Plasma-neutral Interactions as a Method of Constraining Stable Rocky Exoplanet Atmospheres

As we are on the verge of receiving high quality spectra of many exoplanetary atmospheres, the next major challenge is discerning the underlying atmospheric characteristics, such as the surface pressure and temperature profile. Forward modeling of rocky exoplanet atmospheres shows that many different states can produce similar absorption spectra, and thus backing out the "true" state from an observation will be a difficult task that may require the influx of additional information. By considering the implications of a given atmospheric profile on processes such as atmospheric escape and interaction with the stellar wind, additional constraints may be used to confine the valid parameter space of atmosphere states. As a case study, we consider the rocky exoplanet TRAPPIST-1h and, assuming a Titan-like atmosphere, use a 1D diffusion model to explore the parameter space by varying temperature, surface pressure, and eddy diffusion coefficient, to produce over 2000 atmosphere profiles. We then use the GAMERA 3D magnetohydrodynamic (MHD) model to obtain a steady-state plasma solution, and calculate atmospheric loss rates via charge exchange for a subset of the atmosphere profiles. We find that the calculated loss rate scales most strongly with the atmospheric temperature, and that certain atmosphere profiles can be ruled out due to the loss rate being too high to maintain a stable atmosphere. We also discuss the prospect of observing, via Lyman-alpha absorption, the energetic neutral atoms produced via charge exchange. If detectable, this measurement would serve as a powerful tool in estimating the charge exchange rate and therefore the plasma and neutral atmosphere densities and spatial distributions.