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

Chapter 10. SACRED BLACK

Deep sky is, of all visual impressions, the nearest akin to a feeling.


The blue of a cloudless May morning, or the reds and oranges of a sunset at sea, have roused humans to wonder, to poetry, and to science. No matter where on Earth we live, no matter what our language, customs, or politics, we share a sky in common. Most of us expect that azure blue and would, for good reason, be stunned to wake up one sunrise to find a cloudless sky that was black or yellow or green. (Inhabitants of Los Angeles and Mexico City have grown accustomed to brown skies, and those of London and Seattle to gray ones—but even they still consider blue the planetary norm.)

And yet there are worlds with black or yellow skies, and maybe even green. The color of the sky characterizes the world. Plop me down on any planet in the Solar System; without sensing the gravity, without glimpsing the ground, let me take a quick look at the Sun and sky, and I can, I think, pretty well tell you where I am. That familiar shade of blue, interrupted here and there by fleecy white clouds, is a signature of our world. The French have an expression, sacre-bleu!, which translates roughly as “Good heavens!”* Literally, it means “sacred blue!” Indeed. If there ever is a true flag of Earth, this should be its color.

Birds fly through it, clouds are suspended in it, humans admire and routinely traverse it, light from the Sun and stars flutters through it. But what is it? What is it made of? Where does it end? How much of it is there? Where does all that blue come from? If it’s a commonplace for all humans, if it typifies our world, surely we should know something about it. What is the sky?

In August 1957, for the first time, a human being rose above the blue and looked around—when David Simons, a retired Air Force officer and a physician, became the highest human in history. Alone, he piloted a balloon to an altitude of over 100,000 feet (30 kilometers) and through his thick windows glimpsed a different sky. Now a professor at the University of California Medical School in Irvine, Dr. Simons recalls it was a dark, deep purple overhead. He had reached the transition region where the blue of ground level is being overtaken by the perfect black of space.

Since Simons’ almost forgotten flight, people of many nations have flown above the atmosphere. It is now clear from repeated and direct human (and robotic) experience that in space the daytime sky is black. The Sun shines brightly on your ship. The Earth below you is brilliantly illuminated. But the sky above is black as night.

Here is the memorable description by Yuri Gagarin of what he saw on the first spaceflight of the human species, aboard Vostok 1, on April 12, 1961:

The sky is completely black; and against the background of this black sky the stars appear somewhat brighter and more distinct. The Earth has a very characteristic, very beautiful blue halo, which is seen well when you observe the horizon. There is a smooth color transition from tender blue, to blue, to dark blue and purple, and then to the completely black color of the sky. It is a very beautiful transition.

Clearly, the daylit sky—all that blue—is somehow connected with the air. But as you look across the breakfast table, your companion is not (usually) blue; the color of the sky must be a property not of a little air, but of a great deal. If you look closely at the Earth from space, you see it surrounded by a thin band of blue, as thick as the lower atmosphere; indeed, it is the lower atmosphere. At the top of that band you can make out the blue sky fading into the blackness of space. This is the transition zone that Simons was the first to enter and Gagarin the first to observe from above. In routine spaceflight, you start at the bottom of the blue, penetrate entirely through it a few minutes after liftoff, and then enter that boundless realm where a simple breath of air is impossible without elaborate life-support systems. Human life depends for its very existence on that blue sky. We are right to consider it tender and sacred.

We see the blue in daytime because sunlight is bouncing off the air around and above us. On a cloudless night, the sky is black because there is no sufficiently intense source of light to be reflected off the air. Somehow, the air preferentially bounces blue light down to us. How?

The visible light from the Sun comes in many colors—violet, blue, green, yellow, orange, red—corresponding to light of different wavelengths. (A wavelength is the distance from crest to crest as the wave travels through air or space.) Violet and blue light waves have the shortest wavelengths; orange and red the longest. What we perceive as color is how our eyes and brains read the wavelengths of light. (We might just as reasonably translate wavelengths of light into, say, heard tones rather than seen colors—but that’s not how our senses evolved.)

When all those rainbow colors of the spectrum are mixed together, as in sunlight, they seem almost white. These waves travel together in eight minutes across the intervening 93 million miles (150 million kilometers) of space from the Sun to the Earth. They strike the atmosphere, which is made mostly of nitrogen and oxygen molecules. Some waves are reflected by the air back into space. Some are bounced around before the light reaches the ground and they can be detected by a passing eyeball. (Also, some bounce off clouds or the ground back into space.) This bouncing around of light waves in the atmosphere is called “scattering.”

But not all waves are equally well scattered by the molecules of air. Wavelengths that are much longer than the size of the molecules are scattered less; they spill over the molecules, hardly influenced by their presence. Wavelengths that are closer to the size of the molecules are scattered more. And waves have trouble ignoring obstacles as big as they are. (You can see this in water waves scattered by the pilings of piers, or bathtub waves from a dripping faucet encountering a rubber duck.) The shorter wavelengths, those that we sense as violet and blue light, are more efficiently scattered than the longer wavelengths—those that we sense as orange and red light. When we look up on a cloudless day and admire the blue sky, we are witnessing the preferential scattering of the short waves in sunlight. This is called Rayleigh scattering, after the English physicist who offered the first coherent explanation for it. Cigarette smoke is blue for just the same reason: The particles that make it up are about as small as the wavelength of blue light.

