by Carl Sagan
1. Launch of the Voyager 2 spacecraft from the Kennedy Spaceflight Center. Cape Canaveral. Florida, on August 20, 1977. Courtesy NASA.
2. The Voyager Record in its aluminum cover mounted on the spacecraft Courtesy NASA.
3. Crescent Earth, right, and crescent Moon, left, photographed by Voyager 1 on its way out of the solar system. This is the first photograph ever taken of the Earth Moon system together Most of the features seen on the Earth are clouds. The Moon is so much dimmer because it reflects about five times less light per unit area than does the Earth.
4. The Voyager spacecraft as it would appear if well illuminated in interplanetary or interstellar space. An alien spacecraft approaching Voyager a billion years from now, and directing a great searchlight on it, would see something like this—although the spacecraft very likely would have accumulated a number of bumps and bruises in the interim Courtesy NASA.
5. A photograph of the Pioneer 10 and 11 plaque.
6. A photograph taken at the Table Mountain Observatory of the Jet Propulsion Laboratory through a sodium filter, showing the extensive sodium cloud near the Jovian moon Io. Io’s orbit around Jupiter is also shown. Courtesy of T. V. Johnson, Jet Propulsion Laboratory.
7. Schematic diagram of the Voyager spacecraft with some of the spacecraft properties listed. The scanning platform, which contains most of the planet-oriented instruments, is shown at top. Courtesy NASA.
8. Relative sizes of the large satellites of Jupiter and Saturn, compared with Mercury and the Earth’s Moon. The colors and surface features are schematic only.
9. Photograph of Saturn and its rings obtained at New Mexico State University, Las Cruces, New Mexico. Courtesy of Dr. Bradford A. Smith.
10. Pioneer 11 photograph of Jupiter with the Great Red Spot at center. Courtesy NASA.
11. Left, Pioneer 11 photograph toward the Pole of Jupiter, with details computer-enhanced at the Image-Processing Laboratory. JPL. These graceful details in the Jovian clouds had never been suspected to exist before the Pioneer 10 and 11 missions. 12. Right, comparable enhancement of the region around the Great Red Spot, at center. Many of these smaller features have as yet no clear explanation in terms of the meteorology of the atmosphere of Jupiter.
The Sun, the stars, the interstellar medium, the other galaxies—indeed, the universe as a whole—are made chiefly of hydrogen. So are the jovian planets. But the terrestrial planets are anomalous. This difference is thought to be derived from those days in the earliest history of our solar system when the Sun and planets were condensing out of a vast cloud of interstellar gas and dust composed primarily of hydrogen. As the sun turned on, the inner solar system warmed up and the small low-gravity objects that were destined to become the terrestrial planets could not retain hydrogen, the lightest and fastest-moving gas, which trickled away into interplanetary space. In the outer solar system, however, temperatures were low and the forming planets were massive. Hydrogen did not have escape velocity and therefore was retained.
The jovian planets are therefore in some sense similar to the early Earth. If there is a solid, rocky surface to Jupiter it is in the very core, so far below the region we can view as to be permanently inaccessible. We see nothing on Jupiter, Saturn, Uranus or Neptune but atmosphere and clouds. Those atmospheres and clouds may be in some respects similar to Earth near the dawn of its history. It is therefore interesting that regions of bright coloration exist on Jupiter and Saturn. Reds, browns, yellows and oranges as well as blues are in evidence. The white clouds are probably condensed ammonia, condensed water, and their compounds. The highest white clouds are probably a kind of ammonia cirrus on Jupiter. But what could the source of the coloration be? Besides hydrogen, the principal constituents of the atmosphere of Jupiter are helium, ammonia, methane and water. There is some evidence for very small quantities of other materials, some of which (like germane, GeH4, the hydride of germanium) are very exotic. But none of these materials by themselves are colored. Phosphine, PH3, has been spectroscopically detected on Jupiter and red phosphorous compounds may make some contribution to the color of the Great Red Spot—although there are problems with phosphorous as an explanation of the general brownish coloration of the planet. Sulfur and its compounds have not been detected directly, but they must be there—if hydrogen cannot escape from Jupiter, the much more massive sulfur certainly cannot. But the details of the colorations do not seem to be matched by sulfur and its compounds, at least according to the best present studies.
