Martian soil. Micrograph of a patch of soil on Mars, showing spherical grains on a fine layer of sand. The spherical particles may have been formed by the cooling of molten rock, or by the accretion in water of successive layers around a smaller particle. This image was taken by the microscopic imager instrument on the Mars Exploration Rover Opportunity, near the rock outcrop called Stone Mountain. Opportunity reached Stone Mountain on 5 February 2004. This was the most detailed view of the soil to date. The image is a mosaic, formed from several separate images.