The concept of terraforming Mars has been popularized in pop culture and recently by Elon Musk. The objective is to transform Mars into a planet suitable for habitation by living organisms and humans. Conceptually, this may be achieved in different steps: firstly, by increasing the atmospheric pressure of the whole planet to allow human beings to perform outdoor activities, possibly equipped with only an oxygen mask and without a pressurized suit. Secondly, by transforming the environment to make terrestrial organisms able to survive and proliferate. Thirdly, by creating a breathable atmosphere. The process would require a strong thickening of the atmosphere. A method to restore the early Mars CO2 atmosphere had been hypothesized in the 1990s, but after 30 years of Mars exploration, its feasibility is strongly questioned. Even if terraforming became possible, implementing it would violate many principles of modern environmental ethics. Assuming it is done for the purpose of habitation, it would also raise many legal and societal issues, pertaining notably to the relationships between communities on Mars and the Earth’s populations, structures, and rules.