However, if we take a mixture of hydrogen, helium, methane, ammonia, and water and supply energy—ultraviolet light as simulated sun light, or an electrical discharge as simulated lightning—we can make in the laboratory a range of complex organic molecules that have many of the properties of the jovian coloring material. Among these molecules are the amino acids, the building blocks of proteins, and a wide range of other organic molecules employed in life on Earth. Such experiments are highly relevant to the question of the origin of life because Earth’s early environment was hydrogen-rich, and Earth’s early atmosphere probably included methane, ammonia and water vapor. The fact that the stuff of life can be made so readily under such general hydrogen-rich conditions has been an encouragement to those concerned about the possibility of life elsewhere. The possibility that organic matter is readily made on Jupiter and the other jovian planets today is a very exciting prospect. The amount of organic matter that is there must, however, be very small, because the atmosphere of Jupiter is extremely convective and organic matter made high in the atmosphere will in a relatively brief period—say, a month—be carried down to great depths where the temperatures are elevated and the organic matter will be fried. An important and unsolved problem is whether the steady-state abundance of organic matter—the equilibrium between what is made and what is destroyed—can be enough to explain the jovian coloration.
An even more speculative point concerns the possibility of life in the clouds of Jupiter. We have so far made no searches of Jupiter’s environment in enough detail even to begin to investigate such a possibility, but it is not, in my opinion, entirely out of the question. There is a place in the atmosphere of Jupiter—and when the high clouds clear we can sometimes see down to it—where the temperature is about that of the surface of the Earth, where there is probably abundant liquid water in the clouds, and where organic molecules fall from the skies like manna from heaven. Whether life could have arisen and maintained itself in the convective circumstances of Jupiter’s atmosphere is an open question; but the fact that there are such apparently pleasant places on Jupiter encourages us to keep our minds (and our eyes) open.
Jupiter is a source of continuous radio emission and radio bursts, both of which are received by radio telescopes on Earth. It was deduced many years ago that this emission is the result of charged particles—protons and electrons—from the solar wind trapped in a vast radiation belt about Jupiter by an intense jovian magnetic field. When Pioneers 10 and 11 flew through the jovian radiation belts this proposition was dramatically verified. The strong magnetic field on Jupiter may be due to its spinning metallic hydrogen interior; and the details of the trapped radiation belts, the configuration of the magnetic field, and particularly the interaction between the trapped particles and the moons of Jupiter are of very great interest. Here, as with the question of the weather on Jupiter, studies of another planet may significantly illuminate our knowledge of our own. Leakage from Earth’s radiation belts produce the auroras in both polar zones and are strongly connected with such practical matters as radio propagation on Earth and possibly even with the weather.
Jupiter has fourteen or more moons, but only four large ones, called the Galilean satellites after Galileo, their discoverer. The innermost Galilean satellite is called Io, and its orbit constrains it to plow through the great radiation belt of trapped charged particles that surrounds Jupiter. This circumstance somehow enables Io’s position to control the emission of radio bursts toward the Earth. Io itself seems to b
e trailing a vast cloud of sodium, sulfur, potassium and other atoms in a kind of truncated doughnut trailing the moon in its orbit about Jupiter. It has been suggested that Io once had salty oceans; that because of its low gravity the water in those oceans has long since escaped to space; and that the salts remaining behind are being sputtered or splayed off the surface of Io by the charged particles in the jovian radiation belts, thus producing the doughnut-shaped cloud.
The possibility of dried-up ocean basins on Io immediately suggests that the moons in the outer solar system will not be close copies of our own dead and battered hulk of rock which—with, I suppose, a certain affection—we call the Moon. The moons in the outer solar system are very different. Many of them have such low densities that they cannot possibly be composed primarily of rock and must instead be essentially icy. Some have atmospheres. Some—like Iapetus, the ninth moon of Saturn—have enormous differences in brightness between the hemisphere that is pointing in the direction of the satellite’s motion around its planet and the hemisphere facing the other way; in Iapetus’ case, the brightness difference between the leading and trailing hemispheres is a factor of six. There is no even semi-plausible explanation of this circumstance.
