The Martian atmosphere contains oxygen and water vapor, elements essential for life on Earth, but Earthlings can't exactly breathe easy on the red planet. Oxygen and water vapor aren't found in Earthly proportions on Mars; in fact, they are barely traceable.
The air on Mars is 95% carbon dioxide, 2.7% nitrogen, and 1.6% argon. Oxygen is only a fraction of the remaining .7% to which all other gases, neon, xenon, ozone, krypton, carbon monoxide, etc., contribute.
Similarly, water vapor is found in a quantity that is less than one one-thousandth of that found in the Earth's atmosphere - and that's at times of high concentration.
The amount of water vapor in the Martian atmosphere is highly variable: summer marks its highest concentration, during the winter water vapor is hardly detectable - the vapor freezes out of the air, adhering to fine red iron rich clay particles blown by the wind; carbon dioxide condenses on the ice/dust particles and drops to the ground, changing the surface color as the frost accumulates.
Scientists speculate that the atmosphere on Mars was once very different. Certain surface features on Mars suggest that the planet once had running water, but under current conditions water cannot exist in its liquid form on Mars. To help explain these baffling circumstances, scientists suggest that perhaps the Martian atmosphere was once much denser; estimates indicate that Mars may have lost nearly twenty times the amount of carbon dioxide it has today and roughly ten times as much nitrogen.
