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Most people can swim. Even I can, though someone once said unkindly that I give a spirited impression of a baby seal with training flippers. Most animals can also swim, at least passably. And we know that the sea is teeming with life, some of it very intelligent. Dolphins, sea lions, and porpoises are no fools, as we have learned. What, then, are the chances that, somewhere, advanced life forms live in a liquid environment?
By "advanced life forms" I mean beings who can not only communicate, as dolphins admittedly can, but who have a real language and are capable of teaching, building, and so on. Olaf Stapledon's magnificent novel Star Maker describes one such civilization. But, of course, that is only fiction; We have no proof that any intelligent race lives in a liquid environment. For that matter, we have no proof that life exists anywhere beyond the earth, and a few eminent scientists believe that it does not. I will return to that later. In the meantime, what about liquid planets?
Rather surprisingly, there is a liquid planet no farther away than Jupiter, the largest member of the sun's family. One scientist commented that the solar system is made up of the Sun, Jupiter, and assorted debris. This may be an extreme view, but Jupiter is certainly more massive than all the rest of the planets put together Its equatorial diameter is more than 140,800 kilometers, and its huge globe could contain a thousand earths.
The surface we see through our telescopes is gaseous. We can make out dark belts, where atmospheric gas descends toward the planet's surface; bright zones, where the gas is surging upward; and such specific features as the Great Red Spot's huge oval. The surface of Jupiter is always changing, and our view of it alters in minutes. Jupiter spins round quickly. At less than 10 hours, its day is the shortest in the solar system, though the Jovian year is 11.9 times as long as ours.
Even though Jupiter is remote—its mean distance from the Sun is 772.8 million kilometers—it shines brilliantly in our skies. We can never go near Jupiter ourselves because the planet is surrounded by zones of lethal radiation and by an immensely powerful magnetic field.
Yet we have begun to learn what Jupiter is like under its upper layer of gas. For many years, astronomers thought that it could be a miniature sun, lighting, and warming its moons. When this attractive idea was disproved in the 1920s, most authorities decided there would be a rocky core surrounded by a thick layer of ice, which would in turn be overlaid by a gaseous atmosphere.
Today, this has also been assigned to the astronomical scrap heap. We have found that Jupiter radiates more energy than it could if it depended entirely on the Sun. The inner temperature must be thousands of degrees. This does not make Jupiter a miniature star, but it surely rules out any thick layer of subsurface ice.
Life on the Liquid Planet
One thing we have known for more than 40 years is that Jupiter's upper gas is rich in hydrogen, together with such unprepossessing hydrogen compounds as ammonia and methane. Lower down, the situation must be different. According to one model, Jupiter may be made up chiefly of liquid hydrogen, with only a relatively small solid core. If true, this provides a liquid environment. Moreover, there must be a region below the cold upper gas and above the hot inner core where the temperature is much the same as it is in our own seas. It seems likely that most of the fundamental materials needed for life exist there, and this intriguingly suggests that living beings may be swimming happily about in the Jovian hydrogen oceans.
Obviously, any such beings would be fundamentally different from any of the life forms we know on Earth. Yet the conditions there may not be quite so uncompromisingly hostile to life as, for instance, the surface of airless Mercury or even that of the moon, the one world we can prove is sterile without any doubt.
It is easy to let our imaginations run riot. Would the Jovians have fins and flippers? Would they be able to talk? Their eyes would have to be good very little light could penetrate those murky depths. But they could know nothing about the great universe around them.
Skeptical but Hopeful
I admit to being something of a skeptic. I have very little faith that life occupies the Jovian deeps. But one never knows! Unfortunately, it will be very difficult to find out one way or the other. With manned visits impossible, the most we can do is to dispatch an unmanned "entry probe" that would plunge to its destruction in the upper gas, sending back information for as long as possible.
On the other hand, Jupiter has four large satellites and at least nine small ones, Of these, Callisto, the outermost of the large moons, seems to be the most promising target for would-be explorers. It is well over a million kilometers from Jupiter, and is therefore safely outside the radiation zones.
In the far future, a Callisto expedition may land. Its members will do all they can to learn whether anyone lives in Jupiter's seas. I think they will fail. I hope I am wrong.
Callisto is not the only Jovian moon that scientists suspect as possible of containing life. Many believe that Europa is the most likely place to find extraterrestrial life. Learn about the search for life on Jupiter's ice moon in Unmasking Europa: The Search for Life on Jupiter's Ocean Moon.
Unmasking Europa: The Search for Life on Jupiter's Ocean Moon by Richard Greenberg
Unmasking Europa: The Search for Life on Jupiter's Ocean Moon tells the engaging story of Europa, the oceanic moon. The volume includes images of the ocean moon's surface, focusing on the crack patterns, extensive rifts and ridges, and refrozen pools of exposed water filled with rafts of displaced ice. It also tells the inside story of a human enterprise in science that lead to the discovery of this vast new world, which could well harbor life.