Physical features resembling shorelines, gorges, river beds and islands suggest that great rivers once marked the planet. Now, however, there is only a trace of water to be found.
The Martian atmosphere is primarily CO2. The air contains only about 1/1000 as much water as our air, but even this small amount can condense out forming clouds that rise high in the atmosphere. Local patches of early morning fog can form in the valleys.
The Viking Orbiters and Landers far exceeded their design lifetimes of 120 and 90 days. The first to fail was Viking Orbiter 2, which stopped operating after two years when a leak depleted its attitude-control gas. Viking Lander 2 operated for 3 1/2 years, when it finally shut down due to battery degeneration.
Viking Orbiter 1 quit after four years, and Lander 1 functioned for over seven years before closing down. In all, the orbiters and landers returned over 56,000 color and black and white photographs of the surface of Mars.
