Mars

Until the fist flyby of Mars by Mariner 4 in 1965, it was speculated that there might be liquid water on the planet's surface. This was based on observations of periodic variations in light and dark patches, particularly in the polar latitudes, which looked like seas and continents, while dark striations were interpreted by some observers as irrigation channels for liquid water. These straight-line features were later proven not to exist and were instead explained as optical illusions. Still, of all the planets in our solar system other than Earth, Mars is the most likely to habor liquid water, and perhaps life.