Mars is called the Red Planet because it looks red (why else?). Well, more like reddish. Orange. Ancient civilizations noticed the red hue it has and named it accordingly – Mars is the Roman god of (bloody) war, Greek counterpart being Ares. Hebrew name Ma'adim mean "turning red", Her Desher means "the red one" in ancient Egypt etc. But how did they manage to see that it was red and why is it red in the first place?
Its surface is covered with fine sand and dust full of iron oxide (rust) that get picked up by winds which create colossal sandstorms and fill the atmosphere with rust. The rust absorbs green and blue part of the light, reflecting the yellow, orange and red parts of the spectrum, making it visible to us on Earth.
This rust was formed billions of years ago when Mars presumably still had water, oxygen-rich atmosphere and was warm. In these conditions the iron minerals in the ground turned to rust. Now, rusting could be due to superoxide resulting from iron minerals exposed to ultraviolet rays. This is a topic of much debate, but as we continue to explore the planet further, we are expecting to get more answers.
The question remains how come there is so much iron in the martian soil? Geologists suspect that because the planet has low mass compared to that of Earth, it never had strong enough gravity for its iron core to grow as massive as that of Earth. This could mean that a lot of iron was left in the soil during the planet formation period.
So, there you go! Now you know why Mars is called the Red Planet.
Find out more about Mars here.