My vague recollections of high-school chemistry is that ferrous salts tend to be green and soluble, and can be oxidized to insoluble reddish ferric compounds by

My vague recollections of high-school chemistry is that ferrous salts tend to be green and soluble, and can be oxidized to insoluble reddish ferric compounds by atmospheric oxygen - hence bog iron [1] I don't suppose this implies Mars had free oxygen at some point, but it would be fascinating if it did. [1] https://en.wikipedia.org/wiki/Bog_iron https://en.wikipedia.org/wiki/Bog_iron