What causes Mars to be red?
Answers
The simple explanation for the Red Planet's color is that its regolith, or surface material, contains lots of iron oxide — the same compound that gives blood and rust their hue. But why does Mars have so much iron, why is that iron "oxidized," and why does iron oxide look red?
It all started 4.5 billion years ago. When the solar system formed, many of the planets landed a dose of iron. Forged in the heart of long-dead stars, the heavy element swirled around in the cloud of gas and dust that gravitationally collapsed to form the sun and planets. Whereas the bulk of Earth's iron sank to its core when the planet was young and molten, NASA scientists think Mars' smaller size (and weaker gravity) allowed it to remain less differentiated. It does have an iron core, but abundant iron exists in its upper layers, also.
Plain-old iron looks shiny black. The element only takes on a reddish tinge when it has been exposed to oxygen, and enough oxygen at that for it to become iron(III) oxide, an atomic fivesome composed of two iron atoms and three oxygen atoms. So why did so much of the iron on Mars' surface oxidize, or gang up with oxygen?
Well, a lot of rocks on Mars are full of iron, and when they're exposed to the great outdoors, they 'oxidize' and turn reddish - the same way an old bike left out in the yard gets all rusty. When rusty dust from those rocks gets kicked up in the atmosphere, it makes the martian sky look pink.