DIX Planetary Science Seminar
The "cosmic shoreline", a semi-empirical relation that separates airless worlds from worlds with atmospheres, as proposed Zahnle & Catling (2017), is now guiding large-scale JWST surveys aimed at detecting rocky exoplanet atmospheres. We revisit the shoreline by applying existing hydrodynamic escape models applied to Earth-like, Venus-like, and steam atmospheres for rocky exoplanets, and we estimate energy-limited escape rates for CH4 atmospheres. We determine the critical stellar irradiation required for atmospheric retention by calculating time-integrated atmospheric mass loss. Our analysis introduces a new metric for target selection in the Rocky Worlds DDT. Exploring volatile abundances between 0.01% and 1% of planetary mass, we find that any significant variation prevents the definition of a clear-cut shoreline. Additionally, uncertain distributions of high-energy stellar evolution, planet age and formation timing further blur the critical stellar irradiation for atmospheric retention, yielding a broad shoreline. Hydrodynamic escape models find atmospheric retention is markedly more favorable for higher-mass planets orbiting higher-mass stars, remaining plausible for 55 Cancri e despite its extreme stellar irradiation. Dedicated modeling efforts are needed to better constrain the escape dynamics of secondary atmospheres, such as the role of atomic line cooling, especially for Earth-sized planets. Finally, we illustrate how bulk density measurements can be used to statistically test the existence of the cosmic shorelines, emphasizing the need for more precise mass and radius measurements.
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Io's atmosphere is spatially variable in both composition and density, and these variations produce distinct auroral morphologies as the atmosphere interacts with the jovian plasma torus. However, the secular variability of Io's atmospheric composition and the properties of the torus electrons exciting the aurora are not well constrained and in situ data are limited to brief spacecraft flybys. Over the past year, I've used Io's visible wavelength auroral emissions to provide a remote sensing window into its atmosphere and how it interacts with the plasma torus. Using high-resolution spectra of Io's atmospheric emission in eclipse by Jupiter observed from Maunakea, I detected new visible and near-infrared emission lines from atomic neutrals and ions, increasing the number of identified atomic emissions by a factor of 3. I interpreted these emissions in conjunction with broadband images taken during the Cassini flyby of the Jovian system to understand the spatial variability in the observed auroral brightness. I used an emission model to show that the 557.7, 777.4 and 844.6 nm oxygen emission line brightnesses can be explained through excitation by electron impact of 5 eV torus electrons on an atmosphere composed of O, SO₂ and an isoelectronic proxy for SO, though the results also suggested emissions from the molecular components may be restricted to higher altitudes. I also evaluated the connection between auroral brightness and Io's position within Jupiter's plasma sheet but found the connection to be ambiguous, indicating a more complex excitation process that exists for other Galilean satellites like Europa and Callisto.