Could we “terraform” Mars—that is, transform its frozen, thin-aired surface into something more friendly and Earthlike? Should we? The first question has a clear answer: Yes, we probably could. Spacecraft, including the ones now exploring Mars, have found evidence that it was warm in its youth, with rivers draining into vast seas. And right here on Earth, we’ve learned how to warm a planet: just add greenhouse gases to its atmosphere. Much of the carbon dioxide that once warmed Mars is probably still there, in frozen dirt and polar ice caps, and so is the water. All the planet needs to recapture its salad days is a gardener with a big budget.