Extraterrestrial Civilizations - Isaac Asimov [25]
By retaining the infrared radiation of the planet, the carbon dioxide in the atmosphere raises the temperature of the planet, as the glass’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 really was to see, or so it seemed; for later astronomers over the next 100 years saw generally what he had seen in the way of a pattern of light and dark areas.
By that time, though, Maxwell and Boltzmann had come out with their kinetic theory of gases, and it didn’t seem that a body with the mass and gravitational field of Mars ought to have large, open bodies of water. Even at Mars’s low temperature, water vapor must have found it too easy to escape, if the atmosphere were thinner than Earth’s. The suspicion grew, therefore, that Mars must be water poor. It had its ice caps, to be sure, and it might have its marshy and boggy regions—but open seas and oceans seemed unlikely.
What, then, were the dark areas?
They might be areas of vegetation, growing in the boggy regions, while the light areas were sandy desert. It was interesting that when it was summer in a particular hemisphere, and the ice cap shrank as it presumably melted, the darkened areas became more extensive as though the melting ice irrigated the soil and allowed vegetation to spread.
Many people began to take it for granted that Mars was the abode of life.
In the course of his observations of Mars in 1877, moreover, Schiaparelli noticed