Presentation #522.01 in the session Diversity of Planets and Planetary Systems (iPosters).
Molecular diffusion limits the thermal escape of minor atmospheric species in collisional atmospheres (1). For example, hydrogen escape from the atmosphere of Titan appears to be determined by its diffusion through N2 (1, 2). The so-called diffusion-limited escape rate is typically applied to the flux of a minor light gas through an isothermal stationary background atmosphere (3). The flux is proportion to the ratio of the mixing fraction and atmospheric averaged scale height evaluated at the homopause. However, at altitudes above the homopause molecular collisions between light and heavy molecules can affect the population of escaping atoms. Above the homopause the atmosphere becomes increasingly tenuous and rarefied gas dynamic (RGD) simulations are required to model the diffusive flow. Tucker et al. (2013) simulated the H2 diffusive flow flux for the atmosphere of Titan with an RGD model and obtained an escape rate 50 % larger than the diffusion-limited escape estimate. However, to date there has not been a systematic study of rarefaction effects on the diffusion-limited escape flux. We will present RGD results on the effect of atmospheric rarefaction on the diffusion-limited escape rate as a function of mixing ratio, Jeans parameter (gravitational energy / thermal energy) and atmospheric density.
1. Hunten DM. 1973. J. Atmos. Sci. 30:1482–94
2. Tucker OJ, Johnson RE, Deighan JI, Volkov AN. 2013. Icarus. 222(1):149–58
3. David C. Catling JFK, Catling DC, Kasting JF. 2017. Atmospheric Evolution on Inhabited and Lifeless Worlds. Cambridge: Cambridge University Press