Question

why mars is known as red planet ?

Answer 1

because the surface of mars look red due to large amount of ferrous oxide(iron ore)

Answer 2

Well, the composition of the Martian soil has a high amount of iron oxide which is formed due to the reaction of iron with atmospheric oxygen and water molecules, e.i, the water vapour. The iron oxide appears red in nature and thus, give mars its mostly known red colour making it the so called 'Red Planet'
The process by which the iron oxide is formed can give us the theory that Mars was once full of water and probably sustained life according to the scientists, I think I gave a comparitvely big answer then what you asked, but knowledge should keep increasing!