Sent to Mars to conduct a 90-day investigation, both of the Mars Exploration Rover vehicles far outlasted engineers' expectations. They launched in 2003 and both arrived at Mars in January 2004. The Spirit rover operated for five years, finally expiring during a cold Martian winter. The Opportunity rover is still making discoveries in 2014.
The twin Mars rovers are field geology robots, remote-controlled from Earth and capable of examining the rocks microscopically to determine their composition. At the start of each Martian day, called a "sol," controllers on Earth beam instructions to the rover about where to go and what to do. The rovers are capable of moving up to 328 feet (100 meters) per sol.
Scientists chose the rovers' two landing sites because of their expectation these locations would yield signs of past water on Mars. Spirit was chosen to land in the giant Gusev crater because it looks as though it once held a lake of liquid water. Opportunity landed at Meridiani Planum, a plain of iron oxide (hematite). [10 Amazing Mars Discoveries by Spirit & Opportunity]
In its five years of operation, Spirit covered 4.8 miles (8 kilometers) and returned 128,000 photos. In 10 years, Opportunity has covered more than 24 miles (39 kilometers) and returned 187,000 photos.
The Mars Exploration Rovers Spirit and Opportunity have made observations that led scientists to conclude that billions of years ago, the surface of Mars had abundant water and conditions conducive to life.
Soon after landing, Opportunity found round nodules of the mineral hematite, which usually forms in water.
Spirit found rocks 10 times richer in magnesium and iron carbonates than any rocks found before on Mars. The rocks formed when the planet was warmer and wetter.
A broken, dragging wheel on Spirit made a chance discovery. The wheel dug into the Martian soil, revealing a layer of silica dust. This could have been produced in hot springs.
Opportunity found bright veins of the mineral gypsum, formed when flowing water left calcium in cracks in the rocks.
Opportunity found clay that had formed in neutral-pH water, a sign of conditions that would have been habitable for life.
A mysterious rock resembling a jelly doughnut suddenly appeared in front of Opportunity. The rock may have been hit by the rover and thrown, exposing its underside.