Peering into a hole that it dug, the Mars rover Opportunity has turned up new puzzles for scientists to ponder. Since landing Jan. 25, the Opportunity has been exploring the same small crater. One intriguing feature of that crater, in the middle of the vast plain Meridiani Planum, is the small pebbles, many almost perfectly round, scattered over the surface. The rover also saw these pebbles, about the size of BB’s, embedded in an outcrop of bedrock it cruised past last week, and scientists wanted to see what lies in the dirt, too.