How could the surface temperature on Mars have been kept above the frost point of water?
The most likely answer is the familiar greenhouse effect. The gasses in the Martian atmosphere are nearly transparent to most of the Sun’s radiation, and so the atmosphere is not heated directly by the Sun. Instead, like Earth’s atmosphere, it is heated from below by energy re-radiated from the surface at longer infrared wavelengths. Because the gasses do absorb at these wavelengths, they trap heat. Water vapor and carbon dioxide are particularly effective greenhouse gases. Methane, which is also a very effective greenhouse gas is suspected of being a primary constituent of the ancient Martian atmosphere. For Mars a Methane mixing ratio of 1:2 in a thick CO2 atmosphere could maintain global surface temperatures above 32F even under an early, dimmer Sun. The greenhouse effect can be measured by comparing the temperature at which a planet radiates the energy it absorbs from the sun with its observed surface temperature. The Earths effective radiating temperature is -18 degrees Celsius, a
