Pale Blue Dot: A Vision of the Human Future in Space - Carl Sagan, Ann Druyan (1997)


 … by the shore

Of Triton’s Lake …

I will clear my breast of secrets.


Neptune was the final port of call in Voyager 2’s grand tour of the Solar System. Usually, it is thought of as the penultimate planet, with Pluto the outermost. But because of Pluto’s stretched-out, elliptical orbit, Neptune has lately been the outermost planet, and will remain so until 1999. Typical temperatures in its upper clouds are about -240°C, because it is so far from the warming rays of the Sun. It would be colder still, except for the heat welling up from its interior. Neptune glides along the hem of interstellar night. It is so far away that, in its sky, the Sun appears as little more than an extremely bright star.

How far? So far away that it has yet to complete a single trip around the Sun, a Neptunian year, since its discovery in 1846.* It’s so far away that it cannot be seen with the naked eye. It’s so far away that it takes light—faster than which nothing can go—more than five hours to get from Neptune to Earth.

When Voyager 2 raced through the Neptune system in 1989, its cameras, spectrometers, particle and field detectors, and other instruments were feverishly examining the planet, its moons, and its rings. The planet itself, like its cousins Jupiter, Saturn, and Uranus, is a giant. Every planet is an Earthlike world at heart—but the four gas giants wear elaborate, cumbersome disguises. Jupiter and Saturn are great gas worlds with relatively small rocky and icy cores. But Uranus and Neptune are fundamentally rock and ice worlds swaddled in dense atmospheres that hide them from view.

Neptune is four times bigger than the Earth. When we look down on its cool, austere blueness, again we are seeing only atmosphere and clouds—no solid surface. Again, the atmosphere is made mainly of hydrogen and helium, with a little methane and traces of other hydrocarbons. There may also be some nitrogen. The bright clouds, which seem to be methane crystals, float above thick, deeper clouds of unknown composition. From the motion of the clouds we discovered fierce winds, approaching the local speed of sound. A Great Dark Spot was found, curiously at almost the same latitude as the Great Red Spot on Jupiter. The azure color seems appropriate for a planet named after the god of the sea.

Surrounding this dimly lit, chilly, stormy, remote world is—here also—a system of rings, each composed of innumerable orbiting objects ranging in size from the fine particles in cigarette smoke to small trucks. Like the rings of the other Jovian planets, those of Neptune seem to be evanescent—it is calculated that gravity and solar radiation will disrupt them in much less than the age of the Solar System. If they are destroyed quickly, we must see them only because they were made recently. But how can rings be made?

The biggest moon in the Neptune system is called Triton.* Nearly six of our days are required for it to orbit Neptune, which—alone among big moons in the Solar System—it does in the opposite direction to which its planet spins (clockwise if we say Neptune rotates counterclockwise). Triton has a nitrogen-rich atmosphere, somewhat similar to Titan’s; but, because the air and haze are much thinner, we can see its surface. The landscapes are varied and splendid. This is a world of ices—nitrogen ice, methane ice, probably underlain by more familiar water ice and rock. There are impact basins, which seem to have been flooded with liquid before refreezing (so there once were lakes on Triton); impact craters; long crisscrossing valleys; vast plains covered by freshly fallen nitrogen snow; puckered terrain that resembles the skin of a cantaloupe; and more or less parallel, long, dark streaks that seem to have been blown by the wind, and then deposited on the icy surface—despite how sparse Triton’s atmosphere is (about 1/10,000 the thickness of the Earth’s).

All the craters on Triton are pristine—as if stamped out by some vast milling device. There are no slumped walls or muted relief. Even with the periodic falling and evaporation of snow, it seems that nothing has eroded the surface of Triton in billions of years. So the craters that were gouged out during the formation of Triton must have all been filled in and covered over by some early global resurfacing event. Triton orbits Neptune in the opposite direction to Neptune’s rotation—unlike the situation with the Earth and its moon, and with most of the large moons in the Solar System. If Triton had formed out of the same spinning disk that made Neptune, it ought to be going around Neptune in the same direction that Neptune rotates. So Triton was not made from the original local nebula around Neptune, but arose somewhere else—perhaps far beyond Pluto—and was by chance gravitationally captured when it passed too close to Neptune. This event should have raised enormous solid-body tides in Triton, melting the surface and sweeping away all the past topography.

