masses respectively 318 and 95 times that of Earth, are bodies composed largely of hydrogen and helium.
The outer 1,000 kilometers or so of such a planet constitute what could be called an atmosphere. This turbulent gaseous layer has whirling "storms" and banded patterns with clouds that appear to be various compounds of hydrogen with such elements as nitrogen, oxygen, and carbon. (At this stage, the color of Jupiter's Great Red Spot remains something of a mystery.)
There is no solid lower boundary.But deeper down, hydrogen liquefies under the increasing pressure. And at pressures of 1 to 3 million times that of sea-level air pressure on Earth, the liquid hydrogen becomes metallic and electrically conductive. Fluid motions within this material are believed to set up magnetic fields on Jupiter and Saturn, just as motions within Earth's liquid iron core set up the magnetic field of Earth.
At their centers, these giant planets may also have rocky cores several times the size of Earth and with 15 to 20 times its mass.
A distinctive feature of these planets is that they radiate about twice as much heat as they receive from the Sun. The excess heat is thought to represent gravitational energy released as the planets continue to shrink and cool down.
This is illustrated computer studies such as those made with a model of stat formation adapted for planetary gas balls by James B. Pollack of the NASA-Ames Research Laboratory, Allen Grossman of Iowa State University, and Harold Graboske of the University of California's Lawrence-Livermore Laboratory.
Beginning as a sphere several hundred times as large as the final planet, the primordial gas ball contracts and heats. Eventually, it becomes hot enough to split hydrogen molecules into separate atoms. This changes the pressure that has resisted the gas ball's self-gravity. Catastrophic collapse ensues. What now can be called the proto-planet shrinks to within a few times larger than its final diameter in about a year. After that, it shrinks more slowly. But even today, over 4 billion years later, it is still cooling and contracting.
Such a process would account for Jupiter's heat excess fairly well. But it yields too low an estimate for Saturn. Perhaps Saturn, being smaller than Jupiter, has a region could enough for helium to separate from hydrogen and fall toward the planet's center -- a kind of liquid helium rain. In falling, this could release addition gravitational energy as heat.
This kind of planet-forming collapse mimics the condensation of a star like the Sun. And just as planets are thought to have condensed out of the residual solar nebula, so the residue from Jupiter's and Saturn's mini-nebulas formed moons and rings. Planetary scientists hope to learn more about the formation of the solar system itself by studying the giant planet's satellite systems.
A proto-planet would begin to spin very rapidly as it collapsed and would throw off excess material to form an equatorial disk. Close to the planet, which would then be quite hot, all but the more refractory materials in that disk -- materials such as iron or silicate minerals -- would be vaporized. Satellites that condensed in this region would be rocky, with some iron. In colder, more remote parts of the disk, ices of water or of methane and ammonia would condense.
Jupiter's Galilean satellites fit into this scheme. Io, the innermost, is rocky, while Callisto, the outermost, is two-thirds water ice with a rocky center.
At Saturn, things were more complex. Less massive than Jupiter, the young Saturn would give off less heat as it evolved. Furthermore, the present orbits of the inner satellites and rings lie inside of the diameter of the newly collapsed proto-Saturn. Thus Titan and the outer satellites, like Jupiter's farther satellites, should be a mixture of rock and ices, which seems more or less to be the case.
But rings and inner satellites had to wait until Saturn had contracted further. By then, the consolidating planet had incorporated most of the rocky material in its outer layer into itself. Only water vapor was left behind. And indeed, the ring particles appear to be largely water ice.
This concept of satellite formation can also account for the uniqueness of Tital -- the only satellite known to have an atmosphere. It formed far enough from Saturn, its parent planet, and from the Sun that it could be endowed with ices of water, methane, and ammonia.
Titan would have been warm enough for some of its methane and a ammonia to exist as gases and massive enough to hold on to such an atmosphere. In contrast , Neptune's moon Triton, which is of comparable size, would be too cold for such gases to exist, while the Galilean satellites would have been too warm to have had such ices in the first place.
Preliminary analyses of the Voyage data indicate Titan has a largely nitrogen atmosphere, perhaps 1 1/2 times as massive as our own atmosphere. This could have formed when solar ultraviolet radiation split ammonia into nitrogen and hydrogen, with the hydrogen escaping into space.
Methane, a compound of carbon and four hydrogen atoms, would also have lost some of its hydrogen and formed various hydrocarbons such as acetylene, ethylene , ethane, and hydrogen cyanide, which have been identified in its atmosphere. But it is not known whether Titan's surface is buried in 3 kilometers of hydrocarbon snow, as suggested by Darrell F. Storbel of the US Naval Research Laboratory and the Voyager ultraviolet spectroscopy team. Titan's surface could not be seen trough the orange haze, though to be a hydrocarbon smog, that overlies it.
