Unsteady flows in Io’s atmosphere caused by condensation and sublimation during and after eclipse: Numerical study based on a model Boltzmann equation

Abstract

The behavior of Io’s atmosphere during and after eclipse is investigated on the basis of kinetic theory. The atmosphere is mainly composed of sulfur dioxide (SO2) gas, which condenses to or sublimates from the frost of SO2 on the surface depending on the variation of surface temperature (∼90–114 K). The atmosphere may also contain a noncondensable gas, such as sulfur monoxide (SO) or oxygen (O2), as a minor component. In the present study, an accurate numerical analysis for a model Boltzmann equation by a finite-difference method is performed for a one-dimensional atmosphere, and the detailed structure of unsteady gas flows caused by the phase transition of SO2 is clarified. For instance, the following scenario is obtained. The condensation of SO2 on the surface, starting when eclipse begins, gives rise to a downward flow of the atmosphere. The falling atmosphere then bounces upward when colliding with the lower atmosphere but soon falls again. This process of falling and bounce back of the atmosphere repeats during the eclipse, resulting in a temporal oscillation of the macroscopic quantities, such as the velocity and temperature, at a fixed altitude. For a pure SO2 atmosphere, the amplitude of the oscillation is large because of a fast downward flow, but the oscillation decays rapidly. In contrast, for a mixture, the downward flow is slow because the noncondensable gas adjacent to the surface hinders the condensation of SO2. The oscillation in this case is weak but lasts much longer than in the case of pure SO2. The present paper is complementary to the work by Moore et al. (Moore, C.H., Goldstein, D.B., Varghese, P.L., Trafton, L.M., Stewart, B. [2009]. Icarus 201, 585–597) using the direct simulation Monte Carlo (DSMC) method.