Authors: Carl Sagan
Cosmos (25 page)
As the solar system condensed out of instellar gas and dust, Jupiter acquired most of the matter that was not ejected into interstellar space and did not fall inward to form the Sun. Had Jupiter been several dozen times more massive, the matter in its interior would have undergone thermonuclear reactions, and Jupiter would have begun to shine by its own light. The largest planet is a star that failed. Even so, its interior temperatures are sufficiently high that it gives off about twice as much energy as it receives from the Sun. In the infrared part of the spectrum, it might even be
correct to consider Jupiter a star. Had it become a star in visible light, we would today inhabit a binary or double-star system, with two suns in our sky, and the nights would come more rarely—a commonplace, I believe, in countless solar systems throughout the Milky Way Galaxy. We would doubtless think the circumstances natural and lovely.
Deep below the clouds of Jupiter the weight of the overlying layers of atmosphere produces pressures much higher than any found on Earth, pressures so great that electrons are squeezed off hydrogen atoms, producing a remarkable substance, liquid metallic hydrogen—a physical state that has never been achieved on Earth. (There is some hope that metallic hydrogen is a superconductor at moderate temperatures. If it could be manufactured on Earth, it would work a revolution in electronics.) In the interior of Jupiter, where the pressures are about three million times the atmospheric pressure at the surface of the Earth, there is almost nothing but a great dark sloshing ocean of metallic hydrogen. But at the very core of Jupiter there may be a lump of rock and iron, an Earth-like world in a pressure vise, hidden forever at the center of the largest planet.
The electrical currents in the liquid metal interior of Jupiter may be the source of the planet’s enormous magnetic field, the largest in the solar system, and of its associated belt of trapped electrons and protons. These charged particles are ejected from the Sun in the solar wind and captured and accelerated by Jupiter’s magnetic field. Vast numbers of them are trapped far above the clouds and are condemned to bounce from pole to pole until by chance they encounter some high-altitude atmospheric molecule and are removed from the radiation belt. Io moves in an orbit so close to Jupiter that it plows through the midst of this intense radiation, creating cascades of charged particles, which in turn generate violent bursts of radio energy. (They may also influence eruptive processes on the surface of Io.) It is possible to predict radio bursts from Jupiter with better reliability than weather forecasts on Earth, by computing the position of Io.
That Jupiter is a source of radio emission was discovered accidentally in the 1950’s, the early days of radio astronomy. Two young Americans, Bernard Burke and Kenneth Franklin, were examining the sky with a newly constructed and for that time very sensitive radio telescope. They were searching the cosmic radio background—that is, radio sources far beyond our solar system. To their surprise, they found an intense and previously unreported source that seemed to correspond to no prominent star, nebula or
galaxy. What is more, it gradually moved, with respect to the distant stars, much faster than any remote object could.
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After finding no likely explanation of all this in their charts of the distant Cosmos, they one day stepped outside the observatory and looked up at the sky with the naked eye to see if anything interesting happened to be there. Bemusedly they noted an exceptionally bright object in the right place, which they soon identified as the planet Jupiter. This accidental discovery is, incidentally, entirely typical of the history of science.
Every evening before Voyager 1’s encounter with Jupiter, I could see that giant planet twinkling in the sky, a sight our ancestors have enjoyed and wondered at for a million years. And on the evening of Encounter, on my way to study the Voyager data arriving at JPL, I thought that Jupiter would never be the same, never again just a point of light in the night sky, but would forever after be a
place
to be explored and known. Jupiter and its moons are a kind of miniature solar system and exquisite worlds with much to teach us.
In composition and in many other respects Saturn is similar to Jupiter, although smaller. Rotating once every ten hours, it exhibits colorful equatorial banding, which is, however, not so prominent as Jupiter’s. It has a weaker magnetic field and radiation belt than Jupiter and a more spectacular set of circumplanetary rings. And it also is surrounded by a dozen or more satellites.
The most interesting of the moons of Saturn seems to be Titan, the largest moon in the solar system and the only one with a substantial atmosphere. Prior to the encounter of Voyager 1 with Titan in November 1980, our information about Titan was scanty and tantalizing. The only gas known unambiguously to be present was methane, CH
4
, discovered by G. P. Kuiper. Ultraviolet light from the sun converts methane to more complex hydrocarbon molecules and hydrogen gas. The hydrocarbons should remain on Titan, covering the surface with a brownish tarry organic sludge, something like that produced in experiments on the origin of life on Earth. The lightweight hydrogen gas should, because of Titan’s low gravity, rapidly escape to space by a violent process known as “blowoff,” which should carry the methane and other atmospheric constituents with it. But Titan has an atmospheric pressure at least as great as that of the planet Mars. Blowoff does not seem to be happening. Perhaps there is some major and as yet undiscovered atmospheric constituent—nitrogen, for example—which
keeps the average molecular weight of the atmosphere high and prevents blowoff. Or perhaps blowoff is happening, but the gases lost to space are being replenished by others released from the satellite’s interior. The bulk density of Titan is so low that there must be a vast supply of water and other ices, probably including methane, which are at unknown rates being released to the surface by internal heating.
