AliensAre there other forms of life in the universe'？ The scientific search for extraterre
Aliens
Are there other forms of life in the universe'？ The scientific search for extraterrestrial (ET) life forms has been bolstered by two recent discoveries. First, the discovery of life forms in exotic environments on Earth indicates that life is very hearty and can adapt to the strangest and most hostile environments. Second, astronomers found planets orbiting stars besides our sun—over 50 extrasolar planets have been discovered as of 2001. Are there alien life forms on any of these planets？
If alien life does exist, what might it be like？ Would it be simple forms of life such as bacteria, viruses or algae, or more advanced, multi-cellular creatures, perhaps even intelligent beings？ Would aliens be animals, plants or have characteristics of both？ Would they have arms and legs and walk upright as we do？ Would they depend upon vision as their primary sense or use another way to gather information about their surroundings？ Would they "breathe" oxygen or some other gas？
Greetings, Carbon-based Bipeds(两足动物)!
Most of us picture alien life the way it's portrayed in movies, where aliens are commonly depicted as human-like forms because they use actors either to play the roles directly in make up or to be models for computer-generated animation. Also, audiences relate to human-like aliens better than to more exotic, monster like. However, the human body plan bilateral symmetry with one
head, two legs and two arms—stems from when early animals such as lizard, crocodile, or dinosaur colonized the Earth's land masses, and it seems unlikely that such a shape would evolve on an alien world. So, let's forget Hollywood for the moment and look closely at the real science of astrobiology.
Astrobiology is the scientific study of life in the universe. Astrobiologists seek to understand (among other things) how life arose and evolved on Earth, what governs the way life is organized and what makes a planet habitable.
Astrobiology combines the disciplines of biology, chemistry, physics, geology and astronomy. Often, astrobiologists must use the information learned about life on Earth as a guide for studying life elsewhere. Let's examine some of the unusual things that we have learned from life on Earth:
Life in the Extreme
Up until about 30 years ago, it was believed that all life on Earth was dependent upon energy from the sun. Furthermore, it was thought that you would probably not find life where temperatures were extremely hot, like in geysers or hot springs, or extremely cold,' like in the Antarctic desert.
These ideas changed when oceanographers explored hydrothermal vents, openings in the ocean floor where extremely hot, mineral-rich water erupts from the crust. Hydrothermal vents are located several miles below the surface, on the ocean floor, where the surrounding water is at or near freezing, it is absolutely dark and the pressure is high. In organized communities around the bases of these vents, called black smokers, scientists found clams, crabs and exotic, giant tubeworms measuring 6 feet (2 meters) long. The water coming out of these vents is 230 to 662 degrees Fahrenheit (110 to 350 degrees Celsius).
How can these animals survive so far from the sunlight, under these extreme conditions？ In the water, scientists found species of bacteria that split hydrogen sulfide(硫化物)from the water to get energy to make organic compounds. The tubeworms have bacteria in their tissues that help them derive energy from the water. The clams feed on the bacteria, and the crabs feed on the tubeworms.
The discovery of hydrothermal-vent communities showed that it is possible for life to evolve in places without light from the sun, and in other worlds without sufficient light from the parent star. In view of the discovery of hydrothermal vents, it may be possible that life exists on Europa, an icy moon of Ju
A．Y
B．N
C．NG
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