Terraforming

Terraforming, or Earth-shaping, is a theoretical process of modifying a planet's atmosphere to make it habitable for humans. In the case of Mars, terraforming would require artificial thickening of the atmosphere to intensify the process of greenhouse warming (heating the frozen landscape), ice melting to increase the H₂O content of the atmosphere (creating water clouds) and greatly increasing the O₂ density to ultimately make the atmosphere breathable.

Mars and the "Triple Point" of water

The phase diagram for water, clearly displaying water's triple point.

Presently, ice on Mars sublimes as the atmospheric pressure is so low, ice bypasses the liquid phase when heated. Sublimation occurs allowing ice to turn directly into gas (steam). One of the main challenges for future terraforming efforts would be to increase the atmospheric pressure significantly so water can exist as a liquid on the surface of Mars. The atmospheric pressure will therefore need to be greater than the triple point of water (thereby existing as ice, liquid and gas).

It's much easier to understnad when you put it that way!

Long term prospect

The ultimate results of terraforming are disputed. Terraforming may have only a temporary effect, even if the effect lasts for some hundred or thousand years. Eventually, the solar wind may carry away most of the new atmosphere due to the insufficient magnetic fields of Mars. It has been suggested that the cost of terraforming a planet would be prohibative, however to a growing population on the surface of that planet it would most likely be considered a normal colonial function to ensure that daily colonial endeavours have a positive effect on the atmosphere.