Martian soil. Micrograph of soil from the side of a trench dug by the Mars Exploration Rover Opportunity. Spherical objects are seen embedded in very fine grained soil. The rover used one of its wheels to dig the trench in the Meridiani Planum region of Mars. Opportunity may use its scientific instruments to further examine the freshly-exposed soil, in an effort to understand its origin and the formation of the spherical particles. They 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, taken in February 2004, measures 3 cm across.