In August, NASA’s unmanned Phoenix Mars Mission blasted off from the Kennedy Space Center as the first mission of NASA’s Mars Scout Program. Phoenix will reach the Red planet in May 2008, and is part of NASA’s strategy to ”follow the water” on Mars. This will pave the way for manned Martian missions, which President George W. Bush made a NASA priority in 2004. No date has been set for the crewed mission to Mars, but NASA is drawing up plans. The ultimate goal: making Mars a human outpost.

The renewed focus on Mars has rejuvenated the idea of terraforming Mars, which once belonged to the realm of science fiction, but is becoming increasingly possible today. Terraforming—or earthforming—is a Herculean feat of planetwide engineering that will change the Martian atmosphere and allow humans to make uninhabitable Mars into a planet fit for natural life. Space suits will still be necessary for human habitation, but food will available from plants that grow on a more temperate Mars.

”Now we have fairly accurate maps of the Red Planet, and can imagine how it might be modified—terraformed—to make it nearer to our heart’s desire,” said space visionary Arthur C. Clarke recently. A little more than a decade ago, Clarke wrote a book, The Snows of Olympus: A Garden on Mars (W.W. Norton & Co., 1994) that dealt with how to create a biosphere on Mars. He envisioned a huge garden on the slopes of Olympus Mons, the giant Martian volcano.

Mars, the only likely candidate for terraforming in the solar system, has numerous features in common with Earth. Like Earth, it has a rocky composition and marked seasons. Clearly, an environment different from the cold, dry world we see today once existed on Mars, as has been clear from recent NASA missions. Liquid water flowed on the Martian surface in the past: there are vast dry gorges and canyons etched by water and ice. The reddish color of the soil indicates hydration of the surface rocks, and is evidence of the role of water in the evolution of the planet.

Earlier this year the NASA rover Spirit confirmed that Mars was once a wet place. Spirit was exploring an area called Gusev Crater, when it found a piece of nearly pure silica. Silica is a component of window glass, and it is often a main ingredient in sand. The sort of silica found on Mars is usually formed in the presence of a significant amount of water. And if water flowed once on this frigid planet, will it not be possible to make it flow again?

Given that most astronomers and space scientists believe that humans have been responsible for global warming on Earth, some scientists think so.

”Humans are effectively warming the Earth,” says NASA’s Christopher McKay, who says we ”can, and should, do the same on Mars.” McKay has written extensively about terraforming Mars, which he terms ”planetary ecosynthesis.” McKay is one of a handful of NASA scientists who have begun to think about creating a permanent outpost on Mars even though many years may pass before a human sets foot there.

The first step to terraforming Mars would be to warm up the planet. It has a lot of frozen carbon dioxide (dry ice) at its poles. Increasing the temperature from its current –60 °C to just above 0 °C would turn the dry ice into gas. The ensuing thick atmosphere of carbon dioxide would create a greenhouse effect and would allow water to flow on Mars; in principle, plants could then grow slowly.

The first Martian plants would probably be lichens. In a few decades, there would be a seasonally ice-free lake on the summit of Olympus, Mars’s largest volcano. Soon thereafter, a terraformed Mars would witness a veritable green revolution characterized by pines and oaks, allowing it to become our second home.

Of course, questions remain as to what is the best way to warm up Mars. One possibility, espoused by James Oberg in his book New Earths in 1981, is to use giant mirrors in orbit around the planet to increase the temperature by reflecting sunlight onto the poles. NASA researchers are currently investigating large solar sails in an effort to provide spacecraft with solar power. Such solar sails could be adapted for this purpose.

Another warming idea, as proffered by McKay, is to generate gases known as halocarbons that would create a greenhouse atmosphere. They would exclude gases like bromine and fluorine, which can damage an ozone layer. It would involve building a factory on Mars to make halocarbons through chemical reactions. The raw materials would be the Martian soil and the Martian atmosphere. Mars has a thin atmosphere, and many light gases such as hydrogen aren’t trapped by Martian gravity. However, the heavier halocarbon molecules would be trapped, and they would contribute to a greenhouse effect. McKay says that real chemical studies have yet to be done, but he favors this method because we already have ”produced halocarbons on Earth that have contributed to global warming.”

In addition, there are more speculative ideas, such as using nuclear-powered rockets to divert asteroids to crash on Mars. This would create a catastrophe that would release a lot of heat into the atmosphere, which would increase the planet’s temperature dramatically.