Vertical Temperature Structure in Io's Atmosphere from ALMA SO$_2$ Observations

TL;DR

This study used ALMA spectroscopy to derive the first detailed vertical temperature profiles of Io's atmosphere, revealing a cold lower atmosphere and a rising thermosphere, thus providing new insights into Io's thermal energy balance.

Contribution

It presents the first observationally-derived vertical temperature profiles of Io's atmosphere using multi-line ALMA data and advanced modeling techniques to separate thermal and dynamical effects.

Findings

Lower atmosphere temperature: 124-137 K up to 0.5 nbar

Thermospheric temperature rise to hundreds of kelvins

Vertical structure consistent across datasets, with hemispheric differences

Abstract

The structure of Io's atmosphere is controlled by competing processes, from volcanic outgassing and sublimation to radiative cooling and plasma heating. Yet, the lack of an observationally-derived temperature profile has left this balance unconstrained. We used four epochs of Atacama Large Millimeter/submillimeter Array (ALMA) Band 7 (275-373 GHz) and Band 8 (385-500 GHz) SO$_2$ spectroscopy to retrieve Io's vertical atmospheric temperature profiles. To mitigate longstanding degeneracies common in atmospheric retrievals, we performed a simultaneous multi-line analysis combined with line-of-sight disk-resolved Doppler velocity maps and a forward model that included a sub-beam velocity-dispersion term. This modeling approach enabled the separation of thermal and dynamical line-shape contributions. On the leading hemisphere, we retrieved a cold, quasi-isothermal lower atmosphere ($\sim$124-137~K up to $\sim$0.5~nbar), followed by a thermospheric rise reaching hundreds of kelvins by $\sim10^{-2}$~nbar. On the trailing hemisphere, our fits yielded qualitatively similar profiles but consistently retrieved lower SO$_2$ column densities. The lower column densities confined line formation to the first few kilometers, making the trailing hemisphere spectra statistically consistent with an isothermal atmosphere. Across datasets, we retrieved fractional gas coverages of $\sim$35-50$\%$ and sub-beam velocity dispersions of $\sim$25-85$\mathrm{~m~s^{-1}}$, encoding line-of-sight velocity dispersion within a beam element in excess of the disk-resolved Doppler velocity map. Together, these retrievals deliver the first vertically resolved temperature profiles of Io's atmosphere, reveal robust vertical structure on the dayside leading hemisphere, and offer new constraints on Io's thermal energy balance.