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Thermal Evolution of Sublimated Water Vapor and Radiolytic Products in Callisto’s Atmosphere

Presentation #109.05D in the session “Icy Satellites: Surface and Above”.

Published onOct 26, 2020
Thermal Evolution of Sublimated Water Vapor and Radiolytic Products in Callisto’s Atmosphere

Results for 2D molecular kinetics models of single- and multi-component Callisto-like atmospheres are presented. The evolution of these neutral atmospheres is driven by the diurnal changes in surface temperatures, intermolecular collisions, and thermal escape.

Our recent study demonstrated the influence of collisions and thermal escape in single- and multi-component 1D Callisto-like neutral atmospheres. Therein we assumed that the constituents of the atmospheres (O2, CO2, H2) were radiolytic products which thermally desorb from the surface according to the local temperature and on returning to the surface they permeate the porous regolith and become trapped in the radiation-damaged ice. Using the direct simulation Monte Carlo (DSMC) method we calculated translational and internal energy exchanges via intermolecular collisions between test particles. Our results demonstrated that collisions can suppress or enhance H2 thermal escape relative to Jeans escape and collisions between the escaping H2 and the heavier background gases (O2 and CO2) affect the latter’s structure, producing non-isothermal profiles.

Here we expand our previous models to 2D to include the diurnal variation of surface temperatures, as well as include H2O sublimation, which is extremely sensitive to Callisto’s surface temperatures. An additional loss mechanism is implemented for H2 particles in these simulations as the Hill Sphere is set as the upper boundary. Thus, if particle trajectories exceed this boundary, even if they do not have sufficient energy to escape, they are still lost (to Jupiter). In 2D, local and global return and escape rates can be calculated as a means to better understand the distribution of Callisto’s neutral atmosphere. Moreover, thermal winds induced by collisions between particles originating from the warmer regions of the atmosphere with particles originating from the colder regions are observed.

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