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The similarity of Mars 1 Earth has caused much debate on whether life exists or has existed on the planet. However, none of the exploration missions have provided any biological proof. 2 its likeness Mars is a cold, dry, desert-like planet with a thin atmosphere. There is no liquid water, which is an essential ingredient 3 life. There is little atmospheric protection 4 solar or cosmic radiation. The temperatures are so low that only Antarctica on Earth is comparable, although Mars is much colder. The thin atmosphere consists 5 around 95% carbon dioxide. No known species could survive the harsh Martian surface conditions. Scientific evidence suggests that the planet was once more habitable than it is today, but whether life has ever existed there remains a mystery.