Martian mineral spheres. Coloured composite image from the Mars exploration rover Opportunity of spherical haematite (iron ore) balls found in the Martian crater Endurance. The darker coloured ones are nicknamed blueberries, whilst the lighter ones are called popcorn. 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.