Greenhouse gases

Greenhouse gases are gases which are more transparent in the visual spectrum than in the thermal (infrared) spectrum. In the atmosphere of a planet, these gases will hold heat that would be lost to space, in the air. This will increase the surface temperature.

Explanation

Because the sun's emissions peak in the visual range, visible light passes through such gas relatively unhindered and heats up the surface below. (On Mars, the dust in the atmosphere also causes signifiant heating.) When the planet re-emits the energy as blackbody radiation, it is in the infrared range due to the planet's comparatively low temperature.

The greenhouse gases are opaque at these frequencies and will absorb much of the radiation, heating up in the process. Alternatively, they may be reflective in the thermal range and simply reflect the radiation back down. This results in a higher average surface temperature than without the greenhouse gases.

Many natural gases (including CO2, CH4, NH3, NO2) are greenhouse gases, but artificial Super Greenhouse Gases such as Carbon Tetrafluoride can be created which are very long lived in the atmosphere, and will warm the planet hundreds or thousands of times more effectively than carbon dioxide (CO2). People have estimated that it would cost half a billion dollars per year to warm the planet with such gases. (See the Super Greenhouse Gases article for more discussion.)

Water is a powerful greenhouse gas, but if its concentration rises, it soon precipitates out of the atmosphere. Thus though it contributes to global warming, it is a constant and does not force global warming.

Beyond the Earth

For greatest effect, the greenhouse gas must be as opaque as possible in the range of blackbody radiation from the planet and as transparent as possible in the range of its star's peak output. Since Mars has the same sun as the Earth and a similar (if lower) temperature, the effectiveness of greenhouse gasses would not be severely different, but the BEST greenhouse gas will absorb infra-red light at temperatures cooler than Earth. As Mars warms, the mix of gases will approach the ones with the same warming potential as Earth.

Life Generated Greenhouse Gases

Early life on Mars will likely be anorexic bacteria (the brown, smelly Mars period), which produce plenty of methane which is a greenhouse gas.

As cyanobacteria (blue-green algae) are introduced, they will draw down the carbon dioxide, and build up oxygen. This is good and bad. We are losing CO2 (a warming gas), but adding O2 (which will help build up an ozone layer, which will protect other green house gases. An ozone layer will also protect water, people, plastics, and everything else on Mars' surface.

Some life forms will produce small amounts of ammonia, a green house gas.

Lighting in a nitrogen, oxygen atmosphere will produce various nitrogen - oxygen compounds, which are greenhouse gases. In a sterile environment, these compounds participate into the soil and are lost. If life exists in the soil, nitrogen fixing bacteria can move the nitrogen into living tissues, and eventually recycle it back into the air.

Thus life will have both positive and negative effects as far as the build up of greenhouse gases in the atmosphere.