Non-detection of post-eclipse changes in Io's Jupiter-facing atmosphere: Evidence for volcanic support?

We present observations of Io's tenuous lower atmosphere on the Jupiter-facing hemisphere at ultraviolet wavelengths, from 2100 to 2250 Å, using the Hubble Space Telescope's Cosmic Origins Spectrometer, during the period immediately after Io emerges from Jupiter eclipse. The density of an atmosphere dominantly controlled by frost sublimation is highly dependent on the thermal properties of the surface frost and frost temperature, and the changing solar illumination conditions as Io enters and exits eclipses allow us to study the atmospheric response due to frost sublimation and its relative role compared to volcanic output. These new eclipse emergence observations were obtained to better understand the relative roles that frost sublimation and volcanoes play in supporting the overall atmosphere on Io. The observations were obtained on two separate dates, and in both cases, we observe Io post Jupiter eclipse, from approximately 2 min after egress to approximately 2 h post-egress. On both of these occasions, the atmospheric density was observed to be constant with time to within 15%, implying that the atmosphere did not collapse while in Jupiter eclipse. We use a thermal model to explore combinations of volcanic support and sublimation support with a variety of SO2 frost thermal inertias and bond albedos, which are consistent with these new observations. We find that a pure sublimation-supported atmosphere with a single surface component (100% frost coverage) is difficult to reconcile with the observed lack of post-eclipse atmospheric changes, unless the SO2 frost thermal inertia is very high (⩾500 MKS) accompanied with moderate bond albedos (∼0.55). However, atmospheric support by high thermal inertia frost is difficult to reconcile with the dramatic cooling of much of Io's surface in eclipse, which should lead to condensation of the atmosphere on the cold surface components. Alternatively, a Jupiter-facing atmosphere entirely dominated by volcanic emissions in which frost sublimation plays only a minor role fits the data well. The results are surprising given the previous observations of reduced FUV auroral emissions in eclipse that suggest that the atmosphere does collapse in eclipse. Future improvements to incorporate non-frost components into the thermal model will help improve our interpretations.