|MadSci Network: Astronomy|
According to the Mars Rovers' website (1), the gray rocks are hematite, which is a mineral made up of iron and oxygen. The chemical interactions of these 2 elements give the rocks a gray color. In powdered form, it is rust-colored and gives Mars the red color (2) we're so familiar with. (1) http://marsrovers.jpl.nasa.gov/ (2) http://marsrovers.jpl.nasa.gov/spotlight/hematite01.html
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