Why does Mars look red?

Answer

Iron in Martian rock and dust has reacted with oxygen over time, giving the surface and sky a rusty tint.

Context

Martian soil and dust contain iron-bearing minerals that have been oxidized, the same chemical change that turns iron rust-colored on Earth. Orbiters and rovers have identified iron oxides such as hematite in the dust and in many rocks. The dust is extremely fine, with grains only a few micrometers across, and it coats much of the surface and hangs in the atmosphere, tinting both the ground and the daytime sky a butterscotch or salmon color.

The rock beneath the dust is not red. Much of Mars's crust is dark volcanic basalt, and freshly exposed rock in rover images looks gray. The red look is mostly a surface coating, and dust storms can spread it across the whole planet. Sunsets on Mars appear bluish near the Sun, because the fine dust scatters light differently from Earth's air.

How the iron got oxidized is still debated. A 2025 study led by researchers using orbiter and lander data argued that the dust is rich in ferrihydrite, an iron oxide that contains water, which would point to oxidation while liquid water was around, much earlier than the dry conditions of today. Other explanations stress the thin atmosphere, ultraviolet light and reactive chemicals at the surface. The question remains open, and the color itself has been known since ancient observers called Mars the red star.