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Illustration of the atmosphere of Titan, Saturn's largest moon. Titan's atmosphere is primarily composed of nitrogen (about 95 percent) and methane (about 5 percent). Other carbon-rich compounds are also present in smaller amounts. When exposed to the Sun's ultraviolet light and high-energy particles in Saturn's magnetic field, methane and nitrogen molecules in Titan's upper atmosphere are split apart. The resulting fragments recombine to form various organic chemicals that often include nitrogen, oxygen, and other elements that are vital for life on Earth. (KEYSTONE/SCIENCE PHOTO LIBRARY/DETLEV VAN RAVENSWAAY)