Blueberries on Mars

This frame from the microscopic imager on NASA's Mars Exploration Rover Opportunity shows spherules up to about 5 millimeters (one-fifth of an inch) in diameter. The camera took this image during the 924th Martian day, or sol, of Opportunity's Mars-surface mission (Aug. 30, 2006), when the rover was about 200 meters (650 feet) north of 'Victoria Crater.'

Opportunity discovered spherules like these, nicknamed "blueberries," at its landing site in "Eagle Crater," and investigations determined them to be iron-rich concretions that formed inside deposits soaked with groundwater.