Published today in the journal Icarus's Articles in Press page is a new article by Chris Moore, D. B. Goldstein, P.L. Varghese, L.M. Trafton, and B. Stewart titled, "1-D DSMC Simulation of Io's Atmospheric Collapse and Reformation During and After Eclipse." Note that this article requires a personal or institutional subscription to access. The article presents a computer model of Io's atmosphere during an eclipse of the Sun by Jupiter and how non-condensable chemical species in the atmosphere effect gas number densities and collapse times.
Io's thin atmosphere is primarily composed of Sulfur dioxide (SO2), a chemical that also dominates Io's volcanic gases and great gray-white snow fields. The atmosphere of Io is basically in equilibrium vapor pressure with the surface. In other words, the colder the surface temperature, the more SO2 that is condensed out on the surface. The warmer the surface temperature, the more SO2 that sublimates into the atmosphere. In addition to SO2, Sulfur monoxide (SO) and molecular oxygen, disassociative products of SO2, are also present in the atmosphere at much smaller mole fractions.
The sudden onset of eclipse for the sub-Jupiter hemisphere brings dramatic surface temperature changes and with it atmospheric changes. Before an eclipse, the molecules (within one scale height of the surface, or 10 km) are well mixed with about 80% SO2 and 20% non-condensable species like SO and O2. Above 10 km, the molecules are not collisional and therefore have different scale heights depending on molecular weight. The general picture of what happens during an eclipse, as modeled by Moore et al. can be seen in the cartoon at left. As the eclipse begins, a significant amount of the SO2 in Io's lower atmosphere condenses onto the surface as the surface cools rapidly from 120K at the sub-solar point to ~105-110 K. SO2 in the upper atmosphere does not condense out because of the lag time for the upper atmosphere to sense the temperature change at the surface. This lag is further complicated by energy supplied by the plasma torus which keeps the upper atmosphere's temperature "artificially" inflated. As the eclipse progresses, more SO2 condenses out, but the rate slows as more plasma from the torus reaches the surface, keeping the temperature of the SO2 in the lower atmosphere inflated above what it normally would be if it were in perfect vapor pressure equilibrium. In addition, SO, enhanced in mole fraction as much of the SO2 in the lower atmosphere has condensed into a frost on the surface, forms a diffusion layer near the surface, preventing SO2 higher up in the atmosphere from condensing out. After the eclipse, the SO2 frost that had condense during the eclipse starts to sublimate. This vertical motion in the atmosphere pushes the diffusion layer SO higher up in the atmosphere. Within 30 minutes of the end of the eclipse, the lower atmosphere consists primarily of SO2 while the upper atmosphere is composed of 70% SO2 and 30% SO. Over the next half day, the atmosphere re-equlibrates back to the situation prior to the eclipse.
While the surface goes through a similar rapid drop in temperature at sunset, the greater length of night and lateral winds mean that the non-condensable species don't have the same effect as they do during an eclipse.
These changes in the atmosphere effect Io's auroral emission, seen in color by Galileo in 1998 (picture shown at top). Most of the auroral emission comes from the upper atmosphere. The fact that upper atmosphere does not condense is consistent with Geissler et al. 2001's observation that the auroral glows observed by Galileo do not dim with time elapsed since eclipse ingress.
Link: 1-D DSMC simulation of Io's atmospheric collapse and reformation during and after eclipse [dx.doi.org]