In some places the surface is as bright and white as freshly fallen Antarctic snows (and may offer a skiing experience unrivaled in all the Solar System). Elsewhere there’s a tint, ranging from pink to brown. One possible explanation: Freshly fallen snows of nitrogen, methane, and other hydrocarbons are irradiated by solar ultraviolet light and by electrons trapped in the magnetic field of Neptune, through which Triton plows. We know that such irradiation will convert the snows (like the corresponding gases) to complex, dark, reddish organic sediments, ice tholins—nothing alive, but here too composed of some of the molecules implicated in the origin of life on Earth four billion years ago.

In local winter, layers of ice and snow build up on the surface. (Our winters, mercifully, are only 4 percent as long.) Through the spring, they are slowly transformed, more and more reddish organic molecules accumulating. By summertime, the ice and snow have evaporated; the gases so released migrate halfway across the planet to the winter hemisphere and there cover the surface with ice and snow again. But the reddish organic molecules do not vaporize and are not transported—a lag deposit, they are next winter covered over by new snows, which are in turn irradiated, and by the following summer the accumulation is thicker. As time goes on, substantial amounts of organic matter are built up on the surface of Triton, which may account for its delicate color markings.

The streaks begin in small, dark source regions, perhaps when the warmth of spring and summer heats subsurface volatile snows. As they vaporize, gas comes gushing out as in a geyser, blowing off less-volatile surface snows and dark organics. Prevailing low-speed winds carry away the dark organics, which slowly sediment out of the thin air, are deposited on the ground, and generate the appearance of the streaks. This, at least, is one reconstruction of recent Tritonian history.

Triton may have large, seasonal polar caps of smooth nitrogen ice underlying layers of dark organic materials. Nitrogen snows seem recently to have fallen at the equator. Snowfalls, geysers, windblown organic dust, and high-altitude hazes were entirely unexpected on a world with so thin an atmosphere.

Why is the air so thin? Because Triton is so far from the Sun. Were you somehow to pick this world up and move it into orbit around Saturn, the nitrogen and methane ices would quickly evaporate, a much denser atmosphere of gaseous nitrogen and methane would form, and radiation would generate an opaque tholin haze. It would become a world very like Titan. Conversely, if you moved Titan into orbit about Neptune, almost all its atmosphere would freeze out as snows and ices, the tholin would fall out and not be replaced, the air would clear, and the surface would become visible in ordinary light. It would become a world very like Triton.

These two worlds are not identical. The interior of Titan seems to contain much more ice than that of Triton, and much less rock. Titan’s diameter is almost twice that of Triton. Still, if placed at the same distance from the Sun they would look like sisters. Alan Stern of the Southwest Research Institute suggests that they are two members of a vast collection of small worlds rich in nitrogen and methane that formed in the early Solar System. Pluto, yet to be visited by a spacecraft, appears to be another member of this group. Many more may await discovery beyond Pluto. The thin atmospheres and icy surfaces of all these worlds are being irradiated—by cosmic rays, if nothing else—and nitrogen-rich organic compounds are being formed. It looks as if the stuff of life is sitting not just on Titan, but throughout the cold, dimly lit outer reaches of our planetary system.

Another class of small objects has recently been discovered, whose orbits take them—at least part of the time—beyond Neptune and Pluto. Sometimes called minor planets or asteroids, they are more likely to be inactive comets (with no tails, of course; so far from the Sun, their ices cannot readily vaporize). But they are much bigger than the run-of-the-mill comets we know. They may be the vanguard of a vast array of small worlds that extends from the orbit of Pluto halfway to the nearest star. The innermost province of the Oort Comet Cloud, of which these new objects may be members, is called the Kuiper Belt, after my mentor Gerard Kuiper, who first suggested that it should exist. Short-period comets—like Halley’s—arise in the Kuiper Belt, respond to gravitational tugs, sweep into the inner part of the Solar System, grow their tails, and grace our skies.

