We don't know much for certain about the Martian atmosphere either. We do know that the Martian air is paper-thin, less than one percent as dense as Earth's sea-level atmosphere. Under such thin air, liquid water cannot exist and human blood would boil. The atmosphere is 95% carbon dioxide, and its total mass changes drastically with the Martian seasons. In the winter, large quantities of the gas freeze out of the air and coat the polar caps with dry ice. During the summer, the ice caps recede, and the gas returns to the atmosphere.
Maybe these fluctuations help stir up the immense dust storms that cover the planet at least once every Martian year. Hurricane-like winds whip up to speeds of 200 to 300 miles per hour and batter the planet's surface. These storms appear to originate in the southern hemisphere when Mars is closest to the Sun. Apparently, winds are generated by the extreme change of temperatureÑalmost 150¡F from day to night.
But the biggest mystery of the Martian atmosphere is its past. Current theories suggest that it was once much thicker. The existence of a different atmosphere at one time may explain why Mars has so many features that appear to have been formed by running water.
But it also raises many more questions. Why did the atmosphere thin so much? What was Mars like under a different cover? Could our atmosphere undergo similar transformations?
