Free Extraterrestrial Civilizations by Isaac Asimov

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 the greenhouse effect because the glass of a greenhouse does the same thing.
    By retaining the infrared radiation of the planet, the carbon dioxide in the atmosphere raises the temperature of the planet, as theglass’s retention of infrared radiation raises the temperature inside a greenhouse. Because of the very high content of carbon dioxide in Venus’s atmosphere, the surface temperature of the planet is far higher than we would expect it to be from its distance from the Sun alone, especially since ordinarily we would expect its clouds to shield it from much of the Sun’s heat. Venus is the victim of a runaway greenhouse effect.
    The result is that Venus’s surface temperature is about 480° G (900° F), considerably higher than Mercury’s surface temperature. Mercury may be closer to the Sun, but it doesn’t have a heat-conserving atmosphere.
    The surface temperature of Venus is far above the boiling point of water and is, indeed, hot enough to melt lead easily. There can be no liquid water anywhere on the planet. What water it has must exist as vapor in the clouds, and there is evidence that the liquid droplets in the clouds are, to a considerable extent, the extremely corrosive substance sulfuric acid.
    It takes a vivid imagination indeed to conceive of life on such a planet, and Venus must be crossed off as a possible abode for extraterrestrial intelligence.
MARTIAN CANALS
    As for Mars, that from the beginning seemed to have a much better chance for life. Its rotation, its axial tip, its ice caps all seemed hopeful. Its presumed great age gave it, it would seem, a particularly good chance at advanced life.
    About 1830, astronomers began to make serious attempts to map Mars. The first map produced was by a German astronomer, Wilhelm Beer (1797–1850). Others followed, but success was not remarkable. It was hard to see details through two atmospheres, those of Earth and of Mars, from a distance of hundreds of millions of kilometers. Each astronomer who tried to map Mars seemed to end up with a map that was completely unlike the ones produced by his predecessors.
    All agreed, however, that there seemed to be light areas and dark areas, and the notion grew that the light areas represented land surface and the dark areas water surface.
    A particularly good chance for observation came in 1877 when Mars and Earth happened to be in those parts of their orbits that brought them as closely together as they ever got to be. And by then, of course, astronomers had better telescopes than they ever had before.
    One observer with an excellent telescope was the Italian astronomer Giovanni Virginio Schiaparelli (1835–1910). During his observations in 1877, he drew a map of Mars that, once again, looked altogether different from anything that had been drawn before. With his map, though, things settled down. Finally, he saw what there