NASA's twin Mars Exploration Rovers, Spirit and Opportunity, launched toward Mars on June 10 and July 7, 2003, in search of answers about the history of water on Mars and the geology of two distinct landing sites.
The rover Opportunity landed on Meridiani Planum on January 25, 2004, and is still in operation more than 4300 sols later! Meridiani Planum is a vast plain located at the equator, close to the boundary between the ancient Southern Highlands of Mars and the younger Northern Lowlands. Meridiani Planum was chosen as a landing site because observations from orbit by the Mars Global Surveyor spacecraft (mission: 1996-2007) showed the surface was rich in the mineral hematite (Fe2O3), which on Earth often has its origin in aqueous processes. Prior to Opportunity's landing, two hypotheses were put forth for the origin of the layered sequence in Meidiani Planum; one that they are lake deposits and the other that they are volcanic airfall deposits. Thus, there was the possibility that water-laid sediments could be studied by Opportunity.
Now in her thirteenth year of operation, Opportunity has traveled over the plains, explored the interiors of several small craters, and is now studying the rocks on the rim of the ancient, 22 km diameter Endeavour crater. She has traveled over 42.6 km setting a new record for roving on a planetary surface. For most of the mission, Opportunity studied the sulfate-rich aeolian sandstones, named the Burns formation, that make up the upper layers of Meridiani Planum. While most of the sandstones have textures indicated they were deposited by wind action, some show evidence for formation in a shallow, lagoon-like environment. The rocks exposed on the rim of Endeavour crater are impact breccias thrown out by the Endeavour impact and are older than the Burns fm. sandstones. These impact breccias contain evidence for aqueous-based alteration and environments that pre-date for deposition of the Burns formation sandstones.
The rover Spirit, a.k.a. MER-A, landed in Gusev crater on January 4, 2004. Gusev crater is an ancient crater that is about 160 km across. It is located just south of the boundary between the ancient Southern Highlands and the younger Northern Lowlands of Mars. The landing site was chosen because the southern rim of Gusev crater is breached by one of the largest branching valley networks on Mars, Ma'adim Vallis. Gusev is thought to have contained a large lake in ancient times that acted as a settling pool for sediment carried by the water flowing down Ma'adim Vallis. The great length and depth of Ma'adim Vallis indicate a large quantity of sediment likely was transported, and the shallow depth and flat floor of Gusev crater indicated the potential for a thick sedimentary sequence to be present. Ground truth measured by Spirit's instruments demonstrated that the plains on the floor of Gusev are volcanic - the rocks initially studied by Spirit are basalt. Spirit's mobility allowed her to be commanded to drive to and climb the Columbia Hills, a series of low hills rising above the volcanic plains and preserving an older rock record. Observations made in the Columbia Hills region did uncover evidence for aqueous processes in these ancient rocks, mostly acidic and sulfate-rich, but evidence for alkaline, neutral pH aqueous solutions were also uncovered. The last data returned by Spirit on March 24, 2010 were from sol (Mars day) 2209. She had long since blasted past the mission success criteria of 90 sols of operations. In her journey, Spirit traveled 7,730 m, again well in excess of the mission success criteria of 600 m.