Mars has an atmosphere, though it is different than our atmosphere here on Earth. The atmospheric pressure on Mars is only about 1/100 that of Earth's! The atmosphere of Mars is thin, cold, and dry and contains much less oxygen than the atmosphere of Earth. The oxygen content of the Martian atmosphere is only 0.13 percent, compared with 21 percent in Earth's atmosphere. Carbon dioxide makes up 95.3 percent of the gas in the atmosphere of Mars. It also contains nitrogen and argon and very small amounts of water and methane. A lot of people are studying why Mars' atmosphere is so thin. It may be that Mars had a lot of atmosphere in the early part of the planet's history but once its magnetic field disappeared this lack of a protective barrier allowed the sunlight to strip away the upper atmosphere, making the atmosphere thinner. The upper atmosphere of a planet is constantly bombarded by the solar wind, a fast stream of very light particles emanating from the sun. This wind itself is fairly benign, but it also carries a magnetic field. This picks up ions from the upper atmosphere, accelerates them, and then smashes them back into other ions at several hundred kilometers per second, knocking ions out to space. If the planet itself has a magnetic field, it can shield the upper atmosphere from the solar wind. However, because it is a small planet, Mars cooled rapidly so that its inner dynamo disappeared and it lost its original magnetic field quickly.