Investigating Io’s Bulk Atmosphere with Jupiter Eclipses: Conflicting Results on Support Mechanisms?

The moon Io is home to a tenuous atmosphere of molecular SO₂ created from the global and continuous volcanic activity. A pervasive and long lasting question regarding this spatially inhomogeneous atmosphere is whether it is supported primarily by volcanic emission, or by the sublimation of the large reservoir of SO₂ frost found on the surface, and what the relative contributions are between these mechanisms. The source and variability of this SO₂ is important because it significantly contributes to the Io torus and surrounding environment.

Here, we discuss observations of Io’s atmosphere entering and exiting Jupiter eclipse, which helps separate the sublimation component of the atmosphere in vapor pressure equilibrium with the surface from the volcanic component. We present HST-COS UV (220nm) eclipse egress spectra taken in December 2011, and Gemini-TEXES mid-IR (19µm) eclipse ingress spectra from November 2013 at the beginning of JAXA’s Sprint-A/EXCEED mission. The UV spectra do not display significant variation coming out of eclipse, as would be expected if the atmosphere is in vapor pressure equilibrium. The mid-IR spectra, which are the first ever observation of Io’s bulk atmosphere going into and in eclipse, however do show significant change in the SO₂ absorption bands of SO₂. We will discuss the implications of these apparently contradictory results for the mechanisms supporting Io’s atmosphere.