Back in the late nineteenth century, these building blocks of worlds—then mere hypotheses—were called “planetesimals.” The flavor of the word is, I suppose, something like that of “infinitesimals”: You need an infinite number of them to make anything. It’s not quite that extreme with planetesimals, although a very large number of them would be required to make a planet. For example, trillions of bodies each a kilometer in size would be needed to coalesce to make a planet with the mass of the Earth. Once there were much larger numbers of worldlets in the planetary part of the Solar System. Most of them are now gone—ejected into interstellar space, fallen into the Sun, or sacrificed in the great enterprise of building moons and planets. But out beyond Neptune and Pluto the discards, the leftovers that were never aggregated into worlds, may be waiting—a few largish ones in the 100-kilometer range, and stupefying numbers of kilometer-sized and smaller bodies peppering the outer Solar System all the way out to the Oort Cloud.

In this sense there are planets beyond Neptune and Pluto—but they are not nearly as big as the Jovian planets, or even Pluto. Larger worlds may, for all we know, also be hiding in the dark beyond Pluto, worlds that can properly be called planets. The farther away they are, the less likely it is that we would have detected them. They cannot lie just beyond Neptune, though; their gravitational tugs would have perceptibly altered the orbits of Neptune and Pluto, and the Pioneer 10 and 11 and Voyager 1 and 2 spacecraft.

The newly discovered cometary bodies (with names like 1992QB and 1993FW) are not planets in this sense. If our detection threshold has just encompassed them, many more of them probably remain to be discovered in the outer Solar System—so far away that they’re hard to see from Earth, so distant that it’s a long journey to get to them. But small, quick ships to Pluto and beyond are within our ability. It would make good sense to dispatch one by Pluto and its moon Charon, and then, if we can, to make a close pass by one of the denizens of the Kuiper Comet Belt.

The rocky Earthlike cores of Uranus and Neptune seem to have accreted first, and then gravitationally attracted massive amounts of hydrogen and helium gas from the ancient nebula out of which the planets formed. Originally, they lived in a hailstorm. Their gravities were just sufficient to eject icy worldlets, when they came too close, far out beyond the realm of the planets, to populate the Oort Comet Cloud. Jupiter and Saturn became gas giants by the same process. But their gravities were too strong to populate the Oort Cloud: Ice worlds that came close to them were gravitationally pitched out of the Solar System entirely—destined to wander forever in the great dark between the stars.

So the lovely comets that on occasion rouse us humans to wonder and to awe, that crater the surfaces of inner planets and outer moons, and that now and then endanger life on Earth would be unknown and unthreatening had Uranus and Neptune not grown to be giant worlds four and a half billion years ago.

THIS IS THE PLACE for a brief interlude on planets far beyond Neptune and Pluto, planets of other stars.

Many nearby stars are surrounded by thin disks of orbiting gas and dust, often extending to hundreds of astronomical units (AU) from the local star (the outermost planets, Neptune and Pluto, are about 40 AU from our Sun). Younger Sun-like stars are much more likely to have disks than older ones. In some cases, there’s a hole in the center of the disk as in a phonograph record. The hole extends out from the star to perhaps 30 or 40 AU. This is true, for example, for the disks surrounding the stars Vega and Epsilon Eridani. The hole in the disk surrounding Beta Pictoris extends to only 15 AU from the star. There is a real possibility that these inner, dust-free zones have been cleaned up by planets that recently formed there. Indeed, this sweeping-out process is predicted for the early history of our planetary system. As observations improve, perhaps we will see telltale details in the configuration of dust and dust-free zones that will indicate the presence of planets too small and dark to be seen directly. Spectroscopic data suggest that these disks are churning and that matter is falling in on the central stars—perhaps from comets formed in the disk, deflected by the unseen planets, and evaporating as they approach too close to the local sun.

Because planets are small and shine by reflected light, they tend to be washed out in the glare of the local sun. Nevertheless, many efforts are now under way to find fully formed planets around nearby stars—by detecting a faint brief dimming of starlight as a dark planet interposes itself between the star and the observer on Earth; or by sensing a faint wobble in the motion of the star as it’s tugged first one way and then another by an otherwise invisible orbiting companion. Spaceborne techniques will be much more sensitive. A Jovian planet going around a nearby star is about a billion times fainter than its sun; nevertheless, a new generation of ground-based telescopes that can compensate for the twinkling in the Earth’s atmosphere may soon be able to detect such planets in only a few hours’ observing time. A terrestrial planet of a neighboring star is a hundred times fainter still; but it now seems that comparatively inexpensive spacecraft, above the Earth’s atmosphere, might be able to detect other Earths. None of these searches has succeeded yet, but we are clearly on the verge of being able to detect at least Jupiter-sized planets around the nearest stars—if there are any to be found.

