Greenhouse effect was first noted by Joseph Fourier in 1824 and investigated quantitatively by Svante Arrhenius in 1896. The term refers the phenomenon whereby gases in the Earth’s atmosphere (principally the triatomic molecules H2O and CO2) are much more transparent to incoming visible sunlight than to the infrared re-radiation from the surface. This causes the mean temperatures in the lower atmosphere and surface to be elevated relative to an airless world in equilibrium with solar flux: by about 30 K for the surface. Similarly, much greater and lesser greenhouse conditions are seen on Venus (500 K) and Mars (5 K) due to their CO2 atmospheres. On the Earth, an additional feedback loop exists between the condensable species H2O, which is the main greenhouse gas, and the second most important CO2, whose concentration is not limited by saturation. Greenhouse effect this occurs due to the steep dependence of the saturation vapor pressure (SVP) on temperature: small rises in atmospheric temperature due to increased CO2 levels (e. g. by pollution) are amplified by the greater amounts of water vapor that subsequently are retained in the atmosphere greenhouse effect.
On Titan, a parallel situation exists. The atmosphere is dominated by N2 (95-98.6%), with significant amounts of CH4 (5-1.4%) and H2 (0.1%). The action of solar UV photons and energetic particles in the upper atmosphere causes photolysis of methane and nitrogen, leading to diverse organic compounds, including ethane (C2H6), HCN and higher organics including aromatics such as benzene (C6H6), and ultimately macromolecules and organic haze particles. The infrared opacity is mainly due to collision-induced absorption of N2-N2, N2-H2 and especially CH4-N2 pairs: a significant departure from the triatomic absorption that causes greenhouse warning of the inner planets. However the effect is substantial: Titan’s retains 90% of surface radiation, intermediate between the prodigious Venus (99.9%), and the Earth (60%), raising its surface temperature from 82 K to 94 K. A feedback loop also exists, whereby increases in H2 raise the infrared opacity and thereby the greenhouse warming, causing more CH4 to be retained in the atmosphere, still greater opacity, and further warming, greenhouse effect.
Greenhouse effect in fact, the surface would be even warmer still (105 K) if it were not for a competing radiation process: the so-called ‘antigreenhouse effect’. As the name implies, this phenomenon is the reverse of the greenhouse effect: an atmospheric component that absorbs at visible wavelengths but is transparent in the thermal infrared. On Titan, the main antigreenhouse effect is due to the ubiquitous haze layers that obscure the surface at most visible wavelengths, with an additional component due to the near-infrared absorption bands of CH4. On Earth, O3 currently contributes a small antigreenhouse cooling, but in the recent past volcanic eruptions have measurably cooled the planet, and historically large impacts such as the proposed K-T event must have caused far greater global temperature drops. Titan’s haze is therefore a major component of the atmospheric energy balance, and so chemistry as well as physical processes must be considered in this complex system greenhouse effect .