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for a reduced atmosphere on ancient Mars
The blue-toned rocks in the upper-left of the image are depleted in iron because it was removed during weathering on ancient Mars. This is geological evidence that iron was lost from the rocks in reduced conditions.
Both Earth and Mars currently have oxidising atmospheres, which is why iron-rich materials in daily life develop rust (a common name for iron oxide) during the oxidation reaction of iron and oxygen. The Earth has had an oxidising atmosphere for approximately two and a half billion years, but before that, the atmosphere of this planet was reducing – there was no rust.