Last week I wrote about environmental issues growing out of human missions to Mars, and the obligation of the United States (and other space powers) under the 1967 Outer Space Treaty to prevent “harmful contamination” of Mars. But what about beneficial contamination? Mars, as far as we can tell, is a dead world. Even if it turns out to host some forms of life, they are almost certain to be limited to bacteria, akin to the extremophiles that populate places like volcanoes, undersea thermal vents, and deep subsurface rock formations, and their distribution is likely to be similarly circumscribed. Algae would be big, big news. But Mars needn’t remain dead (or near-dead). For several decades people have been looking at “terraforming” Mars by giving it an earthlike – or at least more earthlike – climate. (For the technically inclined, there is a superb engineering textbook on the subject, Martyn Fogg’s Terraforming: Engineering Planetary Environments, a thoroughly practical book published by the thoroughly practical SAE).