Sodium and Potassium Signatures of Volcanic Satellites Orbiting Close-in Gas Giant Exoplanets

TL;DR

This paper proposes that sodium and potassium gas signatures could indicate volcanic activity on exomoons like Io orbiting close-in gas giants, with implications for detecting hidden satellites through transmission spectra.

Contribution

It introduces a model linking exo-Io volcanic gas signatures to detectable sodium and potassium signals, suggesting stable orbits and significant atmospheric loss rates for such satellites.

Findings

Exo-Io can be stable against orbital decay for certain tidal quality factors.

Exo-Io's gas loss rates can be orders of magnitude higher than Io's, producing detectable sodium signatures.

Observed sodium column densities in exoplanet atmospheres can be explained by volcanic activity on hidden exomoons.

Abstract

Extrasolar satellites are generally too small to be detected by nominal searches. By analogy to the most active body in the Solar System, Io, we describe how sodium (Na I) and potassium (K I) $\textit{gas}$ could be a signature of the geological activity venting from an otherwise hidden exo-Io. Analyzing $\sim$ a dozen close-in gas giants hosting robust alkaline detections, we show that an Io-sized satellite can be stable against orbital decay below a planetary tidal $\mathcal{Q}_p \lesssim 10^{11}$. This tidal energy is focused into the satellite driving a $\sim 10^{5 \pm 2}$ higher mass loss rate than Io's supply to Jupiter's Na exosphere, based on simple atmospheric loss estimates. The remarkable consequence is that several exo-Io column densities are on average $\textit{more than sufficient}$ to provide the $\sim$ 10$^{10 \pm 1}$ Na cm$^{-2}$ required by the equivalent width of exoplanet transmission spectra. Furthermore, the benchmark observations of both Jupiter's extended ($\sim 1000$ R$_J$) Na exosphere and Jupiter's atmosphere in transmission spectroscopy yield similar Na column densities that are purely exogenic in nature. As a proof of concept, we fit the "high-altitude" Na at WASP 49-b with an ionization-limited cloud similar to the observed Na profile about Io. Moving forward, we strongly encourage time-dependent ingress and egress monitoring along with spectroscopic searches for other volcanic volatiles.