Io’s SO2 and NaCl Wind Fields from ALMA

Abstract

We present spatially resolved measurements of SO _2 and NaCl winds on Io at several unique points in its orbit: before and after eclipse and at maximum eastern and western elongation. The derived wind fields represent a unique case of meteorology in a rarified, volcanic atmosphere. Through the use of Doppler shift measurements in emission spectra obtained with the Atacama Large Millimeter/submillimeter Array between ~346 and 430 GHz (~0.70–0.87 mm), line-of-sight winds up to ~−100 m s ^−1 in the approaching direction and >250 m s ^−1 in the receding direction were derived for SO _2 at altitudes of ~10–50 km, while NaCl winds consistently reached ~∣150–200∣ m s ^−1 in localized regions up to ~30 km above the surface. The wind distributions measured at maximum east and west Jovian elongations and on the sub-Jovian hemisphere pre- and posteclipse were found to be significantly different and complex, corroborating the results of simulations that include surface temperature and frost distribution, volcanic activity, and interactions with the Jovian magnetosphere. Further, the wind speeds of SO _2 and NaCl are often inconsistent in direction and magnitude, indicating that the processes that drive the winds for the two molecular species are different and potentially uncoupled; while the SO _2 wind field can be explained through a combination of sublimation-driven winds, plasma torus interactions, and plume activity, the NaCl winds appear to be primarily driven by the plasma torus.

Keywords

• Io
• Galilean satellites
• Radio astronomy
• Millimeter astronomy
• Submillimeter astronomy
• Atmospheric dynamics