In 1976, two Viking landers were unsuccessful in finding evidence of life on the surface of Mars.  Mars appeared to be a dead planet.  In 2004, the discovery that liquid water once existed on the surface of Mars by the Mars Exploration Rovers, and the discovery of methane in the Martian atmosphere by the European Mars Express along with Earth-based telescopes, have renewed the vigor in the search for evidence of extinct or extant Martian life.