This magnified look at the martian soil near the Mars Exploration Rover Opportunity's landing site, Meridiani Planum, shows coarse grains sprinkled over a fine layer of sand. The image was captured on the 10th day, or sol, of the mission by the rover's microscopic imager. Scientists are intrigued by the spherical rocks, which can be formed by a variety of geologic processes. The examined patch of soil is 1.2 inches (3 centimeters) across. The circular grain in the lower left corner is approximately 0.12 inch (3 millimeters) across — or about the size of a sunflower seed.
NASA / JPL / US Geological Survey