Furthermore, an atmosphere is necessary for exchange of materials so that the basic molecules for biology are not all used up. On Earth, for example, green plants give off oxygen-a waste product-into the atmosphere. Many respiring animals, like human beings, breathe the oxygen and give off carbon dioxide, which the plants in turn imbibe. Without this clever (and painfully evolved) equilibrium between plants and animals, we would rapidly run out of oxygen or carbon dioxide. For these two reasons-radiation protection and molecular exchange-an atmosphere seems required for life.
Some of the worlds in our solar system have exceedingly thin atmospheres. Our Moon, for example, has at its surface less than one million millionth the atmospheric pressure on Earth. Six places on the near side of the Moon were examined by Apollo astronauts. No top-heavy structures, no lumbering beasts were found. Nearly four hundred kilograms of samples have been returned from the Moon and meticulously examined in terrestrial laboratories. There were no animalcules, no microbes, almost no organic chemicals, or even any water. We expected the Moon to be lifeless, and apparently it is. Mercury, the closest planet to the Sun, resembles the Moon. Its atmosphere is exceedingly thin, and it ought not to support life. In the outer solar system there are many large satellites the size of Mercury or our own Moon, composed of some mix of rock (like the Moon and Mercury) and ices. Io, the second moon of Jupiter, falls into this category. Its surface seems to be covered with a kind of reddish salt deposit. We are very ignorant about it. But because of its very low atmospheric pressure, we do not expect life on it.
Then there are planets with moderate atmospheres. Earth is the most familiar example. Here life has played a major role in determining the composition of our atmosphere. The oxygen is, of course, produced by green-plant photosynthesis, but even the nitrogen is thought to be made by bacteria. Oxygen and nitrogen together comprise 99 percent of our atmosphere, which has evidently been reworked on a massive scale by the life on our planet.
The total pressure on Mars is about one half of one percent that on Earth, but the atmosphere there is composed largely of carbon dioxide. There are small quantities of oxygen, water vapor, nitrogen and other gases. The Martian atmosphere has not obviously been reworked by biology, but we do not know Mars well enough to exclude life there. It has congenial temperatures at some times and places, a dense enough atmosphere, and abundant water locked away in the ground and polar caps. Even some varieties of terrestrial microorganisms can survive there very well. Mariner 9 and Viking found hundreds of dry riverbeds, apparently Indicating a time in the recent geological history of the planet when abundant liquid water flowed. It is a world awaiting exploration.
A third and less familiar example of places with moderate atmospheres is Titan, the largest moon of Saturn. Titan appears to have an atmosphere with a density between that of Mars and Earth. This atmosphere is, however, composed largely of hydrogen and methane, and is surmounted by an unbroken layer of reddish clouds-probably complex organic molecules. Because of its remoteness, Titan has attracted the interest of exobiologists only recently, but it holds the promise of a long-term fascination.
The planets with very dense atmospheres present a special problem. Like Earth, their atmospheres are cold at the top and warmer at the bottom. But when the atmosphere is very thick, the temperatures at the bottom become too hot for biology. In the case of Venus, the surface temperatures are about 480°C; for the Jovian planets, many thousands of degrees Centigrade. All these atmospheres, we think, are convective, with vertical winds vigorously carrying materials both up and down. Life probably cannot be imagined on their surfaces because of the high temperatures. The cloud environments are perfectly clement, but convection will carry hypothetical cloud organisms down to the depth and fry them there. There are two obvious solutions. There might be small organisms that reproduce as fast as they are carried down to the planetary skillet or the organisms might be buoyant. Fish on Earth have float bladders for a similar purpose, and both on Venus and on the Jovian planets, organisms that are essentially hydrogen-filled balloons can be envisioned. For them to float at modest temperatures on Venus, they need to be at least a few centimeters across, but for the same purpose on Jupiter, they must be at least meters across-the size of ping-pong balls and meteorological balloons, respectively. We do not know that such beasts exist, but it is of some little interest to see that they can be envisioned without doing violence to what is known of physics, chemistry or biology.
Our profound ignorance of whether other planets harbor life may end within this century. Plans are now afoot for the chemical and biological examination of many of these candidate worlds. The first step was the American Viking missions, which landed two sophisticated automatic laboratories on Mars in the summer of 1976, almost three hundred years to the month of Leeuwenhoek’s discovery of hay infusoria. Viking found no curious structures 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.
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,
tower and swarm, raining down primeval
bisque, while life waits in the wings.
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.