It was once believed that Mars had an atmosphere much like Earth's and that if we could find a way to get there, we could breath Martian air. As often happens, knowledge brings a new outlook. Composition of the Martian air is 95% Carbon Dioxide, 3% nitrogen and trace amounts of oxygen and water. The density of the Martian atmosphere is only 1% of Earth.

     One surprise was the pale yellowish-pink sky rather than the blue we are so accustomed to. Our sky is blue because incoming sunlight is scattered by the gas molecules in our atmosphere. Blue light is scattered the most, and that is why we see a blue sky. Mars sky is pinkish-orange because dust particles scatter light even more. It is believed that dust particles are suspended in the atmosphere all the time, instead of just when there is a dust storm. The presence of all that dust would further scatter the sunlight giving the Martian sky that characteristic color.

     With all that Carbon Dioxide in Mars' atmosphere, you might expect to find a runaway greenhouse effect like that on Venus, but it is not the case. Mars density is so low that the carbon dioxide creates only a minor greenhouse effect. And Mars is so cold that clouds of dry ice (frozen CO2) and some water crystals drift about in the Martian atmosphere.

     Winds are created by air being heated near the surface around the surface, then rising and moving towards the poles. This is the same general pattern for winds on Earth. The planet's rotation causes the Coriolis effect which deflects the winds so that they blow around the planet nearly parallel to the equator. Martian winds are usually a gentle breeze, but occasionally they can rise to gale force. This occurs seasonally near the poles. The winds pick up a great deal of dust off the surface and sometimes the entire planet is engulfed in one giant dust storm.

     Though there is water crystals in the atmosphere, no rain ever falls on Mars. The atmosphere is too cold and there is too little water in them to produce rain. There is so little water in the atmosphere that if all of it were to fall at once, it would make a layer less than 1/2000th of an inch thick. Despite such little water, fog does form in some valleys and on really cold nights there is frost. Around the poles, there is carbon dioxide snow!

     Mars has acquired its atmosphere through outgassing from the interior. Most of this outgassing took place during the first billion years. Once the planet cooled, it released little gas. The amount of atmosphere a planet has depends on how much it is outgassing and how much of those gasses are lost to space. Since Mars has nearly ceased outgassing, more gasses are lost to space and the atmosphere has thinned considerably since Mars' early history.

     It is believed that Mars had a very different atmosphere in its distant past. At one time, the planet's atmosphere may have been much more like Earth's is now. Apparently, Mars has changed a great deal in it's history.

The Lure of Mars

The Martian Atmosphere

The Surface of Mars

Seasons and Climate on Mars

Water on Mars

The Moons of Mars

Spacecraft To Mars

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Kathy Miles, Author, and Chuck Peters, Systems Administrator