Why Is Mars Red
Why is Mars red? If there isn’t an atmosphere then why would the rust colored oxidation be present?
Because the red is the color of oxygen that’s already been combined with iron.
Take a rusty nail and put it in a vacuum for a thousand years, and it’ll still be a browny-orange color. That’s effectively locked-in as the result of a chemical reaction that took place some time in the past, and you don’t need any future oxygen to keep it that color.
The interior of Mars consist mostly of iron & nickel, so there is plenty of iron for rust to form. The surface of Mars is polluted with old rust, so much so that all of the oxygen is pretty much locked up in the various forms of rust that are all over its surface. It also consist of a high percentage of sulfur as well.
Mars atmosphere did used to have a lot of oxygen along with water on the surface but now consist of mostly carbon dioxide in the atmosphere.
Though, an oxygen atmosphere is a perfect combination to create rust quickly, the same can be done with sulfur dioxide and carbon dioxide.
There are still oxygen molecules present, which the process allows rust to be formed on the thin layer but flake off, thus exposing the layer of iron beneath and creating more rust.