Exploring Io's atmospheric composition with APEX: first measurement of 34SO2 and tentative detection of KCl

TL;DR

This study uses APEX observations to detect and analyze molecular species in Io's atmosphere, including the first tentative detection of KCl and measurement of 34SO2 isotopic ratio, providing insights into volcanic gas composition.

Contribution

It presents the first tentative detection of KCl and measures the 34S/32S isotopic ratio in Io's atmosphere using spectral data from APEX, expanding knowledge of its molecular composition.

Findings

Tentative detection of KCl with a relative abundance of 4x10^{-4} to 8x10^{-3}

First measurement of 34S/32S isotopic ratio in Io's gas phase, twice Earth's and ISM values

Upper limits set for SiO and S2O abundances

Abstract

The composition of Io's tenuous atmosphere is poorly constrained. Only the major species SO2 and a handful of minor species have been positively identified, but a variety of other molecular species should be present, based on thermochemical equilibrium models of volcanic gas chemistry and the composition of Io's environment. This paper focuses on the spectral search for expected yet undetected molecular species (KCl, SiO, S2O) and isotopes (34SO2). We analyze a disk-averaged spectrum of a potentially line-rich spectral window around 345 GHz, obtained in 2010 at the APEX-12m antenna (Atacama Pathfinder EXperiment). Using different models assuming either extended atmospheric distributions or a purely volcanically-sustained atmosphere, we tentatively measure the KCl relative abundance with respect to SO2 and derive a range of 4x10^{-4}-8x10^{-3}. We do not detect SiO or S2O and present new upper limits on their abundances. We also present the first measurement of the 34S/32S isotopic ratio in gas phase on Io, which appears to be twice as high as the Earth and ISM reference values. Strong lines of SO2 and SO are also analyzed to check for longitudinal variations of column density and relative abundance. Our models show that, based on their predicted relative abundance with respect to SO2 in volcanic plumes, both the tentative KCl detection and SiO upper limit are compatible with a purely volcanic origin for these species.