The short answer does not explain where all of the iron oxide comes from, though. There is a larger percentage of iron on the Martian surface than there is on other planets. The exact source is unknown, but many scientists believe that it came from the volcanoes that used to erupt all over the planet. When the Sun’s radiation and the solar wind break down water molecules that are in the Martian atmosphere they interact with, and oxidize, the iron. That process would take billions of years to cover the planet. Scientists have two theories about how the red dust became so thick and spread so widely across the surface.
source: www.universetoday.com...
and.....
QUESTION: Why is Mars red? ANSWER: Oxygen from water and from the carbon dioxide in the atmosphere combined with minerals, mainly iron, in the rocks and dust. It is this oxidised iron, the rust, that gives Mars its red colour.
quest.arc.nasa.gov...
