The Structure of Titan’s N 2 and CH 4 Coronae

Abstract

In this study, we analyze the structures of Titan’s N 2 and CH 4 coronae using a large data set acquired by the Ion Neutral Mass Spectrometer (INMS) instrument on board Cassini . The N 2 and CH 4 densities measured from the exobase up to 2000 km imply a mean exobase temperature of 146 K and 143 K, respectively, which is lower than the mean upper atmospheric temperature by 4 and 7 K. This indicates that on average, Titan possesses a subthermal rather than suprathermal corona. A careful examination reveals that the variability in corona structure is not very likely to be solar driven. Within the framework of the collisionless kinetic model, we investigate how the CH 4 energy distribution near the exobase could be constrained if strong CH 4 escape occurs on Titan. Several functional forms for the CH 4 energy distribution are attempted, assuming two representative CH 4 escape rates of s −1 and s −1 . We find that the double Maxwellian and power-law distributions can reproduce the shape of the CH 4 corona structure as well as the imposed CH 4 escape rate. In both cases, the escape rate is contributed by a suprathermal CH 4 population on the high-energy tail, with a number fraction below 5% and a characteristic energy of 0.1–0.6 eV per suprathermal CH 4 molecule. The coexistence of the subthermal CH 4 corona revealed by the INMS data and substantial CH 4 escape suggested by some previous works could be reconciled by a significant departure in the exobase CH 4 energy distribution from ideal Maxwellian that enhances escape and causes a noticeable redistribution of the corona structure.