When we examine Titan through the telescope we see a barely perceptible reddish disc. Some observers have reported variable white clouds above that disc—most likely, clouds of methane crystals. But what is responsible for the reddish coloration? Most students of Titan agree that complex organic molecules are the most likely explanation. The surface temperature and atmospheric thickness are still under debate. There have been some hints of an enhanced surface temperature due to an atmospheric greenhouse effect. With abundant organic molecules on its surface and in its atmosphere, Titan is a remarkable and unique denizen of the solar system. The history of our past voyages of discovery suggests that Voyager and other spacecraft reconnaissance missions will revolutionize our knowledge of this place.
Through a break in the clouds of Titan, you might glimpse Saturn and its rings, their pale yellow color diffused by the intervening atmosphere. Because the Saturn system is ten times farther from the sun than is the Earth, the sunshine on Titan is only 1 percent as intense as we are accustomed to, and the temperatures should be far below the freezing point of water even with a sizable atmospheric greenhouse effect. But with abundant organic matter, sunlight and perhaps volcanic hot spots, the possibility of life on Titan
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cannot be readily dismissed. In that very different environment, it would, of course, have to be very different from life on Earth. There is no strong evidence either for or against life on Titan. It is merely possible. We are unlikely to
determine the answer to this question without landing instrumented space vehicles on the Titanian surface.
To examine the individual particles composing the rings of Saturn we must approach them closely, for the particles are small—snowballs and ice chips and tiny tumbling bonsai glaciers, a meter or so across. We know they are composed of water ice, because the spectral properties of sunlight reflected off the rings match those of ice in the laboratory measurements. To approach the particles in a space vehicle, we must slow down, so that we move along with them as they circle Saturn at some 45,000 miles per hour; that is, we must be in orbit around Saturn ourselves, moving at the same speed as the particles. Only then will we be able to see them individually and not as smears or streaks.
Why is there not a single large satellite instead of a ring system around Saturn? The closer a ring particle is to Saturn, the faster its orbital speed (the faster it is “falling” around the planet—Kepler’s third law); the inner particles are streaming past the outer ones (the “passing lane” as we see it is always to the left). Although the whole assemblage is tearing around the planet itself at some 20 kilometers per second, the
relative
speed of two adjacent particles is very low, only some few centimeters per minute. Because of this relative motion, the particles can never stick together by their mutual gravity. As soon as they try, their slightly different orbital speeds pull them apart. If the ring were not so close to Saturn, this effect would not be so strong, and the particles could accrete, making small snowballs and eventually growing into satellites. So it is probably no coincidence that outside the rings of Saturn there is a system of satellites varying in size from a few hundred kilometers across to Titan, a giant moon nearly as large as the planet Mars. The matter in all the satellites and the planets themselves may have been originally distributed in the form of rings, which condensed and accumulated to form the present moons and planets.
For Saturn as for Jupiter, the magnetic field captures and accelerates the charged particles of the solar wind. When a charged particle bounces from one magnetic pole to the other, it must cross the equatorial plane of Saturn. If there is a ring particle in the way, the proton or electron is absorbed by this small snowball. As a result, for both planets, the rings clear out the radiation belts, which exist only interior and exterior to the particle rings. A close moon of Jupiter or Saturn will likewise gobble up radiation belt particles, and in fact one of the new moons of Saturn was discovered in just this way: Pioneer 11 found an unexpected gap in
the radiation belts, caused by the sweeping up of charged particles by a previously unknown moon.
The solar wind trickles into the outer solar system far beyond the orbit of Saturn. When Voyager reaches Uranus and the orbits of Neptune and Pluto, if the instruments are still functioning, they will almost certainly sense its presence, the wind between the worlds, the top of the sun’s atmosphere blown outward toward the realm of the stars. Some two or three times farther from the Sun than Pluto is, the pressure of the interstellar protons and electrons becomes greater than the minuscule pressure there exerted by the solar wind. That place, called the heliopause, is one definition of the outer boundary of the Empire of the Sun. But the Voyager spacecraft will plunge on, penetrating the heliopause sometime in the middle of the twenty-first century, skimming through the ocean of space, never to enter another solar system, destined to wander through eternity far from the stellar islands and to complete its first circumnavigation of the massive center of the Milky Way a few hundred million years from now. We have embarked on epic voyages.
