1D DSMC simulation of Io's atmospheric collapse and reformation during and after eclipse
Abstract
A onedimensional Direct Simulation Monte Carlo (DSMC) model is used to examine the effects of a noncondensable species on Io's sulfur dioxide sublimation atmosphere during eclipse and just after egress. Since the vapor pressure of SO _{2} is extremely sensitive to temperature, the frostsupported dayside sublimation atmosphere had generally been expected to collapse during eclipse as the surface temperature dropped. For a pure SO _{2} atmosphere, however, it was found that during the first 10 min of eclipse, essentially no change in the atmospheric properties occurs at altitudes above ∼100 km due to the finite ballistic/acoustic time. Hence immediately after ingress the auroral emission morphology above 100 km should resemble that of the immediate preeclipse state. Furthermore, the collapse dynamics are found to be greatly altered by the presence of even a small amount of a noncondensable species which forms a diffusion layer near the surface that prevents rapid collapse. It is found that after 10 min essentially no collapse has occurred at altitudes above ∼20 km when a nominal mole fraction of noncondensable gas is present. Collapse near the surface occurs relatively quickly until a static diffusion layer many mean free paths thick of the noncondensable gas builds up which then retards further collapse of the SO _{2} atmosphere. For example, for an initial surface temperature of 110 K and 35% noncondensable molefraction, the ratio of the SO _{2} column density to the initial column density was found to be 0.73 after 10 min, 0.50 after 30 min, and 0.18 at the end of eclipse. However, real gas species (SO, O _{2}) may not be perfectly noncondensable at Io's surface temperatures. If the gas species was even weakly condensable (nonzero sticking/reaction coefficient) then the effect of the diffusion layer on the dynamics was dramatically reduced. In fact, if the sticking coefficient of the noncondensable exceeds ∼0.25, the collapse dynamics are effectively the same as if there were no noncondensable present. This sensitivity results because the loss of noncondensable to the surface reduces the effective diffusion layer size, and the formation of an effective diffusion layer requires that the layer be stationary; this does not occur if the surface is a sink. Upon egress, vertical stratification of the condensable and noncondensable species occurs, with the noncondensable species being lifted (or pushed) to higher altitudes by the sublimating SO _{2} after the sublimating atmosphere becomes collisional. Stratification should affect the morphology and intensity of auroral glows shortly after egress.
 Publication:

Icarus
 Pub Date:
 June 2009
 DOI:
 10.1016/j.icarus.2009.01.006
 Bibcode:
 2009Icar..201..585M