During the 40 years that followed the departure of Mariner 4, we learned a lot about Mars from the many spacecraft sent to the Red Planet.
Mars has a very thin atmosphere (about 1% of Earth's pressure), no liquid water, and the incident UV radiation combined with the highly oxidising regolith make Mars' surface a deadly place for life. However, from images showing large river channels and networks, and the Mars Exploration Rovers showing layered sediments and alteration of the layers by water, we learned that the first half billion years of its history, Mars was a warm, wet place with a thick atmosphere. So could Mars be made habitable again?
This is the premise of terraforming changing a planet to make it habitable as Earth (terra = Earth). The idea of terraforming was first suggested in the 1930s purely in the science fiction field. But in the 1960s, scientists started thinking about the idea more seriously. Is this possible? It can be done with current technology?
To answer the question of whether the transformation of Mars to Earth is, we must first look at what is required for life and if Mars has these basics. Mars can not, at the present time, to maintain liquid water due to its low temperatures and thin atmosphere (atmospheric pressure below the triple point of water, pressure below which a material can be present as solid or vapor, regardless temperature).
Apart from water in liquid form, the most basic life on Earth needs an atmosphere with which to exchange gases. More complex organisms have more stringent and numerous requirements - plants need small amounts of oxygen, animals need a higher atmospheric pressure - but micro-organisms are low-maintenance.
Mars has frozen carbon dioxide (CO2 ice) on the edges of the poles, which is absorbed by the ground, which would be released if the planet warmed. The heat will also cause the melting of frozen water have been observed at the poles. Therefore, Mars seems to have the two key ingredients needed to sustain life.
Apart from this, if Mars warmed by some method, there will be a positive feedback in the release of carbon dioxide from the polar caps and regolith, the thickening of the atmosphere, further heating of the planet, water release, and subsequent to, conditions permitting liquid water to maintain the surface.
How could we warm Mars or force the frozen carbon dioxide to be released into the atmosphere? They have proposed many ideas, such as: putting mirrors in orbit around Mars to reflect extra light onto the Martian atmosphere, and so the heat sprinkling poles with dark dust that can reduce the albedo (ie brightness) and thus absorbed more solar energy and then to release super-greenhouse gases in the atmosphere to warm the planet.
There are groups working on making the first two ideas technologically feasible. But we have already implemented the greenhouse gas idea on Earth - making it, at least for now, the most promising terraforming method to Earth.
The gases of the super-greenhouse are molecules very effective at absorbing energy released by the surface of a planet, and then re-radiating upwards into space - to be lost forever - but also downwards the planet's surface, who and heated. They operate in a way similar to the blanket. But we do not want just any blanket!
For example, carbon dioxide would be like a thin sheet whereas a super-greenhouse gas, like, the super-fluoro-propane (C3F8), would be like a thick wool blanket. So we would want to use super-greenhouse gases - with high warming potentials, and long life time (1000s to 10 000s of years) - to reduce the required replenishment rate. A final key point is to choose super-greenhouse gases that do not destroy the current - and future - ozone layer Martian (unlike chlorofluorocarbons, or CFCs).
