Mars Rover Marks an Unexpected Anniversary With a Mysterious Discovery At right, a photo from Mars taken on Jan. 8 by the Opportunity rover shows a rock that was not there when the same patch of ground was photographed, at left, less than two weeks earlier. Ten years ago, NASA's Opportunity rover bounded to the surface of Mars for what was planned to be a three-month exploration. Opportunity is still going today - and still making discoveries. The latest, scientists said on Thursday at a news conference celebrating an anniversary none had expected 10 years ago, is a small rock that seemingly appeared out of nowhere. The rock, whose chemical composition was also unexpected, appears in an image taken Jan. 8. There was no rock in a picture taken of the same spot less than two weeks earlier. "This is strange," said Steven W. Squyres, the principal investigator for Opportunity, during the NASA news conference. But he added, "We don't think anything particularly exotic happened here." Dr. Squyres said the most likely explanation was that as the rover pirouetted at an uphill location, its lame right front wheel, which has not turned for years, dragged across the rock and flicked it out of the ground to its new location. The scientists have not yet spotted the divot where the rock popped out, but that spot may be obscured by the rover's solar panels. Year after year, Opportunity goes farther than anyone dreamed. The expectation had been that it would drive about a kilometer - six-tenths of a mile - before dust accumulated on the solar panels and the batteries drained. Unexpectedly, fortuitous winds periodically cleaned off the solar panels, and Opportunity, as well as its twin, Spirit, continued to operate. Spirit got stuck in a sand dune 2009 and then fell silent in 2010 after it was not able to point its solar panels toward the sun during the winter months. Instead of one kilometer, Opportunity has driven 38.7 kilometers, or about 24 miles, exploring a series of ever larger craters, taking 170,000 pictures along the way. "It's a well-made American vehicle," said Raymond E. Arvidson, the deputy principal investigator. Over all, the rover remains in good health, despite the lame wheel, two scientific instruments that are no longer working, and a robotic arm with arthritislike stiffness. John L. Callas, the project manager, said the rover had also suffered an "amnesia moment," probably caused by aging memory chips. "It's just an operational annoyance," Dr. Callas said. But if it gets precipitously worse, there are some corrective actions we can take. Perhaps a greater obstacle to Opportunity's continued roving might be the limits of NASA's budget. This spring, NASA officials will review all of NASA's spacecraft that have lived beyond their original missions. Opportunity currently costs $14 million a year. "We have to weigh how much money we have and what missions are most productive," said Michael Meyer, the lead scientist for NASA's Mars exploration program. The newly discovered rock is like nothing Opportunity has seen. Dr. Squyres said it appeared to have flipped upside down, possibly exposing its underside for the first time in several billion years. "We're seeing stuff we don't normally get to see," he said. "It looks like a jelly doughnut," he continued: "White around the outside, red in the middle. We've looked at it with our microscope. It's clearly a rock. The composition is strange - high in sulfur, magnesium and manganese. "This is an ongoing story of discovery," Dr. Squyres said. Mars keeps throwing new stuff at us. At news conferences, NASA now solicits questions from the public via Twitter. That prompted the "Star Trek" actor William Shatner to chime in, "Are you going to cover the alien rock throwers?" Dr. Squyres replied that he did not think there were any Martian rock throwers, but another possibility might be that the rock was knocked there by a small asteroid impact nearby. In Friday's issue of the journal Science, scientists report on recent analysis of rocks along the rim of the 14-mile-wide Endeavour Crater where Opportunity is currently exploring. Following readings from Mars Reconnaissance Orbiter, Opportunity headed to rocks that appeared to contain a clay mineral known as iron smectite. "Doesn't mean much to many people," said Dr. Arvidson, who is the first author on the Science paper. But to geologists, it's pretty exciting, because it's a clay mineral that forms in a particular aqueous environment. The rocks, some of them older than the impact that created Endeavour, formed in waters that were much less acidic than existed in most other areas that Opportunity has crossed. That fits with the emerging geological picture of a Mars that was more hospitable in its early days. Dr. Squyres said the scientists had set their next major destination as the top of a hill where orbital measurements show even richer clay deposits. That would take a year or two or travel - if the rover survives both Mars and NASA's budget debates.