The past thirty years have seen a revolution in our understanding of the geology of Mars. In fact, most of this revolution has occurred in just the second half of that period. From the mid 1990's onward, successful missions to Mars have included Mars Global Surveyor, Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, Mars Pathfinder, the Mars Exploration Rovers Spirit and Opportunity, and Phoenix. Among the most important discoveries of this period have been extensive deposits of layered sedimentary rocks, broadly distributed aqueous minerals, and evidence of dynamic modern surface processes. Layered sediments are widespread on Mars. Some show evidence of eolian origin, while others were deposited subaqueously; the latter include clear deltaic deposits. Minerals related to aqueous alteration and precipitation include phyllosilicates, sulfates, carbonates, silica, and others. Recent flow processes have produced steep, sinuous gullies, and recent impacts have exposed subsurface materials including ice. Concentrations of subsurface ice at shallow depths are extraordinarily high at high latitudes. Early Mars was clearly very different from the Mars of today, with widespread groundwater and surface water, aqueous alteration, and deposition of sedimentary materials. Together these conditions provided both environments that may have been suitable for life, and the potential for preservation of evidence of life in the martian geologic record.

The next thirty years hold the promise of substantial future discoveries. The Mars Science Laboratory will take robotic surface exploration to a new level of sophistication. After that, the next step in the exploration of the martian surface may be to collect a suite of carefully chosen samples and return them to Earth for analysis.