Mars (planet), planet in the solar system, named for the Roman god of war. It is the fourth planet from the sun and the third in order of increasing mass. Mars has two small, heavily cratered moons, Phobos and Deimos, which some astronomers consider asteroidlike objects captured by the planet very early in its history. Phobos is about 21 km (about 13 mi) across; Deimos, only about 12 km (about 7.5 mi). Appearance from Earth When viewed without a …showed first 75 words of 1946 total…

…showed last 75 words of 1946 total…Viking landers provide no evidence for the existence of life. A more difficult question is whether life ever existed on Mars, given the strong evidence of climatic change and the indications of a previously warmer, thicker atmosphere. Answering this question will probably involve collecting carefully selected subsurface samples and returning them to earth for detailed analysis. U.S. National Aeronautics and Space Administration has proposed a manned voyage to Mars early in the 21st century.