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Question
What are the requirements that we know there must be for life to exist?
Understanding the requirements for life requires a general definition of life -- which is difficult, given that we have only one example of life to study. Thus my answer can be directed only at "life as we know it", which is life that is chemically similar to that on Earth. The requirements include access to the appropriate raw materials (compounds that include hydrogen, oxygen, carbon, nitrogen, sulfur, and phosphorus). There must be an accessible energy source, either chemical or solar. And there must be liquid water, a requirement that is often used to establish what is meant by a "habitable" planet. Based on our terrestrial experience, the allowable range in temperature is from about -20 to +113 C. Life has also adapted to a wide range in acidity and salinity. These are the conditions that life on Earth (mostly microbial) has evolved to tolerate, but we can't be sure that the same limits apply on other worlds.
May 5, 2003
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