Mars is red because its surface is covered in rust.

Mars has more iron in its top layers than Earth does. When Mars was forming, it was smaller and had weaker gravity than Earth. This kept more iron near the surface instead of letting it all sink to the center. Huge volcanoes like Olympus Mons later erupted and spread this iron across the planet.The red color comes from a mineral called hematite. Billions of years ago, Mars had a thicker atmosphere and liquid water. The oxygen from that water reacted with the iron to create a very fine dust. This dust is as thin as talcum powder and covers almost everything on the surface.The red color is only on the surface. In 2013, the Curiosity rover drilled into a Martian rock and found that the inside was actually gray. The gray color comes from volcanic basalt rock that has not been rusted. The dust is so light that wind storms carry it high into the air, which even makes the Martian sky look pink.