NASA / SCIENCE PHOTO LIBRARY NASA / SCIENCE PHOTO LIBRARY
Martian mineral spheres. Composite 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.
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