What gives the planets their colours?

The colours of the planets depend on how you look at them. With the naked eye, only the closest planets are bright enough for colours to be discernible. Mars is notoriously red because we can see through its thin, transparent atmosphere to a surface covered in rock dust that is rust red thanks to the strongly oxidising environment.

When we look at Venus we see sunlight reflected off its global cloud tops, which are dazzlingly bright and white.

The outer planets – Jupiter, Saturn, Uranus and Neptune – have no solid surface, and the colour we see depends on how much atmosphere overlies permanent cloud layers. Jupiter is bright enough to see its colour with the naked eye, and compared to Mars it is distinctly yellow. A telescope will show you details of Jupiter’s cloud patterns, within which there are colour variations. Some rotating storm systems – most famously Jupiter’s Great Red Spot – are red, for reasons we do not well understand. Suggestions include the presence of phosphorus, sulphur or organic molecules.

If you’re looking at an image sent back by a spacecraft, you need to keep your wits about you. Look at the caption to find out if it is ‘natural colour’ or ‘false colour’. The latter means that it was recorded in a different region of the spectrum to what your eyes can see, or has been subject to computer processing to make the colours more intense. In general, any image with a garish range of colours is not natural. However, if you see an image of Uranus looking green or Neptune looking blue, this is real. The topmost global cloud layer of each consists of condensed methane and is seen through a great depth of methane gas that filters out the red component of sunlight so that only green-blue light makes it down to the clouds before being reflected back.

David Rothery, professor of planetary geosciences at the

Open University