The Celsius and Fahrenheit temperature scales on Earth are defined by: (1) the melting temperature of water ice and (2) the temperature at which the saturation vapor pressure of water equals one Earth atmosphere (1013.25 mb). On Mars the surface pressure is 6 mb. At this pressure, water boils at the same temperature as water ice melts, that is, 0°C and 100°C are the same temperature on Mars! Accordingly, how would you define the Celsius and Fahrenheit temperature scales on Mars?