Broca's Brain - Carl Sagan [100]
A place may be too hot or too cold for life. If the temperatures are very high—say, several thousands of degrees Centigrade—then the molecules that make up the organism will fall to pieces. Thus it is customary to exclude the Sun as an abode of life. On the other hand, if the temperatures are too low, then the chemical reactions that drive the internal metabolism of the organism will proceed at too ponderous a pace. For this reason the frigid wastes of Pluto are customarily excluded as an abode of life. However, there may be chemical reactions which proceed at respectable rates at low temperatures but which are unexplored here on Earth, where chemists dislike working in laboratories at −230°C. We must be careful not to take too chauvinistic a view of the matter.
The giant outer planets of the solar system, Jupiter, Saturn, Uranus, and Neptune, are sometimes excluded from biological considerations because their temperatures are very low. But these temperatures are the temperatures of their upper clouds. Deeper down in the atmospheres of such planets, as in the atmosphere of the Earth, much more clement conditions are to be encountered. And they appear to be rich in organic molecules. By no means can they be excluded.
While we human beings enjoy oxygen, this is hardly a recommendation for it, since there are many organisms that are poisoned by it. If the thin protective ozone layer in our atmosphere, made by sunlight from oxygen, did not exist, we would rapidly be fried by ultraviolet light from the Sun. But on other worlds, ultraviolet sunshades or biological molecules impervious to near-ultraviolet radiation can readily be imagined. Such considerations merely underline our ignorance.
An important distinction among the other worlds of our solar system is the thickness of their atmospheres. In the total absence of an atmosphere it is very difficult to conceive of life. As on Earth, the biology on other planets must, we think, be driven by sunlight. On our planet, the plants eat the sunlight and the animals eat the plants. Were all the organisms on Earth forced (by some unimaginable catastrophe) into a subterranean existence, life would cease as soon as accumulated food stores were exhausted. The plants, the fundamental organisms on any planet, must see the Sun. But if a planet has no atmosphere, not only ultraviolet radiation but X-rays and gamma rays and charged particles from the solar wind will fall unimpeded on the planetary surface and frizzle the plants.
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