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nearby (or sauntering by) which were top-heavy, and no detectable organic molecules. Of three experiments in microbial metabolism, two in both landing sites repeatedly gave what seemed to be positive results. The implications are still under vigorous debate. In addition, we must remember that the two Viking landers examined closely, even with photography, less than one millionth of the surface area of the planet. More observations—particularly with more sophisticated instrumentation (including microscopes) and with roving vehicles—are needed. But despite the ambiguous nature of the Viking results, these missions represent the first time in the history of the human species that another world has been seriously examined for life.

In the following decades it is likely that there will be buoyant probes into the atmospheres of Venus, Jupiter and Saturn, and landers on Titan, as well as more detailed studies of the surface of Mars. A new age of planetary exploration and exobiology dawned in the seventh decade of the twentieth century. We live in a time of adventure and high intellectual excitement; but also—as the step from Leeuwenhoek to Pasteur shows—in the midst of an endeavor which promises great practical benefits.

CHAPTER 13

TITAN,

THE ENIGMATIC MOON

OF SATURN

On Titan, warmed by a hydrogen blanket,

ice-ribbed volcanoes jet ammonia

dredged out of a glacial heart. Liquid

and frozen assets uphold an empire

bigger than Mercury, and even a little

like primitive Earth: asphalt plains and hot

mineral ponds. But

how I’d like to take the waters of Titan, under

that fume-ridden sky,

where the land’s blurred by cherry mist

and high above, like floating wombs,

clouds

tower and swarm, raining down primeval

bisque, while life waits in the wings.

DIANE ACKERMAN,

The Planets (New York, Morrow, 1976)

TITAN IS NOT a household word, or world. We do not usually think of it when we run through a list of familiar objects in the solar system. But in the last few years this satellite of Saturn has emerged as a place of extraordinary interest and prime significance for future exploration. Our most recent studies of Titan have revealed that it has an atmosphere more like the Earth’s—at least in terms of density—than any other object in the solar system. This fact alone gives it new significance as the exploration of other worlds begins in earnest.

Besides being the largest satellite of Saturn, Titan is also, according to recent work by Joseph Veverka, James Elliot and others at Cornell University, the largest satellite in the solar system—about 5,800 kilometers (3,600 miles) in diameter. Titan is larger than Mercury and nearly as large as Mars. And yet there it is in orbit around Saturn.

We might obtain some clues about the nature of Titan by examining the two major worlds in the outer solar system—Jupiter and Saturn. Both have a general reddish or brownish coloration. That is, the upper layer of clouds that we see from the Earth has this hue primarily. Something in the atmosphere and clouds of these planets is strongly absorbing blue and ultraviolet light, so that the light that is reflected back to us is primarily red. The outer solar system, in fact, has a number of objects that are remarkably red. Although we have no color photographs of Titan because it is 800 million miles away and has an angular size smaller than the Galilean satellites of Jupiter, photoelectric studies reveal that it is, in fact, very red. Astronomers who thought about the problem once believed that Titan was red for the same reason that Mars is red: a rusty surface. But then the reason for Titan’s red color would be different from the reason for Jupiter’s and Saturn’s, because we do not see to a solid surface on those planets.

In 1944 Gerard Kuiper detected spectroscopically an atmosphere of methane around Titan—the first satellite found to have an atmosphere. Since then, the methane observations have been confirmed, and at least moderately suggestive evidence for the presence of molecular hydrogen has been provided by