Io's SO2 and NaCl Wind Fields From ALMA

TL;DR

This study uses ALMA to measure and analyze the complex wind fields of SO2 and NaCl on Io, revealing differences in wind behavior and driving mechanisms across various orbital positions.

Contribution

First spatially resolved measurements of Io's SO2 and NaCl winds at different orbital points, showing distinct wind patterns and driving processes for each molecular species.

Findings

SO2 winds reach up to 250 m/s in receding direction

NaCl winds reach up to 200 m/s in localized regions

Wind patterns vary significantly with Io's orbital position

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 (ALMA) between $\sim346$-430GHz ($\sim0.70$-0.87 mm), line-of-sight winds up to $\sim-100$ m s$^{-1}$ in the approaching direction and >250 m s$^{-1}$ in the receding direction were derived for SO$_2$ at altitudes of $\sim10$-50 km, while NaCl winds consistently reached $\sim$|150-200| m s$^{-1}$ in localized regions up to $\sim30$ km above the surface. The wind distributions measured at maximum east and west Jovian elongations, and on the subJovian hemisphere pre- and post-eclipse, 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.