Schematic diagram of the passage of the Voyager 1 spacecraft through the Jupiter system. Notice the close encounters with the moons Io and Ganymede.
Passage of the Voyager 2 spacecraft through the Jupiter system. Note the close encounters with Callisto and Ganymede.
Passage of the Voyager 1 spacecraft through the Saturn system. Note the close encounters with the moons Titan, Dione, and Rhea.
Passage of the Voyager 2 spacecraft through the Saturn system. Note the close encounter of the spacecraft with the moons Enceladus and Mimas and with the rings of Saturn.
What will the surface of a rocky or icy moon that has spent four billion years plowing through an intense radiation belt look like? Nobody knows. The radioactivity in the rocks of a partly rocky, partly icy moon will melt the subsurface ice, producing a kind of slush. But the ice in the outer solar system should be not merely water-ice but also methane-ice and ammonia-ice. What is the long-term geology of such a place? Can there be methane seas and ammonia volcanos? Nobody knows. The craters on the Moon are produced by small asteroids colliding with its surface. They are preserved for billions of years because there is no wind or water, no weathering on the Moon. But when a small asteroid collides with an icy object, the ice should melt. Will that “heal” the crater scar? When we look close up at the moons of the outer solar system, will we find them cratered or not? In particular, will the icy parts be cratered? Will there be new categories of surface features unknown among the terrestrial planets? Nobody knows.
And what of Titan? Titan is the largest moon of Saturn and the largest moon in the solar system. It seems to be too warm for its distance from the Sun, perhaps because it traps heat in its atmosphere. Titan has a significant atmosphere—one much denser than that of Mars. It is composed of methane and perhaps hydrogen and other gases. It seems to be surmounted by a brownish cloud layer which, if it exists, almost everyone believes to be composed of organic matter. The irradiation of the methane alone should produce a complex array of hydrocarbons, perhaps related to asphalt and petroleum. The surface temperature of Titan is so low that organic molecules produced there over its evolutionary history have not been instantly fried like organic molecules in the atmosphere of Jupiter. The surface of this moon may therefore be littered with some of the molecules that on Earth, 4 billion years ago, led to the origin of life. In detail, what is the surface of Titan like?
From the surface of Titan, through a break in the clouds, one might be able to see Saturn, looming and magnificent, pale yellow set in a blue sky, its magnificent rings casting a shadow on the clouded globe of the planet itself. There are several rings of Saturn—no one knows how many. But a number of circumplanetary breaks in the rings have been seen, the most famous of which is the Cassini division separating the A and B rings. The rings of Saturn are astonishingly thin, far thinner relative to their lateral dimensions than a piece of paper. But the rings are not gas or solid sheets, as is sometimes depicted in television or motion pictures. Rather, the rings of Saturn are an immense horde of orbiting snowballs, each perhaps meters across, with bumps, facets and irregularities of much smaller dimensions. However, this conclusion is inferential, and no one has yet seen close up any of the constituent boulders, snowballs or particles that comprise the Saturnian rings.
More detailed views of the trajectories of the Voyager 1 and Voyager 2 spacecraft through the Jupiter and Saturn systems. Voyager 1 was launched later than Voyager 2.
In 1976 an important discovery was made: that the planet Uranus as well as Saturn is surrounded by rings, although the rings of Uranus appear to be composed of extremely dark objects quite unlike the bright snowballs of the rings of Saturn. Such rings may possibly arise when a weakly coherent moon wanders so close to its planet that the planet’s tidal gravitational influence tears it apart. Alternatively, the rings may indicate places where the planetary tides have prevented the formation of satellites; if this is the case, the rings represent residua from the epoch of formation of satellites in the early history of the solar system. The discovery of the Uranus rings was in some sense a relief to many of us in planetary astronomy; we no longer have to worry about why it is that only Saturn has rings. The phenomenon evidently is to some degree a general one.
