High Spatial and Spectral Resolution Observations of the Forbidden 1.707 micron Rovibronic SO Emissions on Io: Evidence for Widespread Stealth Volcanism

TL;DR

This study uses high-resolution observations from Keck telescopes to analyze forbidden SO emissions on Io, revealing widespread stealth volcanism and complex non-LTE processes influencing the emissions.

Contribution

It provides the first detailed spatial and spectral analysis of SO emissions on Io, suggesting the presence of stealth plumes and non-LTE effects not previously documented.

Findings

SO emissions are spatially variable and often not aligned with known volcanoes.

Evidence of stealth plumes produced by silicate melt interactions at depth.

Spectral features indicate non-LTE processes in the emission mechanisms.

Abstract

We present observations obtained with the 10-m Keck telescopes of the forbidden SO rovibronic transition at 1.707 micron on Io while in eclipse. We show its spatial distribution at a resolution of ~0.12" and a spectral resolution of R ~2500, as well as disk-integrated spectra at a high spectral resolution (R~15,000). Both the spatial distribution and the spectral shape of the SO emission band vary considerably across Io and over time. In some cases the SO emissions either in the core or the wings of the emission band can be identified with volcanoes, but the largest areas of SO emissions usually do not coincide with known volcanoes. We suggest that the emissions are caused by a large number of stealth plumes, produced through the interaction of silicate melts with superheated SO2 vapor at depth. The spectra, in particular the elevated wing of the emission band near 1.69 micron, and their spatial distribution strongly suggest the presence of non-LTE processes in addition to the direct ejection of excited SO from the (stealth and other) volcanic vents.