So why is the sunset red? The red of the sunset is what’s left of sunlight after the air scatters the blue away. Since the atmosphere is a thin shell of gravitationally bound gas surrounding the solid Earth, sunlight must pass through a longer slant path of air at sunset (or sunrise) than at noon. Since the violet and blue waves are scattered even more during their now-longer path through the air than when the Sun is overhead, what we see when we look toward the Sun is the residue—the waves of sunlight that are hardly scattered away at all, especially the oranges and reds. A blue sky makes a red sunset. (The noontime Sun seems yellowish partly because it emits slightly more yellow light than other colors, and partly because, even with the Sun overhead, some blue light is scattered out of the sunbeams by the Earth’s atmosphere.)

It is sometimes said that scientists are unromantic, that their passion to figure out robs the world of beauty and mystery. But is it not stirring to understand how the world actually works—that white light is made of colors, that color is the way we perceive the wavelengths of light, that transparent air reflects light, that in so doing it discriminates among the waves, and that the sky is blue for the same reason that the sunset is red? It does no harm to the romance of the sunset to know a little bit about it.

Since most simple molecules are about the same size (roughly a hundred millionth of a centimeter), the blue of the Earth’s sky doesn’t much depend on what the air is made of—as long as the air doesn’t absorbthe light. Oxygen and nitrogen molecules don’t absorb visible light; they only bounce it away in some other direction. Other molecules, though, can gobble up the light. Oxides of nitrogen—produced in automotive engines and in the fires of industry—are a source of the murky brown coloration of smog. Oxides of nitrogen (made from oxygen and nitrogen) do absorb light. Absorption, as well as scattering, can color a sky.

OTHER WORLDS, OTHER SKIES: Mercury, the Earth’s Moon, and most satellites of the other planets are small worlds; because of their feeble gravities, they are unable to retain their atmospheres—which instead trickle off into space. The near-vacuum of space then reaches the ground. Sunlight strikes their surfaces unimpeded, neither scattered nor absorbed along the way. The skies of these worlds are black, even at noon. This has been witnessed firsthand so far by only 12 humans, the lunar landing crews of Apollos 11, 12, and 14–17.

A full list of the satellites in the Solar System, known as of this writing, is given in the accompanying table. (Nearly half of them were discovered by Voyager) All have black skies—except Titan of Saturn and perhaps Triton of Neptune, which are big enough to have atmospheres. And all asteroids as well.

Venus has about 90 times more air than Earth. It isn’t mainly oxygen and nitrogen as here—it’s carbon dioxide. But carbon dioxide doesn’t absorb visible light either. What would the sky look like from the surface of Venus if Venus had no clouds? With so much atmosphere in the way, not only are violet and blue waves scattered, but all the other colors as well—green, yellow, orange, red. The air is so thick, though, that hardly any blue light makes it to the ground; it’s scattered back to space by successive bounces higher up. Thus, the light that does reach the ground should be strongly reddened—like an Earth sunset all over the sky. Further, sulfur in the high clouds will stain the sky yellow. Pictures taken by the Soviet Venera landers confirm that the skies of Venus are a kind of yellow-orange.


Mars is a different story. It is a smaller world than Earth, with a much thinner atmosphere. The pressure at the surface of Mars is, in fact, about the same as the altitude in the Earth’s stratosphere to which Simons rose. So we might expect the Martian sky to be black or purple-black. The first color picture from the surface of Mars was obtained in July 1976 by the American Viking 1 lander—the first spacecraft to touch down successfully on the surface of the Red Planet. The digital data were dutifully radioed from Mars back to Earth, and the color picture assembled by computer. To the surprise of all the scientists and nobody else, that first image, released to the press, showed the Martian sky to be a comfortable, homey blue—impossible for a planet with so insubstantial an atmosphere. Something had gone wrong.

The picture on your color television set is a mixture of three monochrome images, each in a different color of light—red, green, and blue. You can see this method of color compositing in video projection systems, which project separate beams of red, green, and blue light to generate a full-color picture (including yellows). To get the right color, your set needs to mix or balance these three monochrome images correctly. If you turn up the intensity of, say, blue, the picture will appear too blue. Any picture returned from space requires a similar color balance. Considerable discretion is sometimes left to the computer analysts in deciding this balance. The Viking analysts were not planetary astronomers, and with this first color picture from Mars they simply mixed the colors until it looked “right.” We are so conditioned by our experience on Earth that “right,” of course, means a blue sky. The color of the picture was soon corrected—using color calibration standards placed for this very purpose on board the spacecraft—and the resulting composite showed no blue sky at all; rather it was something between ochre and pink. Not blue, but hardly purple-black either.