Titan typifies one of the ways planetary exploration has given scientists new perspective by emphasizing the importance of location in planetary evolution. USGS geologist Harold Mazurski points out that, five years ago, a typical planetary scientist would have said that a planet's mass was the most important factor shaping its geological development. Now, while mass is still considered a major factor, a body's location in the solar system seems more important.
Titan is unique because of where it formed in relation to Saturn and to the Sun. Io is the most volcanically active body known, precisely because it is close to Jupiter. It formed where materials such as sulfur were included in its substance. It is energized by powerful tidal forces and by an electric current flowing from Jupiter with wattage equivalent to more than 20 times the total generating capacity of Earth's power plants. As many as eight volcanoes have been seen in action at one time. Sulfurous fountains spurt to heights of 300 kilometers, while vast lave flows constantly renew the satellite's face.
All of this refocuses attention on Earth's own unique position. Our planet formed at just that distance from the Sun where it received an endowment and underwent an evolution favorable for organic life. Even if evidence of some life forms were found on other bodies, the rich flowering that Earth has known is not expected to have occurred anywhere else in our solar system. Learning how general processes of planetary development operate in diverse ways will help greatly to improve understanding of our unique planetary home.
Saturn's minor satellites are among the new aid to such knowledge. Ranging from Phoebe (140-kilometer diameter) to Rhea and Iapetus (1,500-kilometer diameters), they are intermediate between the terrestrial bodies (including the Galilean satellites and Titan) and the asteroids. They fill in an important part of the planetary spectrum that now can be studied in detail.
Likewise, the rings are spurring a harder look at orbital dynamics and the details of planet formation. Once considered a Saturnian idiosyncrasy, now rings are known to be commonplace. Uranus has them.So does Jupiter. Searches for Neptunian rings are being discussed. It has even been suggested that Earth may once have had one.
The formation and maintenance of ring systems are now known to be an important aspect of planetary evolution. Rings provide clues to how a planet formed. Their continued presence and stability represent an interplay of gravitational forces among the planet, its moons, and the ring particles.
Saturn has the most elaborate rings, revealed in magnificent detail by Voyager 1. Understanding the dynamic of that structure with its hundreds of individual ringlets will enrich orbital theory. Voyager images of those rings have also alerted scientists not to overlook electric and magnetic forces. Mysterious "spokes" appear and disappear in part of the ring structure. At this writing, it was thought they may be electrically charged particles that are lifted slightly out of the ring plane by electric and magnetic forces.
Meanwhile, with Jupiter and now with Saturn, whose atmosphere is just beginning to be studied, planetary meteorologists have challenging new cases to stretch their understanding. The giant planets have atmospheric circulation on a scale far vaster than does Earth. Their gravity is stronger. So, too, is the force caused by their rotation, for they spin a little more than twice as fast as Earth. This is the so-called Coriolis force, which turns wind flows around centers of low or high pressure. Also, in addition to heating by the Sun, these atmospheres are heated more or less uniformity from below.
How tell do general principles of atmospheric dynamics developed largely for Earth work out when applied in these alien environments? It turns out that they appear to work quite well, with suitable adaptations, although there remain challenges to understanding.
Jupiter's white ovals and Great Red Spot behave like strong high-pressure systems on Earth. However, they are larger and live much longer -- centuries in the case of the Great Red Spot. Also, Jupiter's zoned system of easterly and westerly winds can be at least partly reproduced by straightforward scaling up of standard theory.
Gareth Williams of the Geophysical Fluid Dynamics Laboratory at the US National Oceanic and Atmospheric Administration did this with an atmospheric computer model developed for Earth. He scaled up the planetary radius from 6, 400 to 72,000 kilometers and shortened the day to 10 hours. This produced as a series of zonal (parallel to latitude circles) jet stream winds reminiscent of Jupiter's alternating light and dark zones. Jupiter's stronger Coriolis force was primarily responsible.
This is not the whole story, however. Jupiter's jets are rock-steady, persisting for many decades. They appear to be linked to some deep-seated driving mechanism. This is very likely some kind of planet wide convection caused by the global heating from below. If and when this is understood, it will add a new dimension to meteorological theory.
Meanwhile, as William Rossow of the Goddard Institute for Space Studies has noted, it seems remarkable that a simple Earth-derived model such as that of Gareth Williams does appear to account for so much. It strenthens confidence in such theories by showing that they probably do embody generally valid principles.
"That is the value of going to another planet," he says; "you have to stretch and pull and twists theories that are reasonably successful for Earth and see if they will work."