A most important and serendipitous recent discovery is of a bona fide planetary system around an unlikely star, some 1,300 light-years away, found by a most unexpected technique: The pulsar designated B1257+12 is a rapidly rotating neutron star, an unbelievably dense sun, the remnant of a massive star that suffered a supernova explosion. It spins, at a rate measured to impressive precision, once every 0.0062185319388187 seconds, This pulsar is pushing 10,000 rpm.

Charged particles trapped in its intense magnetic field generate radio waves that are cast across the Earth, about 160 flickers a second. Small but discernible changes in the flash rate were tentatively interpreted by Alexander Wolszczan, now at Pennsylvania State University, in 1991—as a tiny reflex motion of the pulsar in response to the presence of planets. In 1994 the predicted mutual gravitational interactions of these planets were confirmed by Wolszczan from a study of timing residuals at the microsecond level over the intervening years. The evidence that these are truly new planets and not starquakes on the neutron star surface (or something) is now overwhelming—or, as Wolszczan put it, “irrefutable”; a new solar system is “unambiguously identified.” Unlike all the other techniques, the pulsar timing method makes close-in terrestrial planets comparatively easy and more distant Jovian planets comparatively difficult to detect.

Planet C, some 2.8 times more massive than the Earth, orbits the pulsar every 98 days at a distance of 0.47 astronomical units* (AU); Planet B, with about 3.4 Earth masses, has a 67-Earth-day year at 0.36 AU. A smaller world, Planet A, still closer to the star, with about 0.015 Earth masses, is at 0.19 AU. Crudely speaking, Planet B is roughly at the distance of Mercury from our Sun; Planet C is midway between the distances of Mercury and Venus; and interior to both of them is Planet A, roughly the mass of the Moon at about half Mercury’s distance from our Sun. Whether these planets are the remnants of an earlier planetary system that somehow survived the supernova explosion that produced the pulsar, or whether they formed from the resulting circumstellar accretion disk subsequent to the supernova explosion, we do not know. But in either case, we have now learned that there are other Earths.

The energy put out by B1257+12 is about 4.7 times that of the Sun. But, unlike the Sun, most of this is not in visible light, but in a fierce hurricane of electrically charged particles. Suppose that these particles impinge on the planets and heat them. Then, even a planet at 1 AU would have a surface around 280 Celsius degrees above the normal boiling point of water, greater than the temperature of Venus.

These dark and broiling planets do not seem hospitable for life. But there may be others, farther from B1257+12, that are. (Hints of at least one cooler, outer world in the B1257+12 system exist.) Of course, we don’t even know that such worlds would retain their atmospheres; perhaps any atmospheres were stripped away in the supernova explosion, if they date back that far. But we do seem to be detecting a recognizable planetary system. Many more are likely to become known in coming decades, around ordinary Sun-like stars as well as white dwarfs, pulsars, and other end states of stellar evolution.

Eventually, we will have a list of planetary systems—each perhaps with terrestrials and Jovians and maybe new classes of planets. We will examine these worlds, spectroscopically and in other ways. We will be searching for new Earths and other life.

ON NONE OF THE WORLDS in the outer Solar System did Voyager find signs of life, much less intelligence. There was organic matter galore—the stuff of life, the premonitions of life, perhaps—but as far as we could see, no life. There was no oxygen in their atmospheres, and no gases profoundly out of chemical equilibrium, as methane is in the Earth’s oxygen. Many of the worlds were painted with subtle colors, but none with such distinctive, sharp absorption features as chlorophyll provides over much of the Earth’s surface. On very few worlds was Voyager able to resolve details as small as a kilometer across. By this standard, it would not have detected even our own technical civilization had it been transplanted to the outer Solar System. But for what it’s worth, we found no regular patterning, no geometrization, no passion for small circles, triangles, squares, or rectangles. There were no constellations of steady points of light on the night hemispheres. There were no signs of a technical civilization reworking the surface of any of these worlds.

