The greenhouse and antigreenhouse effects on Titan.

Abstract

There are many parallels between the atmospheric thermal structure of the Saturnian satellite Titan and the terrestrial greenhouse effect; these parallels provide a comparison for theories of the heat balance of Earth. Titan's atmosphere has a greenhouse effect caused primarily by pressure-induced opacity of N2, CH4, and H2. H2 is a key absorber because it is primarily responsible for the absorption in the wave number 400 to 600 cm-1 "window" region of Titan's infrared spectrum. The concentration of CH4, also an important absorber, is set by the saturation vapor pressure and hence is dependent on temperature. In this respect there is a similarity between the role of H2 and CH4 on Titan and that of CO2 and H2O on Earth. Titan also has an antigreenhouse effect that results from the presence of a high-altitude haze layer that is absorbing at solar wavelengths but transparent in the thermal infrared. The antigreenhouse effect on Titan reduces the surface temperature by 9 K whereas the greenhouse effect increases it by 21 K. The net effect is that the surface temperature (94 K) is 12 K warmer than the effective temperature (82 K). If the haze layer were removed, the antigreenhouse effect would be greatly reduced, the greenhouse effect would become even stronger, and the surface temperature would rise by over 20 K.