New high-definition pictures of the soil in a shallow crater on Mars are raising more questions than they’re answering. At a NASA news briefing Wednesday at the Jet Propulsion Laboratory in Pasadena, Calif., experts said the new pictures, taken on Meridiani Planum by the rover Opportunity’s microscopic imager, show fine grains, almost like sand, as well as coarser grains of many different shapes, some of them round.