The Jovian planets are prolific broadcasters of radio waves—generated in part by the abundant trapped and beamed charged particles in their magnetic fields, in part by lightning, and in part by their hot interiors. But none of this emission has the character of intelligent life—or so it seems to the experts in the field.

Of course our thinking may be too narrow. We may be missing something. For example, there is a little carbon dioxide in the atmosphere of Titan, which puts its nitrogen/methane atmosphere out of chemical equilibrium. I think the CO2 is provided by the steady pitter-patter of comets falling into Titan’s atmosphere—but maybe not. Maybe there’s something on the surface unaccountably generating CO2 in the face of all that methane.

The surfaces of Miranda and Triton are unlike anything else we know. There are vast chevron-shaped landforms and crisscrossing straight lines that even sober planetary geologists once mischievously described as “highways.” We think we (barely) understand these landforms in terms of faults and collisions, but of course we might be wrong.

The surface stains of organic matter—sometimes, as on Triton, delicately hued—are attributed to charged particles producing chemical reactions in simple hydrocarbon ices, generating more complex organic materials, and all this having nothing to do with the intermediation of life. But of course we might be wrong.

The complex pattern of radio static, bursts, and whistles that we receive from all four Jovian planets seems, in a general way, explicable by plasma physics and thermal emission. (Much of the detail is not yet well understood.) But of course we might be wrong.

We have found nothing on dozens of worlds so clear and striking as the signs of life found by the Galileo spacecraft in its passages by the Earth. Life is a hypothesis of last resort. You invoke it only when there’s no other way to explain what you see. If I had to judge, I would say that there’s no life on any of the worlds we’ve studied, except of course our own. But I might be wrong, and, right or wrong, my judgment is necessarily confined to this Solar System. Perhaps on some new mission we’ll find something different, something striking, something wholly inexplicable with the ordinary tools of planetary science—and tremulously, cautiously, we will inch toward a biological explanation. However, for now nothing requires that we go down such a path. So far, the only life in the Solar System is that which comes from Earth. In the Uranus and Neptune systems, the only sign of life has been Voyager itself.

As we identify the planets of other stars, as we find other worlds of roughly the size and mass of the Earth, we will scrutinize them for life. A dense oxygen atmosphere may be detectable even on a world we’ve never imaged. As for the Earth, that may by itself be a sign of life. An oxygen atmosphere with appreciable quantities of methane would almost certainly be a sign of life, as would modulated radio emission. Someday, from observations of our planetary system or another, the news of life elsewhere may be announced over the morning coffee.

THE VOYAGER SPACECRAFT are bound for the stars. They are on escape trajectories from the Solar System, barreling along at almost a million miles a day. The gravitational fields of Jupiter, Saturn, Uranus, and Neptune have flung them at such high speeds that they have broken the bonds that once tied them to the Sun.

Have they left the Solar System yet? The answer depends very much on how you define the boundary of the Sun’s realm. If it’s the orbit of the outermost good-sized planet, then the Voyager spacecraft are already long gone; there are probably no undiscovered Neptunes. If you mean the outermost planet, it may be that there are other—perhaps Triton-like—planets far beyond Neptune and Pluto; if so, Voyager 1 and Voyager 2 are still within the Solar System. If you define the outer limits of the Solar System as the heliopause—where the interplanetary particles and magnetic fields are replaced by their interstellar counterparts—then neither Voyager has yet left the Solar System, although they may do so in the next few decades.* But if your definition of the edge of the Solar System is the distance at which our star can no longer hold worlds in orbit about it, then the Voyagers will not leave the Solar System for hundreds of centuries.

Weakly grasped by the Sun’s gravity, in every direction in the sky, is that immense horde of a trillion comets or more, the Oort Cloud. The two spacecraft will finish their passage through the Oort Cloud in another 20,000 years or so. Then, at last, completing their long good-bye to the Solar System, broken free of the gravitational shackles that once bound them to the Sun, the Voyagers will make for the open sea of interstellar space. Only then will Phase Two of their mission begin.

Their radio transmitters long dead, the spacecraft will wander for ages in the calm, cold interstellar blackness—where there is almost nothing to erode them. Once out of the Solar System, they will remain intact for a billion years or more, as they circumnavigate the center of the Milky Way galaxy.

We do not know whether there are other spacefaring civilizations in the Milky Way. If they do exist, we do not know how abundant they are, much less where they are. But there is at least a chance that sometime in the remote future one of the Voyagers will be intercepted and examined by an alien craft.

