Presentation #313.02 in the session “Io & Venus: The Surfaces and Atmospheres of Two Volcanic Worlds”.
We provide a progress report on the longitudinal and seasonal variations of Io’s SO2 dominated atmosphere, as documented by our 20-year ongoing program of consistent observations using the Texas Echelon Cross-Echelle Spectrograph (TEXES) instrument on the NASA Infrared Telescope Facility (IRTF), which began in November 2001. During this time Jupiter and Io have passed through aphelion twice, in 2005 and 2016, and perihelion once, in 2010. During the first Jovian year we detected a strong modulation of anti-Jupiter hemisphere 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 during the second Jovian year have so far shown similar behavior to the first year, with sustained low atmospheric density through perihelion comparable to the density seen a Jupiter year earlier. This repeatability reinforces our inference of a sublimation control. We look forward to monitoring the expected atmospheric peak at the upcoming 2023 perihelion with higher temporal resolution, to better understand how the atmosphere responds to perihelic warming of surface frost.