Mars appears pale orange in the night sky. The Babylonians, Greeks, and Romans all named it after their gods of war. In reality, Mars is a small planet-only half the size of Earth-but there are similarities. Mars, like Earth, has a 24-hour day, polar caps, and an atmosphere. Not surprisingly, Mars has always been the most popular candidate as a site for possible extraterrestrial life. Many scientists believe that some form of life-or at least evidence of past life-may remain within the planet, but no life could survive on the surface. The atmosphere is too thin to block out deadly ultraviolet rays. Mars is also farther from the Sun than Earth, making it much colder.