Titan's upper atmosphere: Composition and temperature from the EUV solar occultation results

The temperature and composition of the upper atmosphere of Titan have been inferred by observing an occultation of the sun by Titan, using the Voyager 1 ultraviolet spectrometer. The temperature is 176±20 K near the evening terminator and 196±20 K near the morning terminator. The major constituent is N2 with a density of 2.7±0.2 × 108 cm−3 at 3840 km. The mixing ratio of CH4 is 8±3% at a radial distance of 3700 km near the evening terminator where [CH4] ≃1.2 × 108 cm−3. On the morning terminator the [CH4] ≃ 1.2 × 108 cm−3 level is about 20 km lower in the atmosphere. The acetylene mixing ratio above 3400 km is measured at the 1 to 2% level. Below 3300 km it decreases to between 0.1 and 0.3%. A layer of absorbing molecules, possibly polymers, is present at both morning and evening terminators. Near the evening terminator the layer is located between 3350 and 3600 km. Near the morning terminator it is located about 100 km lower in the atmosphere. A simple photochemical model suggests that the homopause is located at 3500 ± 70 km with an eddy diffusion coefficient of 1−0.7+2 × 108 cm² s−1, which decreases to ∼10³ cm² s−1 in the lower stratosphere as [N2]−2/3.