Searching for Life Elsewhere in the Solar System

In the not too distant past, it was quite popular of think of Venus as literally Earth's twin. People thought that since Venus's diameter, density, and gravity were only slightly less than Earth's, the cloud-enshrouded planet must be similar—especially since it had an obvious atmosphere and was a little nearer the Sun. Visions of a planet with oceans, tropical forests, giant reptiles, and possibly even primitive humans frequently appeared in science-fiction stories during the first half of the 20th century.

However, since the 1960s, visits by numerous American and Russian spacecraft dispelled all these pre-space age romantic fantasies. The data from these spacecraft missions clearly showed that the cloud-enshrouded planet was definitely not a prehistoric world that mirrored a younger Earth. Except for a few physical similarities of size and gravity, scientists now known that Earth and Venus are very different worlds. For example, the surface temperature on Venus approaches 932°F (500°C), its atmospheric pressure is more than 90 times that of Earth, it has no surface water, and its dense, hostile atmosphere with sulfuric acid clouds and an overabundance of carbon dioxide (about 96 percent) represents a runaway greenhouse of disastrous proportions.

Spacecraft missions throughout the solar system have resulted in a renaissance of interest and understanding about the formerly mysterious worlds (planets and moons) in this solar system. Celestial bodies once believed to have been life-bearing worlds have proven to be disappointingly barren, while other worlds now hold out the promise of sustaining perhaps primitive forms of extraterrestrial life. Besides Mars (see chapter 4), the leading candidate is the Jovian moon, Europa.

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