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 of 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.
SIXTY-TWO WORLDS FOR THE THIRD MILLENNIUM: KNOWN MOONS OF THE PLANETS (AND ONE ASTEROID)—
LISTED IN ORDER OF DISTANCE FROM THEIR PLANET
EARTH, 1
MARS, 2
IDA, 1
JUPITER, 16
SATURN, 18
URANUS, 15
NEPTUNE, 8
PLUT0, 1
Moon
Phobos
Dactyl
Metis
Pan
Cordelia
Naiad
Charon
Deimos
Adrastea
Atlas
Ophelia
Thalassa
Amalthea
Prometheus
Bianca
Despina
Thebe
Pandora
Cressida
Galatea
to
Epimetheus
Desdemona
Larissa
Europa
Janus
Juliet
Proteus
Ganymede
Mimas
Portia
Triton
Callisto
Enceladus
Rosalind
Nereid
Leda
Tethys
Belinda
Himalia
Telesto
Puck
Lysithea
Calypso
Miranda
Elara
Diane
Ariel
Ananke
Helene
Umbriel
Carme
Rhea
Titania
Pasiphae
Titan
Oberon
Sinope
Hyperion
Iapetus
Phoebe
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 hiking 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 rusk. 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 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 Voyager data 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 9 balloon, 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-jaunt, 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.
CHAPTER 11 EVENING AND MORNING STAR
This is another world which is not of men.
—LI BAI, "QUESTION AND ANSWER IN THE MOUNTAINS" (CHINA, TANG DYNASTY, CA. 730)
You can see it shining brilliantly in the twilight, chasing the Sun down below the western horizon. Upon first glimpsing it each night, people were accustomed to make a wish ("upon a star"). Sometimes the wish came true.
Or you can spy it in the east before dawn, fleeing the rising Sun. In these two incarnations, brighter than anything else in the sky except only the Sun and the Moon it was known as the evening and the morning star. Our ancestors did not recognize it was a world, the same world, never too far from the Sun because it is in an orbit about it interior to the Earth's. Just before sunset or just after sunrise, we can sometimes see it near some fluffy white cloud, and then discover by the comparison that Venus has a color, a pale lemon-yellow.
You peer through the eyepiece of a telescope—even a big telescope, even the largest optical telescope on Earth—and you can make out no detail at all. Over the months, you see a featureless disk methodically going through phases, like the Moon: crescent Venus, full Venus, gibbous Venus, new Venus. There is not a hint of continents or oceans.
Some of the first astronomers to see Venus through the telescope immediately recognized that they were examining a world enshrouded by clouds. The clouds, we now know, are droplets of concentrated sulfuric acid, stained yellow by a little elemental sulfur. They lie high above the ground. In ordinary visible light there's no hint of what this planet's surface, some 50 kilometers below the cloud tops, is like, and for centuries the best we had were wild guesses.
You might conjecture that if we could take a much finer look there might be breaks in the clouds, revealing day by day, in bits and pieces, the mysterious surface ordinarily hidden from our view. Then the time of guesses would be over. The Earth is on average half cloud-covered. In the early days of Venus exploration, we saw no reason that Venus should be 100 percent overcast. If instead it was only 90 percent, or even 99 percent, cloud-covered, the transient patches of clearing might tell us much.
In 1960 and 1961, Mariners 1 and 2, the first American spacecraft designed to visit Venus, were being prepared. There were those, like me, who thought the ships should carry video cameras so they could radio pictures back to Earth. The same technology would be used a few years later when Rangers 7, 8, and 9 would photograph the Moon on the way to their crash landings—the last making a bull's-eye in the crater Alphonsus. But time was short for the Venus mission, and cameras were heavy. There were those who maintained that cameras weren't really scientific instruments, but rather catch-as-catch-can, razzle-dazzle, pandering to the public, and unable to answer a single straightforward, well-posed scientific question. I thought myself that whether there are breaks in the clouds was one such question. I argued that cameras could also answer questions that we were too dumb even to pose. I argued that pictures were the only way to show the public—who were, after all, footing the bill—the excitement of robotic missions. At any rate, no camera was flown, and subsequent missions have, for this particular world, at least partly vindicated that judgment: Even at high resolution from close flybys, in visible light it turns out there are no breaks in the clouds of Venus, any more than in the clouds of Titan.* These worlds are permanently overcast.
* For Titan, imaging revealed a succession of detached hazes above the main layer of aerosols. So Venus works out to be the only world in the Solar System for which spacecraft cameras working in ordinary visible light haven't discovered something important. Happily, we've now returned pictures from almost every world we've visited. (NASA's International Cometary Explorer, which raced through the tail of Comet Giacobini-Zimmer in 1985, flew blind, be devoted to charged particles and magnetic fields.)
