Mars has long been known as the “Red Planet” of our solar system for its distinctive red colour. Its earliest recorded name, “Har decher”, literally meant “Red One” in Egyptian. But why exactly is Mars red?
Mars’ surface is dominated by a red-orange tint visible from space. The colour is largely due to iron-oxide, also known as rust, which makes up the dust, rocks, and soil covering the planet. But to understand how this amount of iron-oxide formed, we must first understand the deep geological history of the planet.
Mars, many, many, years ago, is thought to have had a lot of water – perhaps even oceans. Mars also has a much smaller mass than Earth, which means its escape velocity – the speed a molecule needs to leave the planet – is significantly lower. This makes it far easier for gas molecules, like hydrogen, to escape into space. With a thinner atmosphere, water vapour on the planet could rise higher, where ultraviolet light from the Sun splits these water molecules into hydrogen and oxygen. The lighter hydrogen atoms are then able to escape the planet’s weak gravitational pull, while the heavier oxygen atoms remain behind.
Oxygen is extremely reactive, particularly with iron-rich minerals on Mars’ surface. This, naturally, led to the formation of iron oxide, similar to how Earth’s banded iron formations were created when oxygen first emerged in our atmosphere. The result was the planet-wide coating of red rust.
Further evidence of this process can be found in Mars’ enriched deuterium to hydrogen ratio. On Earth, roughly 1 in 6,000 hydrogen atoms is deuterium, also known as “heavy hydrogen,” which has one proton and one neutron. On Mars, this ratio is about five times higher. Since UV radiation would preferentially strip away lighter hydrogen atoms, the enrichment of deuterium suggests that Mars once had a substantial amount of hydrogen – most likely from water. Meanwhile, the oxygen was largely absorbed into the surface, forming large amounts of iron oxide.
But Mars isn’t just red on the surface, but appears to have a reddish hue in its atmosphere too. Images captured by the Mars Pathfinder and Mars Exploration Rover missions suggest that, to the human eye, the Martian sky is also red! This reddish tint is primarily due to the absorption of sunlight by fine dust particles, particularly in the 0.4-0.6 μm wavelength range. Additionally, photon scattering by these dust particles, especially in the near-infrared range around 3 μm, also adds to the sky’s hue, overpowering the effects of Rayleigh scattering typically caused by gas molecules (where light is scattered by particles much smaller than the wavelength of the light).
Overall, the vibrant red colour of Mars not only makes the planet uniquely beautiful, but also shows interesting insight into its history, geology, and atmosphere. But its colour is just the beginning – with a future of exploration ahead of us, the Red Planet’s past will become more clear than ever.
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