Why is Mars red?
Short answer: The soil on Mars contains a lot of iron oxide, a red chemical compound.
Long answer: Both the surface and atmosphere of Mars have a reddish hue, but neither are particularly strong red. Photos from the surface are often more brown than red. However, some red tinge is present across most of the planet, making it look noticeably red from a distance. Photos of Mars from space are typically brownish red.
What causes the redness? The Martian soil ("regolith") contains a lot of iron oxide. Specifically, oxidized ("ferric") iron in the form of red hematite (Fe2O3), also known as iron(III) oxide. This is a chemical compound containing two iron atoms and three oxygen atoms. It's the same compound that causes the red colour of rust and blood.
Pure iron is a shiny silver-grey colour, but it quickly turns darker and changes colour when exposed to air. Enough oxygen will result in iron oxide.
Iron is a common element in the inner Solar System but Mars has an unusually high proportion of iron on the surface, where it has been able to interact with air and liquids. The high level of surface iron could be due to the lower gravity and the early history of Mars. When planets form, heavier elements are more inclined to sink and end up deep underground. Mars is smaller than Earth or Venus, with a lower gravity, so its iron may not have sunk as much and was able to stay more spread out through the planet's layers.
Trivia
- Mars is the only red planet in our Solar System.
- Mars may not have always been red. It may once have been more of a charcoal colour, as well as having large bodies of water.
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Author: Dave Owen