Extraterrestrial Civilizations - Isaac Asimov [37]
Since the outer giants are so low in density, their mass (the quantity of matter they contain, roughly speaking) is lower than one might think from their size. The most massive is Jupiter, with 318 times the Earth’s mass; and the least massive is Uranus, with 14.5 times the Earth’s mass.
From such considerations alone, it is clear that the properties and nature of the outer giants is enormously different from Earth’s. Is life conceivable on them?
On March 2, 1972, a probe, Pioneer 10, was launched for a rendezvous with Jupiter. On December 3, 1973, it passed Jupiter at a distance of only 135,000 kilometers (85,000 miles) from its surface.
During the four days it took Pioneer 10 to fly by Jupiter, its instruments picked up radiation, counted particles, measured magnetic fields, noted temperatures, and analyzed sunlight passing through Jupiter’s atmosphere.
After Pioneer 10 had triumphantly passed Jupiter, a second probe, Pioneer 11, a close duplicate of the first one, was approaching the planet. It had left Earth on April 5, 1973, and passed Jupiter at a distance of 42,000 kilometers (26,000 miles) from its surface on December 2, 1974. It passed over Jupiter’s north polar region, which human beings cannot see from Earth.
Both probes sent back photographs and other useful information. From that information, astronomers feel that rock and metal make up a very small quantity of Jupiter’s total structure. Apparently, Jupiter would seem to consist chiefly of hydrogen, with a small admixture of helium, and traces (in comparison) of the other volatiles. Just as Earth is essentially a spinning ball of rock and metal, so Jupiter is a spinning ball of hot liquid hydrogen. (Ordinarily, liquid hydrogen boils at extremely low temperatures, but under the enormous pressures within Jupiter it apparently reaches far higher temperatures.)
The outermost skin of Jupiter’s ball of liquid is cold, but the temperature rises rapidly with depth. At 950 kilometers (600 miles) below the visible cloud surface, the temperature is already 3,600° C (6,500° F).
In the uppermost cool layer of the planet there is water, ammonia, methane, and other volatiles, including small percentages of hydrocarbons with two or three carbon atoms in the molecule.
Naturally, there is probably circulation in the planetary liquid of Jupiter as there is in Earth’s oceans. There may be vast columns of the Jupiter-liquid sinking and warming, while other columns, equally vast, are rising and cooling.
Here the arguments for life are intriguing. Water is certainly present in the fluid, and while it may be present in small percentages, on vast Jupiter even a small percentage is a large quantity in absolute terms. Even though the water is completely overwhelmed by the hydrogen, there could easily be more water by far on Jupiter than on Earth.
Then, too, there is methane and ammonia in addition to water, and the three could combine to form the kind of organic molecules we associate with life. It would take energy to force the combination, but considering Jupiter’s enormous internal heat, that would be no problem.
We could easily imagine living cells, and perhaps complicated multicellular animals, living in the Jovian ocean, maintaining themselves at a level of comfortable temperature, swimming up in a descending column or down in an ascending column, or perhaps switching from one to another when necessary.
It doesn’t seem hard to believe, really, and it would even be life-as-we-know-it; though, of course, we couldn’t really be certain until we could figure out some way of actually exploring the Jupiter-ocean.
Although we have not yet explored any of the other outer giants as we have Jupiter (though several probes are en route to Saturn after having passed Jupiter), there seems no reason to doubt that