Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and other Celestial Bodies
Article II Outer space, including the moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means.
In other words, the UN Treaty expressly forbids any claim of national appropriation. So, my question is - why can't I appropriate my piece of Mars for myself?