In 1971, Mariner 9 became the first spacecraft to orbit Mars. It found the planet shrouded in a global dust storm that obstructed the view of all but the tallest surface features. As the dust cleared, Mariner 9 mapped volcanoes, canyons, and polar caps.
From 1976 to 1980, the Viking Orbiters gave us our first global view of Mars. They revealed a diversity of landforms related to impact, volcanic, and other geological processes, and strong evidence that water carved many large features in the distant past.
Mars Global Surveyor
Mars Global Surveyor studied Mars from 1997 to 2006. The long duration of its mission allowed it to observe ongoing surface processes. Its Mars Orbiter Camera (MOC) provided both high-resolution images and a long-term record of lower-resolution global views. The spacecraft also produced a planet-wide topographic map and provided evidence of an early magnetic field.
Mars Odyssey began mapping Mars in 2002 and has provided data on its surface composition and radiation environment. It discovered large amounts of near-surface water ice around the poles, determined the radiation hazards for future human explorers, and has served as a communication relay for the two Mars Exploration Rovers.
Europe's Mars Express orbiter arrived at the planet in 2003. It carries seven instruments that map surface terrain and composition and study the subsurface and atmosphere.
Mars Reconnaissance Orbiter
Since 2006, the Mars Reconnaissance Orbiter has studied the Martian surface, subsurface, and atmosphere. Its high-resolution camera (HiRISE) images the surface with greater detail than has ever been seen before from orbit. The Shallow Subsurface Radar (SHARAD) reveals geologic layering as deep as 2 kilometers (1.2 miles) underground. Other instruments study mineralogy, global weather, and atmospheric properties.