While still in the early stages of space exploration, it makes sense to try to prevent the inadvertent introduction of terrestrial life to other worlds while researchers pursue their search for extraterrestrial life. But others argue that sometime later in this century, humanity should begin the process of terraforming other worlds, most probably beginning with Mars. British planetary scientist Martyn Fogg provides a good definition of terraforming as “a process of planetary engineering, specifically directed at enhancing the capacity of an extraterrestrial planetary environment to support life. The ultimate in terraforming would be to create an uncontained planetary biosphere emulating all the functions of the biosphere of the Earth—one that would be fully habitable for human beings."
Mars as it is is not a promising home for Earth life; its average temperature is -60°C, well below Earth’s average of 15°C; the pressure of its carbon dioxide atmosphere is one-hundredth that of Earth’s; and it lacks an ozone layer so its surface is blasted by DNA destroying UV rays from the sun. Can it be made more hospitable? Science fiction author Jack Williamson coined the word terraforming in a 1942 short story in Astounding Science Fiction. Arthur C. Clarke further developed the idea of terraforming in his 1951 novel The Sands of Mars. In 1973, young astronomer Carl Sagan devised a proposal for melting Mars’ South Pole by darkening it. This would boost carbon dioxide in its atmosphere, creating a greenhouse effect, which would warm the planet and allow water to flow.
In a research review in 1998, Martyn Fogg evaluated various suggested technical means to begin terraforming Mars. A runaway greenhouse effect releasing carbon dioxide might be jumpstarted by pumping potent man-made greenhouse gases like perfluorocarbons into the atmosphere or by directing extra sunlight onto the South Pole using a space mirror 250 kilometers in diameter. Once started, Fogg estimates it would take 100 years to build up a thick atmosphere and warm the planet enough so that anaerobic Earth life could successfully colonize the planet. Candidates for pioneering terrestrial microbes include the dessication-resistant cyanobacterium Chroococcidiopsis, the lime-boring cyanobacterium Matteia, and the ionizing-radiation resistant heterotrophic bacterium Deinococcus radiodurans. In addition, the Planetary Society is flying 10 hardy organisms as part of its Living Interplanetary Flight Experiment (LIFE) aboard the Phobos-Grunt mission. They will be returned with soil from Phobos to see how they fare in long exposure to space.Bailey then links to a pdf discussing "The Ethics of Terraforming" [PDF]