By Carole Stott
Mars is the planet that is most like Earth. It is about half the size of Earth, spins round in a period of a little over 24 and a half hours, experiences seasons, and has polar ice caps, but the planet is not habitable for humans. It is too cold for liquid water, there is no atmospheric layer to stop harmful radiation reaching the surface and there is no oxygen to breathe. Yet, sometime in the future all this may change.
In 1952 the science fiction writer Arthur C Clarke wrote The Sands of Mars . It was one of the first novels to discuss transforming Mars's present surface into a landscape that could support human life. This process of deliberate change is called terraforming. The change would be slow, taking many centuries, but eventually the planet would be hospitable for humans and other life forms.
Terraforming would be achieved by triggering the release of significant amounts of carbon dioxide into the Martian atmosphere.
A result of this would be greenhouse-warming of the planet and in turn an increase in its surface temperature. Once the temperature moved above the melting point of ice, liquid water would once again flow on Mars. One way to start off the process would be to put giant mirrors in orbit around the planet. They would direct sunlight to the polar ice caps which would heat up and release carbon dioxide into the atmosphere.
Although today Mars is an inhospitable planet; cold and barren, with dust storms and seared by ultraviolet light, it was not always so. When Mars was young, 4 billion years ago, rain fell, rivers flowed and its environment was more Earth-like. During its first half billion years its atmosphere was denser and similar to the young Earth's but it was to develop differently. Mars's atmosphere escaped, the planet could not hold on to it because it is a cooler place with a lower surface gravity. The loss of the protection of the atmosphere led to surface cooling, the water freezing and another drop in temperature to produce the frigid world seen today.