Uranus and Neptune are so far from Earth that we know very little about them. Even their periods of rotation are still subject to some debate. Almost all the planets in the solar system rotate with axes of rotation to some extent perpendicular to the plane of their orbits about the sun. But Uranus is quite different; its axis of rotation lies almost in the plane of its orbit, as if it is rolling around the Sun on a surface, like a billiard ball. In the 1980s the axis of rotation of Uranus will be pointed toward the inner solar system—that is, toward Earth and the Sun. In that period the Sun’s rays, feeble as they are at Uranus’ great distance from the Sun, will be beating down more or less directly on one of the planetary poles, a situation quite different from our familiar one in which the sunlight is more intense at the equator than at the pole. A study of the weather on Uranus would be exceedingly interesting.
The five moons of Uranus revolve around the planet in the plane of the planet’s equator, so that in the middle 1980s the orbits of the Uranian moons will circle the planet like the rings of a bull’s-eye from the vantage point of a spacecraft approaching from Earth. Uranus and Neptune are significantly more dense than Jupiter and Saturn, which means that they must have less hydrogen, the least dense gas, and more of the heavier elements. But how hydrogen could have been depleted in the icy darkness of the outer solar system during the early flickering history of the Sun is an almost total mystery.
There are at least two other denizens of the outer solar system about which we know even less than we do about Uranus and Neptune: these are Pluto, the outermost known planet, and Chiron, a newly discovered small planet or large asteroid that circles the Sun between the orbits of Saturn and Uranus. We are not even sure of the sizes of these objects, much less such matters as composition or interior structure. Beyond Pluto is a realm of outer darkness from which the Sun appears merely as a bright star and which is inhabited by billions of slowly orbiting snowballs, each about a mile across. When these snowballs occasionally enter the inner solar system and heat up, the snows vaporize and a great tail is ejected by the solar wind away from the Sun. The snowball is then called a comet. But in their usual realm these objects are much less exuberant.
The distance from Earth to the Sun is called, immodestly by astronomers from the planet Earth, one “astronomical unit.” It is 93 million miles or 150 million kilometers and is abbreviated A.U. The main belt of asteroids extends out to about four astronomical units. Thus the terrestrial planets extend from Mercury at about 0.4 A.U. from the Sun to the asteroids at about 4 A.U. from the Sun. But
Neptune is 30 A.U. from the Sun, and the comets extend to 100,000 A.U. The inner solar system where we live and about which we know most is an insignificant province in the vastness of the Sun’s empire. No one knows just where the solar system ends. The distance from Earth to the nearest star is a few hundred thousand A.U., and it is even conceivable that there are distant comets that are simultaneously in orbit—perhaps in figure-eight trajectories—both about our Sun and about one or more of the stars in the nearby Alpha Centauri system. But interplanetary space is filled with the solar wind and associated magnetic fields. Interstellar space has its own charged particles and magnetic fields. One useful definition of the boundary of the solar system is the place where the pressure exerted on the interstellar gas by the solar wind is compensated for by the interstellar magnetic field. Such a place is called the heliopause, the place where the Sun’s influence—at least in this regard—stops. But no one has at yet measured where the heliopause is or the very interesting character of the interplanetary particles and fields at that transition.
It is clear, to paraphrase Isaac Newton, that in our spacecraft exploration of the inner solar system we have been playing on the seashore when the vast ocean of the solar system lay all undiscovered before us. But that situation is about to change dramatically. The Voyager spacecraft are scheduled to take the first systematic close-up looks at Jupiter and its fourteen or so moons in 1979; at Saturn, its ring system and its ten or so moons in 1980 and 1981; and perhaps at Uranus in 1986. Controlled by its own onboard computer as well as subject to instructions from the Earth, and crammed with an array of scientific instruments, the two Voyager spacecraft should revolutionize our knowledge of the outer solar system. The spacecraft are accelerated by Jupiter’s gravity to reach Saturn in much less time than would otherwise be possible, and Saturn’s gravity—if this option is ultimately adopted—is similarly utilized to reach Uranus. It is because of these gravity-assisted trajectories that the Voyager spacecraft eventually will leave the solar system, an accident of celestial mechanics that led to the Voyager records being placed aboard the spacecraft.