The reason is pretty straightforward actually; devoid of anything metaphorical. Mars has a reddish appearance, and so that’s why it was called the Red Planet. In fact, it was Mars’ appearance that inspired people to give it its current name. It would be safe to presume that the color of war, being red (due to all the blood that gets spilled, why else), was the basis for it.
Thus, in the case of Mars’ names, the name Red Planet paved the way for the name Mars; not the other way around. The planet was already described as having a reddish appearance as early as the time of the Ancient Egyptians. Back then, they called it Har decher, which basically means “Red One”.
Mars’ reddish color is quite visible to the naked eye. This color is due to the presence of iron oxide dust, a.k.a. rust. While this doesn’t mean that Mars is made up of rust, it would be safe to say that the planet is fully covered with it.
However, studies have shown that, in most places, the rusty layer is only about a few millimeters thick. Even in the Tharsis area, where this iron oxide dust has been found to be most abundant, the layer of iron oxide dust there only goes to a depth of about 2 meters. That’s pretty shallow compared to the radius of the entire planet.
So where did Mars get all this rust? Rust needs water, right? I’m sure you Martian believers out there are now thinking, “aha! this should prove that Mars was once flowing with water and all that’s left, after all the water gone, is rust.
Very recent studies have shown that it is possible to form the red dust without a significant role from a great body of water.
Researchers from the Mars Simulation Laboratory have actually stumbled upon a strange way of forming red dust. By tumbling quartz samples in glass flasks for many months, effectively tumbling them about ten million times, the researchers were able to produce fine dust out of the sand.
When they added powdered magnetite to the samples, the dust became redder as they were tumbled some more. We actually have that story here in Universe Today: Why is Mars red?….. In the meantime, I think we’ve already explained why Mars is called the Red Planet.
There’s more from NASA: “Unmasking the Face on Mars” and “Mars Shoreline Tests: Massifs in the Cydonia Region”
Here are two episodes at Astronomy Cast that you might want to check out as well:
Stellar Roche Limits, Seeing Black Holes, and Water on Mars
The Search for Extraterrestrial Intelligence