Io's Post-Eclipse Atmosphere: Evidence Against Atmospheric Collapse During Eclipse

Io's atmosphere is supported by a combination of direct volcanic injection of SO2 and SO2 frost sublimation from its surface. To determine the relative contributions of these two components on the overall density of the atmosphere, observations of Io's transition into and out of Jupiter eclipse provide useful constraints on the degree of frost sublimation support, as sublimation is a strong function of surface temperature which is highly affected by insolation. Previous observations of Io's atmosphere during ingress and egress have focused on atomic species seen in the far-ultraviolet. However, these auroral emissions are a product of magnetospheric particle interactions and therefore probe atmospheric density changes less directly and more globally. Here, we present 2011 Hubble Space Telescope Cosmic Origins Spectrograph, spectra of the 2100 - 2350 Å SO2 bands as Io exits eclipse. These SO2 bands directly probe Io's bulk atmosphere. Two separate orbits show no changes in the band depths from eclipse egress to 2 hours after egress. This is in contrast to previous observations of FUV atomic S and O line strengths decreasing in eclipse and increasing post-egress. Our observations might indicate the atmosphere facing Jupiter is dominated by volcanic emission rather than frost sublimation, though this is not easy to reconcile with the FUV results. We present a model of possible combinations of frost thermal inertia and albedo that might be consistent with a static atmosphere, and provide alternative explanations for the lack of response.