Why Is Mars Red?

Reflected sunlight from the Martian surface is stronger at red wavelengths and weaker at blue ones. Orbiting and landed spectrometers record that pattern, and it matches iron-bearing oxide minerals in laboratory measurements.

Martian dust is rich in iron. Where that iron has combined with oxygen it forms oxide minerals that reflect red light — the same family of chemistry as rust on Earth. Global dust storms redistribute that material, so the colour is not a thin local coating but a planet-wide veneer.

Reflectance spectroscopy from orbit, in-situ analysis by rovers, and independent landform evidence — channels, deltas and layered sediments — that liquid water once shaped the surface.

The surface is dominated by oxidised iron-bearing dust, and parts of the geological record were shaped by liquid water in the distant past.

How long surface water persisted, how much of it there was, and whether conditions ever supported life remain open. Oxidation chemistry alone answers none of these.