Early in the history of the Solar System, Mars had an environment which was both much warmer and wetter than we find today. Can we imagine any schemes to change Mars back to that kind of environment which could support life?
 
Terraforming = changing the environment of a distant world to make it Earth-like.

McKay (1999): "... could Mars's environment be made hospitable to colonization by Earth-based life-forms? ... the answer is probably yes: With today's technology, we could transform the climate on the planet Mars, making it suitable once more for life. Such an experiment would allow us to examine, on a grand scale, how biospheres grow and evolve."

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