Mars is often called the ‘Red Planet’ for plain reasons. But what gives our neighboring planet this distinct hue? While Earth is sometimes referred to as the ‘blue marble’ because it’s mostly covered in oceans and has a thick atmosphere giving it a blue appearance, Mars is covered in lots of iron oxide. These are the same compounds that give blood or rust their distinct colour. In light of this, it’s no coincidence that Mars, which on occasions can be seen as a bright red ‘star’, was named after the Greek god of war.
It’s not entirely clear how all that iron oxide wound up on the planet’s surface, but one thing’s for sure: it all started some 4.5 billion years ago when debris, gas, and dust began coalescing into planets. Among these materials was lots of iron, forged in the heart of long-dead stars.
Earth and Mars both have a lot of iron, but while the heavy element sank to Earth’s core when the planet was still young and mushy, scientists think iron was less homogenously incorporated into Mars due to its weaker gravity and smaller size. That’s not to say Mars doesn’t have an iron core too, but there’s still a lot of the metal in the upper crust to be found.
Yet, iron by itself isn’t red — it’s silvery all the way to shiny black. What happened is that all of this surface iron became oxidized, forming iron oxide or rust — a compound made of two iron atoms and three oxygen atoms. But how come so much of Mars’ surface iron got oxidized? Here’s where scientists are still not out yet, but there’s reason to believe this massive oxidation happened when Mars had flowing water and a thick atmosphere, possibly not too different to what we have on Earth today. Leaving an iron-rich pot or spoon outside or in water for a lot of time will make it rust. A very similar process covered Mars in iron oxide.
An alternate theory, first proposed by Albert Yen of NASA’s Jet Propulsion Laboratory and based on data gathered by the 1997 Pathfinder mission, says a great deal of that iron oxide comes from meteorites.
In 2009, Danish researchers performed a study which found water isn’t necessary to produce a lot of iron oxide. Instead, crumbling quartz crystals — the kind found in the Martian regolith — leaves oxygen-rich surfaces exposed. This could easily have happened during the Martian dust storms which are so intense they sometimes kick up so much dust that can be seen with telescopes on Earth. Sunlight can also break down carbon dioxide and other molecules from the atmosphere producing oxidants like hydrogen peroxide and ozone.
Not the whole planet is red, though. Some regions look bright red, while others will appear even black because not everything is covered in iron oxide dust. Thanks to the rovers NASA landed on the planet, like Opportunity, Spirit, or Curiosity, the latest to touch down, we now have unprecedented views from Mars’ surface, but also of its subsurface. When the Phoenix Lander drilled just a few centimeters below the iron oxide rich surface, the ground was brown.
The sky on Mars is red too
Here on Earth, the sky appears blue because of a physical phenomenon called Raleigh scattering. Because shorter wavelengths of light like violet and blue are scattered more by the molecules in the atmosphere, the blue photons appear to come from all directions. On Mars, the opposite is true as the dust that litters the planet’s light atmosphere scatters red photons, making the sky red. This can also happen on Earth when the air is heavily polluted or covered in smoke.
Oddly enough, sunsets on Mars appear blue.
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