Extraterrestrial Civilizations - Isaac Asimov [24]
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, and so 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 neither use photosynthesis nor depend on other forms of life that use photosynthesis. These forms of life are all at the bacterial level, however, and there is no indication that now or ever has any form of life beyond the bacterial existed on Earth without need, direct or indirect, of photosynthesis.
It might also be argued that Earth need not form a rule in this respect. Suppose a form of life got its energy from the Sun and made use of carbon dioxide, but somehow stored the oxygen instead of emitting it into the atmosphere. In due course of time, it made use of the oxygen for the purpose of combining it with carbon atoms and restoring carbon dioxide to the atmosphere. In that way, you could have photosynthesis while retaining a carbon dioxide atmosphere.
This is not beyond the bounds of possibility, but—
Carbon dioxide has the property of absorbing infrared radiation. It allows the high-energy visible light of the Sun to pass through and strike the surface of a planet, but then absorbs the low-energy (and invisible) infrared radiation the planet reemits to space at night. This is called