Both of NASA’s Mars Exploration Rovers have completed their originally planned mission and are tackling extra-credit assignments.

“Spirit and Opportunity have completed all the primary objectives of the mission. The terrific success achieved is a tribute to a superb team whose commitment to excellence, and keeping the public engaged, is hard to match,” said Orlando Figueroa, director of the Mars Exploration Program, NASA Headquarters, Washington.

Opportunity finished its 90th martian day of surface operations on Monday. That was the last of several criteria set in advance for full mission success. Spirit passed its 90-day mark on April 5. Both rovers have met all goals for numbers of locations examined in detail, distances traveled, and scientific measurements with all instruments. Both rovers are healthy. In early April, NASA approved funding for extending operation of Spirit and Opportunity through September.

“This brings Opportunity’s primary mission at Meridiani Planum to a resounding and successful close. It’s stunning to think through the short history of this vehicle,” said Matt Wallace, Opportunity mission manger at NASA’s Jet Propulsion Laboratory, Pasadena, Calif., where rover assembly began barely two years ago. In its three-month primary mission, Opportunity drove 811 meters (more than half a mile) and sent home 15.2 gigabits of data about Mars, including 12,429 images.

Opportunity found other rock exposures in recent days similar to the ones near its landing site that yielded evidence for a body of salty water covering the area long ago. Instead of spending many days to examine those rocks, controllers told the rover to go to the rim of a 130-meter-wide (approximately 430-foot-wide) crater informally amed “Endurance.”

When Opportunity sends home a view into Endurance Crater, expected within a few days, scientists and engineers will begin deciding whether the rover should try to enter that crater. “We’re coming up on a major branch point in the mission,” said Dr. Scott McLennan of the State University of New York, Stony Brook, N.Y., a member of the rovers’ science team. “Can we get down into Endurance? Can we get back out?” he questioned.

Last week, Opportunity paused beside a crater dubbed “Fram,” less than one-tenth the size of Endurance Crater. It examined a rock studded with small, iron-rich spherules that are one part of the evidence for past water in the region. The rover used its rock abrasion tool to grind a hole. This allowed examination of the interior of the rock, called “Pilbara.”

McLennan said, “Pilbara is a dead ringer for McKittrick,” a rock target in the outcrop Opportunity examined in February and March. Another rock at Fram showed hints, it might provide the best-yet evidence about how minerals precipitated out of solution as the ancient body of water evaporated. “It’s something that would be of interest to come back and study more if we don’t see something of even greater interest along our way,” he said. Images of Endurance Crater from a distance seem to show much thicker layers of outcrop than Opportunity has been able to reach so far.

Improvement to the rovers’ mobility from new software has expanded options for planning their explorations. Spirit and Opportunity have driven farther in April than in the previous three months combined. Spirit has traveled more that 1.2 kilometers (three-fourths of a mile), and has another 1.8 kilometers (more than a mile) to go before reaching highlands informally named “Columbia Hills.” Scientists hope to examine rock layers older than the volcanic plain Spirit has been crossing. This week, Spirit is crossing from an area dominated by material dispersed by crater-forming impacts into an area with fewer rocks.

“We are transitioning into a geologically different region. Nothing could be more striking evidence of this than the view ahead of a landscape that has fewer and smaller rocks than the region explored so far,” said Dr. Dave Des Marais, a rover science team member from NASA Ames Research Center, Moffett Field, Calif. Scientists are using Spirit’s observations at ground level to check ideas about the region’s geology based on observations from orbiting spacecraft. That could improve interpretation of orbital data for the whole planet. Spirit will systematically survey the soils, rocks and other features on the plain as it continues toward Columbia Hills, with an arrival planned for mid to late June.

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