Cosmos - Carl Sagan [23]
Nothing in such experiments is unique to the Earth. The initial gases, and the energy sources, are common throughout the Cosmos. Chemical reactions like those in our laboratory vessels may be responsible for the organic matter in interstellar space and the amino acids found in meteorites. Some similar chemistry must have occurred on a billion other worlds in the Milky Way Galaxy. The molecules of life fill the Cosmos.
But even if life on another planet has the same molecular chemistry as life here, there is no reason to expect it to resemble familiar organisms. Consider the enormous diversity of living things on Earth, all of which share the same planet and an identical molecular biology. Those other beasts and vegetables are probably radically different from any organism we know here. There may be some convergent evolution because there may be only one best solution to a certain environmental problem—something like two eyes, for example, for binocular vision at optical frequencies. But in general the random character of the evolutionary process should create extraterrestrial creatures very different from any that we know.
I cannot tell you what an extraterrestrial being would look like. I am terribly limited by the fact that I know only one kind of life, life on Earth. Some people—science fiction writers and artists, for instance—have speculated on what other beings might be like. I am skeptical about most of those extraterrestrial visions. They seem to me to rely too much on forms of life we already know. Any given organism is the way it is because of a long series of individually unlikely steps. I do not think life anywhere else would look very much like a reptile, or an insect or a human—even with such minor cosmetic adjustments as green skin, pointy ears and antennae. But if you pressed me, I could try to imagine something rather different:
On a giant gas planet like Jupiter, with an atmosphere rich in hydrogen, helium, methane, water and ammonia, there is no accessible solid surface, but rather a dense, cloudy atmosphere in which organic molecules may be falling from the skies like manna from heaven, like the products of our laboratory experiments. However, there is a characteristic impediment to life on such a planet: the atmosphere is turbulent, and down deep it is very hot. An organism must be careful that it is not carried down and fried.
To show that life is not out of the question in such a very different planet, my Cornell colleague E. E. Salpeter and I have made some calculations. Of course, we cannot know precisely what life would be like in such a place, but we wanted to see if, within the laws of physics and chemistry, a world of this sort could possibly be inhabited.
One way to make a living under these conditions is to reproduce before you are fried and hope that convection will carry some of your offspring to the higher and cooler layers of the atmosphere. Such organisms could be very little. We call them sinkers. But you could also be a floater, some vast hydrogen balloon pumping helium and heavier gases out of its interior and leaving only the lightest gas, hydrogen; or a hot-air balloon, staying buoyant by keeping your interior warm, using energy acquired from the food you eat. Like familiar terrestrial balloons, the deeper a floater is carried, the stronger is the buoyant force returning it to the higher, cooler, safer regions of the atmosphere. A floater might eat preformed organic molecules, or make its own from sunlight and air, somewhat as plants do on Earth. Up to a point, the bigger a floater is, the more efficient it will be. Salpeter and I imagined floaters kilometers across, enormously larger than the greatest whale that ever was, beings the size of cities.
The floaters may propel themselves through the planetary atmosphere with gusts of gas, like a ramjet or a rocket. We imagine them arranged in great lazy herds for as far as the eye can see, with patterns on their skin, an adaptive camouflage implying that