To exist, life has three main requirements: chemical elements, a source of energy, and a medium that allows for chemical reactions to occur. The first two requirements are readily available throughout the universe. The third requirement causes more speculation -- what is an appropriate medium for the types of chemical reactions needed to create and sustain life?

On Earth, liquid water is the essential medium. But some scientists speculate that liquid water is not the only solvent that could sustain life. For example, perhaps liquid ammonia could fulfill the role of water. In our solar system, liquid ammonia is common, and it is similar to water as a solvent. However, it would be difficult for life to survive in an ammonia environment because the temperature range in which this chemical exists as a liquid -- -78°C to -33°C (-108°F to -27°F) -- is very narrow and very cold. At such low temperatures, chemical reactions happen incredibly slowly, which makes it unlikely for life to arise.

Nevertheless, The National Academy of Sciences organized a group of scientists to address the question of the limits of organic life of planetary systems. Rather than restrict the search for life to carbon and water-based organisms, the committee is evaluating the possibility of "weird life." When stretched beyond the rules of standard biology, the search for extraterrestrial life opens up. Thus, without the restriction of liquid water, even the jovian planets -- the four gas giants within Earth's solar system: Jupiter, Saturn, Uranus, and Neptune -- could potentially provide a habitable environment for unusual life.

One of the best candidates for unusual life is Saturn's moon, Titan. In January 2005, the Huygens probe landed on Titan and found strong evidence of methane rain and seas. Liquid hydrocarbons such as methane cannot dissolve DNA, the genetic material for all life on Earth, but they are good solvents that can support complex chemical reactions. If life were to survive in a methane environment, it would be an entirely unfamiliar form of life -- one without DNA. Waterless life is not impossible, but it has yet to be found. The challenge in searching for life that is completely different from what we know is that we have no idea what to look for!

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