If we had the ability to terraform Mars, would it be morally permissible to do it? This article surveys three preservationist arguments for the conclusion that we should not terraform Mars and three interventionist arguments that we should. The preservationist arguments appeal to a duty to conserve objects of special scientific value, a duty to preserve special wilderness areas, and a duty not to display vices characteristic of past colonial endeavors on Earth. The interventionist arguments appeal to a duty to (...) fulfill our pioneering nature, a duty to extend the lifespan of our species, and a duty to restore the ecosystems Mars may once have housed. The preservationist arguments are stronger than the interventionist arguments; terraforming Mars is probably morally wrong. (shrink)