This article offers two arguments for the conclusion that we should refuse on moral grounds to establish a human presence on the surface of Mars. The first argument appeals to a principle constraining the use of invasive or destructive techniques of scientific investigation. The second appeals to a principle governing appropriate human behavior in wilderness. These arguments are prefaced by two preliminary sections. The first preliminary section argues that authors working in space ethics have good reason to shift their focus away from theory-based arguments in favor of arguments that develop in terms of pretheoretic beliefs. The second argues that of the popular justifications for sending humans to Mars only appeals to scientific curiosity can survive reflective scrutiny.