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Demystifying Mars Part III: Roving Martians
An artist's drawing of a lone rover on Mars. Mars Exploration Rover project scientist Joy Crisp believes her robotic geologists owe much to the work of Mars Global Surveyor and Odyssey. "Mars Global Surveyor and Odyssey were very important in selecting the landing sites. They were key to our success, both scientifically and for landing," said Crisp.

Image to right: The Mars Exploration Rovers have each spent over 600 days rolling across Mars and into the history books. Credit: NASA

The Mars Exploration Rovers, called Spirit and Opportunity, are direct descendants of the Pathfinder/Sojourner system. But this time, each pillowy lander carried a larger all-terrain rover. The golf cart-sized rovers were self-sufficient, robotic dune buggies that carried their own science, power and communications equipment.

Spirit and Opportunity landed on opposite sides of the martian globe in January 2004. Spirit set down in a large meteor impact crater called Gusev Crater. Half a world away, Opportunity targeted the wide, open desert plain of an area called Meridiani Planum. Both rovers arrived to continue looking for geological evidence that Mars had once been wet with standing or flowing water.

Opportunity divined the first signs of water on a thin outcrop of rocks. "Water was around for a while because it deposited the rocks and later soaked them," said Crisp. It turns out the chunk of bedrock had once been swimming in water.

What tipped off Crisp and her team were small, spherical bits dotting the bedrock. They suspected these bits are most likely another form of concretions that form when iron precipitates out of water that soaked the rocks. Found alongside the concretions were little nooks and crevices identified as "lenticular voids." The pockmarks are often left behind after water-borne crystals break down from moving water or chemical changes. Perhaps the strongest evidence for water came when chemical analysis showed the rock was rich with a type of salt called jarosite. On Earth, rocks containing that much jarosite either form in water or are highly altered by long exposures to water. The presence of the salt suggested the rock may once have rested in an acidic lake.

The rovers found round balls of minerals called concretions. A short time later, Spirit also found evidence of past water on the other side of Mars. While exploring Gusev Crater, the rover studied a rock named Mazatzal that was made up of layers and stripes of different minerals. Geologists suspected the rock had once been wetted by groundwater and the stripes were cracks filled by minerals carried in by a thin trickle of water that squeezed through.

Image to left: Concretions are little round balls that form when minerals solidify out of water. Credit: NASA

Along with discovering evidence of water, the two rovers have amazed scientists and Mars watchers by managing to explore the planet far longer than expected. "The plan was for 90 days, but we've been pleasantly surprised that they've kept going and going," said Crisp. Both Spirit and Opportunity have each survived more than 600 days. The truth is, though, some days the right to keep going have been hard-won.

After its first 17 days on Mars, Spirit abruptly and mysteriously stopped communicating with Earth. And NASA was doubly worried knowing that Opportunity was landing only five days later and could fall prey to the same failure as its twin. Initial indications were that Spirit’s flight software began to malfunction. The glitch caused the rover to cease communications and reboot its computer 60 times in three days.

NASA initially believed it could take weeks to revive the comatose rover. But just three tireless days later, engineers determined Spirit's problems rested with storing computer files in the rover's "flash memory." Flash memory – like the kind used to store numbers and pictures on cell phones – holds a rover's computer files even when turned off. Engineers solved the problem by temporarily storing files onboard a different portion of Spirit's memory and reinstalling the flight software into the flash memory. Engineers soon had Spirit back to work and knew how to avoid the same problem with the soon-to-arrive Opportunity.

Engineers test escape techniques with a spare rover. While Opportunity managed to avoid Spirit's issue, the rover literally ran into trouble of its own. On its 446th day exploring Mars, Opportunity bogged down in a sand drift. About 8 feet long and 1 foot tall, the drift's 15-degree slope proved a little too steep for the rover to climb. The dune appeared to be made of fine-grain sand that offered poor traction for Opportunity's wheels to bite into. Not wanting to make matters worse and dig into deeper trouble, NASA engineers stopped the rover's spinning wheels and set to work devising an escape plan.

Image to right: Back on Earth, engineers try to simulate the conditions plaguing Opportunity back on Mars. Credit: NASA

The engineers constructed a sandbox and experimented by filling it with different mix of materials to simulate the sand trap that ensnared Opportunity. Then they drove their test rover into the sandbox, got it stuck and worked out how to set if free under its own power. Before long, the engineers had a plan in hand and took what they learned to the rover control room to slowly ease Opportunity out of the dune, sending the rover on its way.

Like the Spirit and Opportunity rovers trundling across the martian surface, NASA's march across the sparse and enigmatic planet continues. The Mars Reconnaissance Orbiter -- launched in August 2005 -- is streaking toward the planet to join Mars Global Surveyor and Odyssey in orbit. In the years to come, more inquisitive spacecraft will follow them to investigate Mars, perhaps solving the mystery of what happened to the planet's water and even preparing for the arrival of the ultimate scientific explorer: humans.

Continue your own virutal exploration of Mars:
+ Mars information from World Book Encyclopedia
+ Mars Pathfinder/Sojourner Mission
+ Mars Global Surveyor Mission
+ Mars Odyssey Mission
+ Mars Exploration Rover Mission

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Charlie Plain
NASA's John F. Kennedy Space Center