Some big, some small; some gaseous, some rocky. Our Solar System neighbours have countless features, but one of them has always grabbed our attention: their colour. Fire-red Mars; bright golden Venus; Uranus and its greenish-bluish hue… What’s behind these colours? Here’s a first clue: it all depends on the kind of light that gets into your eyes.
Firstly, it must be said that the colour of planets can change, depending on whether we observe them from space or from Earth. Their atmosphere is key: depending on the main elements in it, different colour wavelengths may be ‘repelled’.
In the case of Venus, several Soviet probes discovered that its surface is made of grey rock, like Mercury’s. However, its dense atmosphere, packed with sulphuric acid clouds, prevent solar rays from reaching the surface. So, from Earth we only perceive the Venusian atmosphere’s orange and yellow tones.
Mars colour is its surface’s. Its atmosphere is so thin (made up exclusively of carbon dioxide) that the planet surface is clearly seen from the Earth. The Martian soil is composed mostly of iron ore, which gets a unique reddish hue when oxidizes.
The story of Uranus is quite peculiar, to say the least. Unlike rocky Mars, Venus, and Earth, the seventh and second to last planet of the Solar System is entirely made of gas. Its atmosphere is the coldest in the whole system, and abounds in methane, a compound with the ‘superpower’ to absorb the red colour wavelengths of the visible light spectrum, reflecting only the greens and blues.