Mars has an atmospheric pressure of 6.1 millibars; whereas, Earth's atmospheric pressure is around 1,000 millibars. Observations of terrestrial volcanoes support that view that gasses are released from typical hot magma. The atmospheres on Earth and Mars are so drastically different today because the initial volatile substances have had different fates on the two planets. The Carbon Dioxide Cycle on Mars was documented by the Viking landers, which recorded seasonal fluctuations in the atmospheric surface pressure. The pressure tends to rise when carbon dioxide evaporates from the poles and to drop when carbon dioxide freezes. The pressure is highest in the southern hemispheres spring and summer (northern fall and winter) and lowest in the southern hemispheres winter. Viking 2 (red) recorded higher pressures than Viking 1 (black) because it was at a lower altitude. Click on the chart to get a larger copy.

Most of Earth's water is now in the oceans, because the amount of gases released during Earths evolution were far more than the atmosphere could hold. The presence of oceans in turn provides an active hydrological cycle that removes the carbon dioxide from the atmosphere. Water evaporates from the oceans and when it rains out of the atmosphere it takes carbon dioxide with it. Most of the out gassed carbon dioxide is now trapped in the ocean floor as calcium carbonate (limestone). In contrast, nitrogen has become the dominant element of the Earth's atmosphere, because it is chemically inert and relatively insoluble in water. Oxygen, which makes up 20.9 % percent of the Earth's atmosphere, owes its abundance to the presence of photosynthesizing plants. If there were no volcanic recycling on Earth, weathering processes would convert the small amount of carbon dioxide in the atmosphere into carbonate rocks within several thousand years.

On Mars too, more water was vented than its atmosphere could hold. It has been estimated that the total out gassed water could cover the planet with an ocean tens to hundreds of meters deep. Today Mars is much too cold to support any type of ocean or any form of stable liquid water. The most obvious reason is that Mars is almost half again as far from the Sun as the Earth is and receives less than half as much solar energy. Yet distance from the Sun cannot be the only factor, for it seems Mars was once warm enough to support flowing water on its surface. The lack of plate tectonics may have been the reason for Mars' short lived lush beauty. As volcanic activity decreased, the atmospheric carbon dioxide level and then the greenhouse effect declined.

How could the surface temperature on Mars have been kept above the frost point of water? The most likely answer is the familiar greenhouse effect. The gasses in the Martian atmosphere are nearly transparent to most of the Sun's radiation, and so the atmosphere is not heated directly by the Sun. Instead, like Earth's atmosphere, it is heated from below by energy re-radiated from the surface at longer infrared wavelengths. Because the gasses do absorb at these wavelengths, they trap heat. Water vapor and carbon dioxide are particularly effective greenhouse gases. Methane, which is also a very effective greenhouse gas is suspected of being a primary constituent of the ancient Martian atmosphere. For Mars a Methane mixing ratio of 1:2 in a thick CO2 atmosphere could maintain global surface temperatures above 32F even under an early, dimmer Sun.

The greenhouse effect can be measured by comparing the temperature at which a planet radiates the energy it absorbs from the sun with its observed surface temperature. The Earths effective radiating temperature is -18 degrees Celsius, and its global mean temperature is 15 degrees. The atmospheric greenhouse thus warms the surface 33 degrees. Mars, on the other hand, radiates at -56 degrees, just three degrees below its surface temperature. Its greenhouse effect is modest now because its atmosphere is thin.

For the temperature on Mars to have been above 0 degrees Celsius 4 billion years ago there must have been greenhouse gasses present... and a lot of them. The obvious candidates are carbon dioxide and water; computer models indicate that at least 1,000 millibars of Carbon dioxide would have been necessary.

Assuming that much carbon dioxide was out gassed, where has it all gone? Well, large amounts may have been absorbed into the surface layer of fine grained soil created by meteoritic bombardment, called the regolith. In the late 70's research at the Jet Propulsion Laboratory showed that carbon dioxide adheres to soil grains like those believed to make up the regolith. Carbon dioxide also resides in the polar caps but most of it passes from the solid to the vapor state every summer. Another possibility is that much of the carbon dioxide on Mars is contained in carbonate rocks like Earth. The regolith may also bind water chemically or physically, as it is thought to bind carbon dioxide. This binding would create a huge layer of permafrost covering the surface of Mars.