Exo-Io Simulations of Toroidal Exospheres

Although exomoons, natural satellites beyond our solar system, are still undetectable in direct searches with state-of-the-art instruments, their existence has been hypothesized to explain various inconsistencies in exoplanetary spectra. Exogenic sources of sodium and potassium have been considered at multiple exoplanets, where abundances exceed the exoplanet’s source rates, and hydrostatic exoplanet atmospheres are limited in their ability to explain increased line broadening seen in Na & K spectra, where an orbiting body naturally provides broadening with variable ±∼10-20 km/s.
A semi-analytic atmospheric escape and evolution model dishoom approximates the minimum mass flux needed for an exomoon to provide volcanic material for the absorption of star light. We develop a 3-D test-particle Monte Carlo simulation module called SERPENS (Simulating the Evolution of Ring Particles Emergent from Natural Satellites) to be coupled to dishoom. SERPENS is designed to be highly adaptive, open-source, and easy to use. We simulate the neutral outgassing and evolution of a satellite at multiple candidate exoplanet-exomoon systems including HD189733 b II, HD209458 b I, WASP-49 A b I, HAT-P-1 b I, and WASP-96 b I, in order to provide a number density n[cm−3] and line-of-sight column density N[cm−2] map of the particle environment in a non-hydrostatic medium, characteristic of a volcanic exosphere akin to Jupiter’s Na exosphere fueled by Io. The neutral species maps are then fed into a non-hydrostatic radiative transfer model, Prometheus, which computes an exospheric spectrum that can be directly compared to ongoing ground and space-based spectra of candidate exomoon systems. We model masses ranging from Earth, Io, and Enceladus to emulate long-term effects of mass loss and present the respective particle distributions. Photoionization is set as the prime constraint for the lifetime of atoms and molecules.
In contrast to previous works, our code SERPENS focuses on exomoons and their imprint as a neutral
and plasma torus. SERPENS is designed to eject particles via sputtering and thermal evaporation at
regular time intervals allowing us to simulate an evolving cloud/torus. Multiple species including Na, K
and SO2, as well as their chemical networks, are supported.
Our results demonstrate how exomoons similar to Io, referred to as exo-Ios, can affect line-of-sight column densities depending on the phase of the exomoon at the time of observation. This means that it is possible to model time-variable spectra by taking into account the phase of the exomoon.