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Illustration of an ancient Mars (left) that had a thicker atmosphere and liquid water and present day Mars (right). Mars formed around 4.6 billion years ago. The young planet would have had enough water to fill an ocean occupying almost half of the northern hemisphere, reaching depths greater than 1.6 kilometres. Currently, Mars is a rocky desert world with no surface water. The atmosphere is mostly carbon dioxide and Martian surface temperatures are well below freezing. Most of the water and atmosphere were lost to space, or locked up underground in deposits of ice. (KEYSTONE/SCIENCE PHOTO LIBRARY/CARLOS CLARIVAN/SCIENCE PHOTO LIBRARY)