The concept of terraforming—making a barren world suitable for widespread life—is well developed in science fiction. The term was first used in a science-fiction story published in 1942. It implies the creation of a copy of Earth, which need not be the goal, but the word caught on. (It is definitely more euphonious than the suggested alternatives of “ecopoiesis” or “planetary ecosynthesis.”) In the ’90s the award-winning science-fiction trilogy by Kim Stanley Robinson, Red Mars, Green Mars, and Blue Mars centered on the science and ethics of terraforming. But terraforming is no longer just science fiction. [...]
From what we can tell, Mars has the key materials to construct a biosphere: water, nitrogen, and carbon dioxide. Missions over the past decade have established that the high latitudes are rich in water ice, and the Curiosity rover recently detected nitrate in the soil (about 0.03 percent by mass). Carbon dioxide is the main wildcard. Mars may have vast layers of carbonate minerals, but this form of carbon dioxide is not easily released as gas. The success of terraforming would hinge on the south polar cap and the polar soil. They may contain enough carbon dioxide to bulk up the atmosphere only slightly, or they could store enough to create a pressure on Mars equal to the sea-level pressure on Earth.
In the latter case, studies of the climate of Mars indicate that it has two points of stability. Its current climate, with a thin atmosphere, a thick polar cap, and an average surface temperature of –60 degrees Celsius, is stable. But Mars has a second stable climate state, with a thick atmosphere, a thin polar cap, and temperatures of 15 degrees Celsius. That is what makes terraforming Mars feasible. If we could push its climate from the first state to the second, the clement conditions would be self-sustaining.
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