Exomoon Phase Curves: Toroidal Exosphere Simulations of Exo‐Ios Orbiting 8 Exoplanets in Alkali Spectroscopy

• Published in: Journal of geophysical research. Planets Vol. 129; no. 3
• Main Authors: Meyer zu Westram, M., Oza, A. V., Galli, A.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-03-2024
• Subjects: Algorithms; Alkali metals; Astronomy; Charged particles; Clouds; Computer simulation; Debugging; evaporative exospheres; Evolution; exomoons; exoplanets; Exosphere; Extrasolar planets; Gas density; Jupiter; Monte Carlo simulation; Monte‐Carlo; Moon; Natural satellites; Outgassing; Photoionization; Planetary atmospheres; Planetary orbits; plasma torus; Potassium; Ring structures; Satellite observation; Satellite tracking; Satellites; Sodium; Software engineering; Solar system; Spatial variability; Spectroscopy; Spectrum analysis; Sulfur; Sulfur dioxide; Surface temperature; Toruses
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2023JE007935

Toroidal atmospheres and exospheres characterized at exoplanets may be fueled by volcanically active exomoons, often referred to as exo‐Ios. We study the neutral outgassing and volatile evolution of a close‐orbiting, evaporating satellite at eight candidate exoplanet‐exomoon systems WASP‐49,‐96,‐69,‐17 b, XO‐2N b, HAT‐P‐1 b, HD‐189733 b, and HD‐209458 b by developing a 3‐D test‐particle Monte Carlo simulation, Simulating the Evolution of Ring Particles Emergent from Natural Satellites. The module is coupled to dishoom, approximating the minimum mass‐flux needed to reproduce observations of alkali line profiles identified in dozens of transmission spectra. We focus on sputtered neutral sodium, limited by photoionization and radiative effects. By considering Earth‐, Io‐, and Enceladus‐like masses, we systematically simulate the imprint of a non‐hydrostatic medium (characteristic of volcanic exospheres) in density and velocity space using a novel Delaunay tesselation field estimator algorithm. Our results demonstrate how exomoons can considerably modulate gas density observations probed near exoplanet transit, depending on the orbital phase of the putative satellite at the time of observation. The density evolution, therefore, manifests on orbital timescales as “exomoon phase curves” from shadow to occultation. We find two regimes of density evolution, characteristic of a: (a) localized cloud and (b) an azimuthally symmetric exoring/torus, degenerate with an exoplanet atmosphere, ranging from ∼109.5±0.5 cm−2 to ∼1015±0.25 cm−2 at our leading candidate WASP‐69b I. In certain orbital architectures, the smallest evaporating satellite mass surprisingly generates the brightest sodium signal, fueling optimism for discovering photometrically indiscernible rocky exomoons. We suggest long baseline monitoring of alkali and SO2 systems in spectroscopy to search for the temporal and spatial variability predicted here.
Plain Language Summary
Intense gravitational tidal forces from Jupiter on its rocky moon Io, drives rock melting and over a ∼ kiloton/second of sulfur dioxide and alkali metal volcanism. Io is therefore the most geologically active body in the Solar System, with infrared‐bright lavas despite its surface temperature of −130°C. The roughly lunar‐sized moon is bombarded by charged particle radiation that knocks off volcanic material, thereby fueling a conducting ring of plasma orbiting Jupiter every ∼10 hr along with extended clouds of sodium and potassium radiating across the electromagnetic spectrum. Motivated by dozens of observations of alkalis at exoplanets, we posit that some of these systems may host hidden extrasolar Ios. Exo‐Ios at 8 exoplanet systems are simulated with a novel code in 3‐D (SERPENS) treating Earth, Io, and Enceladus‐mass satellites. Our open‐source software tracks the evolution of orbiting neutral particles and simulates high‐velocity banana‐shaped clouds and extrasolar ring structures, consistent with data. We urge the astronomy community to search for exo‐Ios, and have provided exomoon phase curves, predicting the disappearance of metallic signatures due to the temporal and spatial variability modeled. Our results are surprisingly optimistic in that the smallest exomoon mass generates the brightest signal in certain scenarios, not unlike Saturn's E‐ring.
Key Points
A 3‐D MC code simulates the sputtering & outgassing of an exomoon at 8 candidate systems to provide density maps of the neutral environment
Exomoon phase‐curves that is, spatial & temporal variations of orbiting alkali clouds & tori imprinted in transmission spectra are predicted
Our open access module Simulating the Evolution of Ring Particles Emergent from Natural Satellites allows one to simulate the evaporation of an exomoon and its ability to form a volcanic exoring/exosphere