The vertical structure of Titanʹs upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements

Data acquired by the Ion Neutral Mass Spectrometer (INMS) on the Cassini spacecraft during its close encounter with Titan on 26 October 2004 reveal the structure of its upper atmosphere. Altitude profiles of N2, CH4, and H2, inferred from INMS measurements, determine the temperature, vertical mixing rate, and escape flux from the upper atmosphere. The mean atmospheric temperature in the region sampled by the INMS is 149 ± 3   K , where the variance is a consequence of local time variations in temperature. The CH4 mole fraction at 1174 km is 2.71 ± 0.1 % . The effects of diffusive separation are clearly seen in the data that we interpret as an eddy diffusion coefficient of 4 −3 +4 × 10 9   cm 2 s −1 , that, along with the measured CH4 mole fraction, implies a mole fraction in the stratosphere of 2.2 ± 0.2 % . The H2 distribution is affected primarily by upward flow and atmospheric escape. The H2 mole fraction at 1200 km is 4 ± 1 × 10 −3 and analysis of the altitude profile indicates an upward flux of 1.2 ± 0.2 × 10 10   cm −2 s −1 , referred to the surface. If horizontal variations in temperature and H2 density are small, this upward flux also represents the escape flux from the atmosphere. The CH4 density exhibits significant horizontal variations that are likely an indication of dynamical processes in the upper atmosphere.