This is the right color of the Martian sky. Much of the surface of Mars is desert—and red because the sands are rusty. There are occasional violent sandstorms that lift fine particles from the surface high into the atmosphere. It takes a long time for them to fall out, and before the sky has fully cleaned itself, there’s always another sandstorm. Global or near-global sandstorms occur almost every Martian year. Since rusty particles are always suspended in this sky, future generations of humans, born and living out their lives on Mars, will consider that salmon color to be as natural and familiar as we consider our homey blue. From a single glance at the daytime sky, they’ll probably be able to tell how long it’s been since the last big sandstorm.

The planets in the outer Solar System—Jupiter, Saturn, Uranus, and Neptune—are of a different sort. These are huge worlds with giant atmospheres made mainly of hydrogen and helium. Their solid surfaces are so deep inside that no sunlight penetrates there at all. Down there, the sky is black, with no prospect of a sunrise—not ever. The perpetual starless night is perhaps illuminated on occasion by a bolt of lightning. But higher in the atmosphere, where the sunlight reaches, a much more beautiful vista awaits.

On Jupiter, above a high-altitude haze layer composed of ammonia (rather than water) ice particles, the sky is almost black. Farther down, in the blue sky region, are multicolored clouds—in various shades of yellow-brown, and of unknown composition. (The candidate materials include sulfur, phosphorus, and complex organic molecules.) Even farther down, the sky will appear red-brown, except that the clouds there are of varying thicknesses, and where they are thin, you might see a patch of blue. Still deeper, we gradually return to perpetual night. Something similar is true on Saturn, but the colors there are much more muted.

Uranus and especially Neptune have an uncanny, austere blue color through which clouds—some of them a little whiter—are carried by high-speed winds. Sunlight reaches a comparatively clean atmosphere composed mainly of hydrogen and helium but also rich in methane. Long paths of methane absorb yellow and especially red light and let the green and blue filter through. A thin hydrocarbon haze removes a little blue. There may be a depth where the sky is greenish.

Conventional wisdom holds that the absorption by methane and the Rayleigh scattering of sunlight by the deep atmosphere together account for the blue colors on Uranus and Neptune. But analysis of Voyagerdata by Kevin Baines of JPL seems to show that these causes are insufficient. Apparently very deep—maybe in the vicinity of hypothesized clouds of hydrogen sulfide—there is an abundant blue substance. So far no one has been able to figure out what it might be. Blue materials are very rare in Nature. As always happens in science, the old mysteries are dispelled only to be replaced by new ones. Sooner or later we’ll find out the answer to this one, too.

ALL WORLDS WITH NONBLACK SKIES have atmospheres. If you’re standing on the surface and there’s an atmosphere thick enough to see, there’s probably a way to fly through it. We’re now sending our instruments to fly in the variously colored skies of other worlds. Someday we will go ourselves.

Parachutes have already been used in the atmospheres of Venus and Mars, and are planned for Jupiter and Titan. In 1985 two French-Soviet balloons sailed through the yellow skies of Venus. The Vega 1balloon, about 4 meters across, dangled an instrument package 13 meters below. The balloon inflated in the night hemisphere, floated about 54 kilometers above the surface, and transmitted data for almost two Earth days before its batteries failed. In that time it traveled 11,600 kilometers (nearly 7,000 miles) over the surface of Venus, far below. The Vega 2 balloon had an almost identical profile. The atmosphere of Venus has also been used for aerobraking—changing the Magellan spacecraft’s orbit by friction with the dense air; this is a key future technology for converting flyby spacecraft to Mars into orbiters and landers.

A Mars mission, scheduled to be launched in 1998, and led by Russia, includes an enormous French hot air balloon—looking something like a vast jellyfish, a Portuguese man-of-war. It’s designed to sink to the Martian surface every chilly twilight and rise high when heated by sunlight the next day. The winds are so fast that, if all goes well, it will be carried hundreds of kilometers each day, hopping and skipping over the north pole. In the early morning, when close to the ground, it will obtain very high resolution pictures and other data. The balloon has an instrumental guide-rope, essential for its stability, conceived and designed by a private membership organization based in Pasadena, California, The Planetary Society.

Since the surface pressure on Mars is approximately that at an altitude of 100,000 feet on Earth, we know we can fly airplanes there. The U-2, for example, or the SR-71 Blackbird routinely approaches such low pressures. Aircraft with even larger wingspans have been designed for Mars.

The dream of flight and the dream of space travel are twins, conceived by similar visionaries, dependent on allied technologies, and evolving more or less in tandem. As certain practical and economic limits to flight on Earth are reached, the possibility arises of flying through the multihued skies of other worlds.

IT IS NOW ALMOST POSSIBLE to assign color combinations, based on the colors of clouds and sky, to every planet in the Solar System—from the sulfur-stained skies of Venus and the rusty skies of Mars to the aquamarine of Uranus and the hypnotic and unearthly blue of Neptune. Sacre-jaune, sacre-rouge, sacre-vert. Perhaps they will one day adorn the flags of distant human outposts in the Solar System, in that time when the new frontiers are sweeping out from the Sun to the stars, and the explorers are surrounded by the endless black of space. Sacre-noir.

*Like “gosh-darned” and “geez,” this phrase was originally a euphemism for those who considered Sacre-Dieu!, “Sacred God!,” too strong an oath, the Second Commandment duly considered, to be uttered aloud.