Presentation #112.03 in the session Io.
We have been monitoring the longitudinal and seasonal variations of Io’s SO2 dominated atmosphere for 21 years, using consistent observations of its 19 μm absorption features with the Texas Echelon Cross-Echelle Spectrograph (TEXES) instrument on the NASA Infrared Telescope Facility (IRTF). During this time Jupiter and Io have passed through aphelion twice, in 2005 and 2016, and perihelion once, in 2010. The next Jupiter perihelion is imminent, in January 2023. During the first Jovian year we detected a strong modulation of atmospheric density with both longitude and heliocentric distance, with a denser atmosphere on the anti-Jovian hemisphere and a large density peak near perihelion (Tsang et al. 2013, Icarus 226, 1177). The perihelion peak provides strong evidence for sublimation support of the atmosphere, at least on the anti-Jovian hemisphere, and constrains the thermal inertia of the surface frost. Continued observations through aphelion during the second Jovian year have shown similar behavior to the first year, with sustained low atmospheric density comparable to the density seen one Jupiter year earlier. This repeatability reinforces our inference of dominant sublimation control. We will report on recent observations, through July 2022, of Io’s atmospheric response to the approaching 2023 perihelion, with improved time resolution than at the previous perihelion, to better understand how the atmosphere responds to the warming of surface frost.