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Or, to make a different comparison, a fertilized egg takes as long to wander from the fallopian tubes and implant itself in the uterus as Apollo 11 took to journey to the Moon; and as long to develop into a full-term infant as Viking took on its trip to Mars. The normal human lifetime is longer than Voyager will take to venture beyond the orbit of Pluto.
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We even know what gifts they brought the Court. The Empress was presented with “six little chests of divers pictures.” And the Emperor received “two fardels of cinnamon.”
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In 1979 Pope John Paul II cautiously proposed reversing the condemnation of Galileo done 346 years earlier by the “Holy Inquisition.”
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The courage of Galileo (and Kepler) in promoting the heliocentric hypothesis was not evident in the actions of others, even those residing in less fanatically doctrinal parts of Europe. For example, in a letter dated April 1634, René Descartes, then living in Holland, wrote:
Doubtless you know that Galileo was recently censured by the Inquisitors of the Faith, and that his views about the movement of the Earth were condemned as heretical. I must tell you that all the things I explained in my treatise, which included the doctrine of the movement of the Earth, were so interdependent that it is enough to discover that one of them is false to know that all the arguments I was using are unsound. Though I thought they were based on very certain and evident proofs, I would not wish, for anything in the world, to maintain them against the authority of the Church.… I desire to live in peace and to continue the life I have begun under the motto
to live well you must live unseen
.
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This exploratory tradition may account for the fact that Holland has, to this day, produced far more than its per capita share of distinguished astronomers, among them Gerard Peter Kuiper, who in the 1940’s and 1950’s was the world’s only full-time planetary astrophysicist. The subject was then considered by most professional astronomers to be at least slightly disreputable, tainted with Lowellian excesses. I am grateful to have been Kuiper’s student.
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Isaac Newton admired Christiaan Huygens and thought him “the most elegant mathematician” of their time, and the truest follower of the mathematical tradition of the ancient Greeks—then, as now, a great compliment. Newton believed, in part because shadows had sharp edges, that light behaved as if it were a stream of tiny particles. He thought that red light was composed of the largest particles and violet the smallest. Huygens argued that instead light behaved as if it were a wave propagating in a vacuum, as an ocean wave does in the sea—which is why we talk about the wavelength and frequency of light. Many properties of light, including diffraction, are naturally explained by the wave theory, and in subsequent years Huygens’ view carried the day. But in 1905, Einstein showed that the particle theory of light could explain the photoelectric effect, the ejection of electrons from a metal upon exposure to a beam of light. Modern quantum mechanics combines both ideas, and it is customary today to think of light as behaving in some circumstances as a beam of particles and in others as a wave. This wave-particle dualism may not correspond readily to our common-sense notions, but it is in excellent accord with what experiments have shown light really does. There is something mysterious and stirring in this marriage of opposites, and it is fitting that Newton and Huygens, bachelors both, were the parents of our modern understanding of the nature of light.
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Galileo discovered the rings, but had no idea what to make of them. Through his early astronomical telescope, they seemed to be two projections symmetrically attached to Saturn, resembling, he said in some bafflement, ears.
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A few others had held similar opinions. In his
Harmonice Mundi
Kepler remarked “it was Tycho Brahe’s opinion concerning that bare wilderness of globes that it does not exist fruitlessly but is filled with inhabitants.”
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Such tales are an ancient human tradition; many of them have had, from the beginning of exploration, a cosmic motif. For example, the fifteenth-century explorations of Indonesia, Sri Lanka, India, Arabia and Africa by the Ming Dynasty Chinese were described by Fei Hsin, one of the participants, in a picture book prepared for the Emperor, as “The Triumphant Visions of the Starry Raft.” Unfortunately, the pictures—although not the text—have been lost.
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Frequently pronounced “eye-oh” by Americans, because this is the preferred enunciation in the
Oxford English Dictionary
. But the British have no special wisdom here. The word is of Eastern Mediterranean origin and is pronounced throughout the rest of Europe, correctly, as “ee-oh.”
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Because the speed of light is finite (see
Chapter 8
).
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The view of Huygens, who discovered Titan in 1655, was: “Now can any one look upon, and compare these Systems [of Jupiter and Saturn] together, without being amazed at the vast Magnitude and noble Attendants of these two Planets, in respect of this little pitiful Earth of ours? Or can they force themselves to think, that the wise Creator has disposed of all his Animals and Plants here, has furnished and adorn’d this Spot only, and has left all those Worlds bare and destitute of Inhabitants, who might adore and worship Him; or that all those prodigious Bodies were made only to twinkle to, and be studied by some few perhaps of us poor Fellows?” Since Saturn moves around the sun once every thirty years, the length of the seasons on Saturn and its moons is much longer than on Earth. Of the presumed inhabitants of the moons of Saturn, Huygens therefore wrote: “It is impossible but that their way of living must be very different from ours, having such tedious Winters.”