Why is the planet Mars red?
The short answer is that it isn’t. For most of the planet, the red layer only covers a couple of millimeters and at its deepest, two meters. The red color comes from various oxides of iron (hematite mostly) in very, very fine particles, and trace amounts of other elements including titanium, chlorine and sulfur. One possible way the dust was created was by harder basalt rocks, which contain more feldspar, grinding against the softer basalt to create fine dust particles. All of that iron had to come from somewhere: volcanoes. The best information that we have is that the surface of mars below the red layer is made up of hardened, low viscose lava: basalt. The concentration of iron in Mars’ basalt is higher than that of Earth, which is why Earth is much less red.
