Beginning in the late 1960s with the Mariner 4, 6, and 7 fly-bys, spacecraft began imaging Mars from closer perspectives.
Mariner 9 (1971-72) was the planet's first orbiter, and it successfully imaged the entire globe at 2 to 3 km/pixel resolution. These early spacecraft revealed the planet's Moon-like cratered highlands, immense volcanoes, huge tectonic rifts, and, strikingly, evidence for flowing water in the planet's ancient past, based on identification of branching valley network systems and broad outflow channels.
The next major phase of spacecraft exploration in the late 1970s and early 1980s involved the two Viking Orbiters and two Viking Landers. The orbiters imaged the surface at tenfold to hundredfold increased detail, and the landers performed the first biological experiments in search of Martian life (*, Klein and others, 19921).
The most recent phase of Mars exploration, since the late 1990s, has led to significant increase in the types, spatial resolution, and amount of data returned both from Mars orbit and the surface (*, Barlow, 2008; Bell, 2008). Post-Viking orbiters include Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter.
These have delivered high-resolution visible, thermal, and multispectral imaging; laser altimetry; radar, gravity, and magnetic sounding; and other measurements of the Martian surface, atmosphere, and crust. Rovers and stationary landers, including Mars Pathfinder, Spirit and Opportunity Mars Exploration Rovers, Mars Phoenix, and the Curiosity Mars Science Laboratory, have performed increas- ingly sophisticated surface investigations.
Pamphlet and map: Geologic Map of Mars, USGS