Hydrogen and methane in Titan's atmosphere: chemistry, diffusion, escape, and the Hunten limiting flux principle

One of Professor Donald M. Hunten's lasting contributions to the field of planetary atmospheres was the principle of the (Hunten) limiting flux, where the escape of light species is limited by the rate at which they can diffuse through the atmosphere. While his limiting flux expression has been well tested for hydrogen's escape from the Earth's atmosphere (e. g., Hunten and Strobel (J. Atmos. Sci. 31, 305 (1974)); Hunten and Donahue (Ann. Rev. Earth Planet Sci. 4, 265 (1976))), it has not been tested for Titan's atmosphere, which was the original motivation for the principle. The Cassini-Huygens mission has provided sufficient data on the variation of the H-2 mole fraction with altitude to test its applicability and validity. Only in the vicinity of the homopause does the limiting flux expression yield the actual H-2 escape flux, because the mole fraction varies with altitude. This paper deals also with our current understanding of the three major constituents of Titan's atmosphere (N-2, CH4, and H-2) from the various measurements by instruments on the Cassini orbiter and the Huygens probe. Specific problems addressed are additional required sources of H-2, the CH4 escape rate, and the possible role of energetic electron and ion precipitation from Saturn's magnetosphere.