Especially powerful greenhouse gases, such as sulfur hexafluoride, chlorofluorocarbons (CFCs), or perfluorocarbons (PFCs), have been suggested both as a means of initially warming Mars and of maintaining long-term climate stability. These gases are proposed for introduction because they generate a greenhouse effect thousands of times stronger than that of . Fluorine-based compounds such as sulphur hexafluoride and perfluorocarbons are preferable to chlorine-based ones as the latter destroys ozone. It has been estimated that approximately 0.3 microbars of CFCs would need to be introduced into Mars' atmosphere in order to sublimate the south polar glaciers. This is equivalent to a mass of approximately 39 million tonnes, that is, about three times the amount of CFCs manufactured on Earth from 1972 to 1992 (when CFC production was banned by international treaty). Maintaining the temperature would require continual production of such compounds as they are destroyed due to photolysis. It has been estimated that introducing 170 kilotons of optimal greenhouse compounds (CF3CF2CF3, CF3SCF2CF3, SF6, SF5CF3, SF4(CF3)2) annually would be sufficient to maintain a 70-K greenhouse effect given a terraformed atmosphere with earth-like pressure and composition.