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Spirit and Opportunity

Spirit and Opportunity, the twin Mars Exploration Rovers (MERs), landed on Mars in early 2004. Part of NASA's long-term Mars Exploration Program, the mission has succeeded well beyond the hopes of its creators. Designed to last 90 martian days (a martian day, called a sol, is 24 hours and 39 minutes), the two robot geologists surprised everyone by surviving for many years. And they've beamed back amazing data, including hundreds of thousands of images, many of which include signs that water once flowed on the red planet. Sadly, though, no communication has been received from Spirit since March 2010.

Mars Rover
Artist's conception of a MER rover.
Image courtesy of NASA/JPL-Caltech

Spirit landed in Gusev crater, a 100-mile-wide basin thought to have once contained a body of water. On the opposite side of Mars, Opportunity landed on the Meridiani Planum, a flat plain that when viewed from orbit seemed to be covered in hematite—an iron compound that forms in water. The landing sites were chosen to help the rovers achieve their primary scientific goal: to examine rocks and soil for evidence that there had been liquid water on the martian surface. Mars today is too cold and its atmosphere too thin for liquid surface water to exist. But if there once had been water—which on Earth is an essential ingredient for life—Mars may have been a place where life was possible.

Rolling along on six wheels, these golf-cart-sized rovers can travel more than 300 feet (100 m) a day—and on occasion have gone considerably farther. Navigational and hazard-avoidance cameras help the rovers make their way around. Like all good field geologists, the rovers have excellent tool kits with which to conduct "hands-on" investigations. These tools and other high-tech instruments help the rovers determine the mineral composition of a rock. In turn, scientists back on Earth use that information to study how the rock was formed and if water played a role in its formation.

The MERs have met their goal: They've provided indisputable proof that water once flowed on Mars. Shortly after it landed, Opportunity found ripple marks and layered bedrock, evidence of a shallow, ancient sea. Spirit took longer to find evidence of water on its side of the planet, but then it discovered rocks with some of the highest sulfur content ever found on Mars, and scientists concluded that the sulfate salts were deposited by water. In addition, large amounts of hematite were confirmed in the form of "blueberries"—BB-sized pellets of water-formed rock.

Blueberries
This image, taken at a region called "Berry Bowl" near Opportunity's landing site, shows the hematite "blueberries."
Image credit: NASA/JPL/Cornell/USGS

Spirit and Opportunity's success seems all the sweeter because of the obstacles that were overcome. When the rovers' solar panels became covered with the ubiquitous martian dust, it looked as if the mission would quickly come to an end. But then dust devils—miniature tornados—engulfed the rovers, scouring away the grime on the solar panels.

Even with the solar panels fully operational, though, no one expected the rovers to survive a martian winter—after all, winter on Mars offers precious little sun. Ingenious engineers came up with the idea of parking the rovers on hills, their panels angled to catch what little sunlight was available.

And best of all, the hardy and tenacious Opportunity is still on the job, helping earth-bound scientists delve further into the mysteries of the red planet.

For a detailed look at the tools used by these rovers, see MER Tools of the Trade.