Martian mineral spheres. Image from the Mars exploration rover Opportunity of spherical haematite (iron ore) balls found in a Martian crater. The average diameter of the balls is four millimetres and analysis of the surrounding rock suggests that they were formed as deposits from minerals in Martian surface water. Part of the purpose of the rover's mission was to find evidence of the existence of water on Mars. The round, bright, patch is an area of drilling by the rover. Photographed on the 10th March 2004.