The Structure of Titan’s N₂ and CH₄ 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 1.2 10 25 s −1 and2.2 10 27 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.

Monte Carlo calculations of the atmospheric sputtering yields on Titan

“…With such a choice, the total number of escaping CH 4 molecules from Titan's upper atmosphere remains above 400 under normal incidence for which the sputtering yield is expected to be minimized (Johnson et al 2000), ensuring that statistically robust results with a typical uncertainty of no more than 5% be achieved for all model runs. The number of escaping N 2 molecules should be substantially larger since the N 2 column density is more than a factor of 3 higher than the CH 4 column density above Titan's exobase (De La Haye et al 2007;Jiang et al 2017).…”

“…As stated in Sect. 1, the issue of CH 4 escape on Titan is still under debate and it is crucial to carefully evaluate the relative contributions from a range of escaping mechanisms including atmospheric sputtering studied here (e.g., De La Haye et al 2007;Yelle et al 2008;Strobel 2009;Tucker & Johnson 2009;Bell et al 2010Bell et al , 2011Cui et al 2012;Schaufelberger et al 2012;Jiang et al 2017). For illustrative purposes, we show in Fig.…”

Atmospheric composition of exoplanets based on the thermal escape of gases and implications for habitability

“…Continuous supply from these primordial sources from early formation times resulted in the large amounts of N 2 observed in its present atmosphere. N 2 escape from Titan's atmosphere is an existing process as a result of various mechanisms and has been observed (as well as CH 4 escape) by Cassini in Titan's corona [51]. Because of its low gravitational potential, the Galilean moon Europa is incapable of holding most atmospheric species, including N 2 , in its atmosphere, leading to a tenuous atmosphere.…”

Present-day atomic hydrogen escape on Mars and its variability

“…对于可能存在的"非热"成分, 常规做法依然是使 用金斯公式来计算逃逸通量 [139,142,143] , 即假设"非热" 成分也满足MB分布. 然而值得注意的是, 这并不意 味着"非热"成分逃逸在本质上依然是金斯逃逸.…”