Accordingly, as each Voyager left Earth for the planets and the stars, it carried with it a golden phonograph record encased in a golden, mirrored jacket containing, among other things: greetings in 59 human languages and one whale language; a 12-minute sound essay including a kiss, a baby’s cry, and an EEG record of the meditations of a young woman in love; 116 encoded pictures, on our science, our civilization, and ourselves; and 90 minutes of the Earth’s greatest hits—Eastern and Western, classical and folk, including a Navajo night chant, a Japanese shakuhachi piece, a Pygmy girl’s initiation song, a Peruvian wedding song, a 3,000-year-old composition for the ch’in called “Flowing Streams,” Bach, Beethoven, Mozart, Stravinsky, Louis Armstrong, Blind Willie Johnson, and Chuck Berry’s “Johnny B. Goode.”

Space is nearly empty. There is virtually no chance that one of the Voyagers will ever enter another solar system—and this is true even if every star in the sky is accompanied by planets. The instructions on the record jackets, written in what we believe to be readily comprehensible scientific hieroglyphics, can be read, and the contents of the records understood, only if alien beings, somewhere in the remote future, find Voyager in the depths of interstellar space. Since both Voyagers will circle the center of the Milky Way Galaxy essentially forever, there is plenty of time for the records to be found—if there’s anyone out there to do the finding.

We cannot know how much of the records they would understand. Surely the greetings will be incomprehensible, but their intent may not be. (We thought it would be impolite not to say hello.) The hypothetical aliens are bound to be very different from us—independently evolved on another world. Are we really sure they could understand anything at all of our message? Every time I feel these concerns stirring, though, I reassure myself: Whatever the incomprehensibilities of the Voyager record, any alien ship that finds it will have another standard by which to judge us. Each Voyager is itself a message. In their exploratory intent, in the lofty ambition of their objectives, in their utter lack of intent to do harm, and in the brilliance of their design and performance, these robots speak eloquently for us.

But being much more advanced scientists and engineers than we—otherwise they would never be able to find and retrieve the small, silent spacecraft in interstellar space—perhaps the aliens would have no difficulty understanding what is encoded on these golden records. Perhaps they would recognize the tentativeness of our society, the mismatch between our technology and our wisdom. Have we destroyed ourselves since launching Voyager, they might wonder, or have we gone on to greater things?

Or perhaps the records will never be intercepted. Perhaps no one in five billion years will ever come upon them. Five billion years is a long time. In five billion years, all humans will have become extinct or evolved into other beings, none of our artifacts will have survived on Earth, the continents will have become unrecognizably altered or destroyed, and the evolution of the Sun will have burned the Earth to a crisp or reduced it to a whirl of atoms.

Far from home, untouched by these remote events, the Voyagers, bearing the memories of a world that is no more, will fly on.

*It takes so long to circuit the Sun because its orbit is so vast, 23 billion miles around, and because the force of the Sun’s gravity—which keeps it from flying out into interstellar space—is at that distance comparatively feeble, less than a thousandth what it is in the Earth’s vicinity.

*Robert Goddard, the inventor of the modern liquid-fueled rocket, envisioned a time when expeditions to the stars would be outfitted on and launched from Triton. This was in a 1927 afterthought to a 1918 handwritten manuscript called “The Last Migration.” Considered much too daring for publication, it was deposited in a friend’s safe. The cover page bears a warning: “The[se] notes should be read thoroughly only by an optimist.”

*The Earth, by definition, is 1 AU from its star, the Sun.

*Radio signals that both Voyagers detected in 1992 are thought to arise from the collision of powerful gusts of solar wind with the thin gas that lies between the stars. From the immense power of the signal (over 10 trillion watts), the distance to the heliopause can be estimated: about 100 times the Earth’s distance from the Sun. At the speed it’s leaving the Solar System, Voyager 1 might pierce the heliopause and enter interstellar space around the year 2010. If its radioactive power source is still working, news of the crossing will be radioed back to the stay-at-homes on Earth. The energy released by the collision of this shock wave with the heliopause makes it the most powerful source of radio emission in the Solar System. It makes you wonder whether even stronger shocks in other planetary systems might be detectable by our radio telescopes.