Next time you go for a stroll on Mars, be sure you don’t leave any litter behind. A plan to keep parts of the red planet in their pristine state could see seven areas turned into ‘planetary parks’, regulated just like national parks here on Earth. The scheme has been proposed by Charles Cockell, a microbiologist for the British Antarctic Survey in Cambridge, and Gerda Horneck, an astrobiologist from the German Aerospace Centre in Cologne, Germany.