The martian movie opens with a red planet. what chemical process is responsible for mars red color

The reason for red color of mars is the presence of large amount of chemical substance called Iron oxide.

Iron oxide, the chemical which is responsible for reddish color of rust. However, the chemical process involved in the rusting is due to the water ice which is chemically bound in surface rock and the chemical reaction between the atmospheric oxygen and surface rock caused the rusting of the surface.

There is formation of iron oxide and thus the surface of mars appears red.