Read Online Extraterrestrial Civilizations by Isaac Asimov - Free Book Online Page B

us technologically, but often as decadent and weary of life—in their old age as a species.
    On the other hand, many stories were written of a junglelike Venus, or one with a plantetary ocean—in either case filled with primitive life forms. In 1954, I myself published a novel,
Lucky Starr and the Oceans of Venus
, in which the planet was described as having a planetary ocean. But only two years later our thoughts about Venus were revolutionized.
    After World War II, astronomers gained a large number of new and extraordinarily useful tools for the exploration of the worlds ofthe Solar system. They could send out microwaves to the surfaces of distant planets, receive the reflections, and from the properties of those reflections deduce the nature of the surface even if they could not see them optically. They could receive radio waves sent out by the planets themselves. They could send out rocket-powered probes that could skim by the planet or even land on their surfaces and send back useful data (as in the case of the mapping of Mercury’s surface by
Mariner 10
).
    In 1956, the American astronomer Robert S. Richardson analyzed radar reflections from Venus’s surface beneath the cloud layer and found it was rotating, very slowly, in the wrong direction—clockwise.
    In that same year, a team of astronomers under Cornell H. Mayer received radio waves from Venus and were astonished to find that the intensity of those waves was equivalent to what would be expected from an object far hotter than Venus was thought to be. If this were so, there could be no planetary ocean on Venus; indeed no liquid water of any kind (and there went my poor novel when it was only two years old).
    On December 14, 1962, an American Venus probe,
Mariner 2
, passed close by Venus’s position in space, monitored its radio-wave emission, and confirmed the earlier report. On June 12, 1967, a Soviet Venus probe,
Venera 4
, entered Venus’s atmosphere and sent back confirming data while descending for an hour and a half.
Venera 5
and
6
, landing on Venus’s surface on May 16 and 17, 1969, put the matter beyond all doubt.
    Venus has an extraordinarily dense atmosphere, about 95 times as dense as Earth’s. Venus’s atmosphere, what’s more, is 95 percent carbon dioxide, the molecules of which have a mass of 44. (Carbon dioxide had been detected in Venus’s atmosphere by more ordinary methods as long before as 1932.)
    It is natural enough for a planet to have an atmosphere containing carbon dioxide. Our own atmosphere has a small quantity of carbon dioxide (0.03 percent) and that small quantity is essential to the growth of plant life.
    The photosynthesis of green plants uses the energy of the Sun to combine carbon dioxide molecules with water molecules to form the components of plant tissue—sugar, starch, cellulose, fats, proteins, andso on. In the process, though, free oxygen is formed in excess and is discharged into the atmosphere.
    It is generally thought, in fact, that at some time in the distant past, the Earth’s atmosphere was far richer in carbon dioxide than it is now, and that free oxygen was absent. (We’ll get back to this matter later in the book.) Earth’s early atmosphere, then, was somewhat like Venus’s present one, but less dense; and it is only the action of photosynthesis that gradually removed the carbon dioxide and replaced it with oxygen.
    From the fact that Venus’s atmosphere is so rich in carbon dioxide and so poor in oxygen (none has been detected), we can deduce at once that photosynthesis in its Earthly form is absent from the planet or, at the very least, has not been established for long.
    This would seem to indicate that there are no green plants of any consequence on the planet, and therefore no animal life (which depends ultimately on plants for food), and therefore no intelligence.
    It might be argued that photosynthesis is not essential to life and, indeed, it isn’t. On Earth there are forms of life that