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Mixing in the atmospheres of other worlds
To interpret new observations of exoplanets using telescopes, a better understanding of how gases at high pressures and temperatures mix in their atmospheres is required. The goal of this project is to develop more accurate models for mixtures of major gases in planetary atmospheres at extreme conditions and apply them to interpret recent spectra collected for sub-Neptune planets.
The study of exoplanets and their atmospheres has been revolutionised by the advent of the James Webb Space Telescope (JWST), and will continue to grow as other, ground-based telescopes (e.g., the ELT) come online in the coming years. These technological advances have enabled us to spectroscopically characterise the atmospheres of not only giant planets, but a new class of planet, sub-Neptunes (roughly 2 – 3 Earth radii), to unprecedented precision. Common gaseous species such as CH4 and CO2 have recently been detected in the atmosphere of one such sub-Neptune, K2-18b with JWST, together with the tentative detection of other more exotic gases, such as dimethylsulfide (DMS), a potential biomarker (Madhusudhan et al. 2023). However, these spectra are degenerate with respect to composition, and combinations of other molecules at equilibrium (Shorttle et al. 2024) or produced by photochemical reactions (Wogan et al. 2024) may also explain the variation in the observed atmospheric spectrum.
Our ability to discern between these possibilities, and thus determine whether such atmospheres are germane to hosting life, is limited by our lack of understanding of how gases mix at high pressures and temperatures. Owing to the masses and radii of typical sub-Neptunes, pressures at the atmosphere-interior interface are thought to be of the order of several gigapascal (GPa; Kite et al. 2020; Vazan et al. 2022). At these conditions, gases in the atmosphere do not mix ideally (that is, their physical properties are not linear functions of their concentrations). As such, the relative abundances of gas species in the atmospheres of sub-Neptunes may differ considerably from that expected for ideal mixtures, thereby leading to substantial uncertainties in the interpretation of atmospheric spectra of these planets.
References
Kite, E. S., Fegley Jr, B., Schaefer, L., & Ford, E. B. (2020). Atmosphere origins for exoplanet sub-neptunes. The Astrophysical Journal, 891(2), 111.
Madhusudhan, N., Sarkar, S., Constantinou, S., Holmberg, M., Piette, A. A., & Moses, J. I. (2023). Carbon-bearing molecules in a possible hycean atmosphere. The Astrophysical Journal Letters, 956(1), L13.
Shorttle, O., Jordan, S., Nicholls, H., Lichtenberg, T., & Bower, D. J. (2024). Distinguishing oceans of water from magma on mini-Neptune K2-18b. The Astrophysical Journal Letters, 962(1), L8.
Vazan, A., Sari, R. E., & Kessel, R. (2022). A new perspective on the interiors of ice-rich planets: ice–rock mixture instead of ice on top of rock. The Astrophysical Journal, 926(2), 150.
Wogan, N. F., Batalha, N. E., Zahnle, K. J., Krissansen-Totton, J., Tsai, S. M., & Hu, R. (2024). JWST observations of K2-18b can be explained by a gas-rich mini-Neptune with no habitable surface. The Astrophysical Journal Letters, 963(1), L7.
The study of exoplanets and their atmospheres has been revolutionised by the advent of the James Webb Space Telescope (JWST), and will continue to grow as other, ground-based telescopes (e.g., the ELT) come online in the coming years. These technological advances have enabled us to spectroscopically characterise the atmospheres of not only giant planets, but a new class of planet, sub-Neptunes (roughly 2 – 3 Earth radii), to unprecedented precision. Common gaseous species such as CH4 and CO2 have recently been detected in the atmosphere of one such sub-Neptune, K2-18b with JWST, together with the tentative detection of other more exotic gases, such as dimethylsulfide (DMS), a potential biomarker (Madhusudhan et al. 2023). However, these spectra are degenerate with respect to composition, and combinations of other molecules at equilibrium (Shorttle et al. 2024) or produced by photochemical reactions (Wogan et al. 2024) may also explain the variation in the observed atmospheric spectrum. Our ability to discern between these possibilities, and thus determine whether such atmospheres are germane to hosting life, is limited by our lack of understanding of how gases mix at high pressures and temperatures. Owing to the masses and radii of typical sub-Neptunes, pressures at the atmosphere-interior interface are thought to be of the order of several gigapascal (GPa; Kite et al. 2020; Vazan et al. 2022). At these conditions, gases in the atmosphere do not mix ideally (that is, their physical properties are not linear functions of their concentrations). As such, the relative abundances of gas species in the atmospheres of sub-Neptunes may differ considerably from that expected for ideal mixtures, thereby leading to substantial uncertainties in the interpretation of atmospheric spectra of these planets.
References
Kite, E. S., Fegley Jr, B., Schaefer, L., & Ford, E. B. (2020). Atmosphere origins for exoplanet sub-neptunes. The Astrophysical Journal, 891(2), 111. Madhusudhan, N., Sarkar, S., Constantinou, S., Holmberg, M., Piette, A. A., & Moses, J. I. (2023). Carbon-bearing molecules in a possible hycean atmosphere. The Astrophysical Journal Letters, 956(1), L13. Shorttle, O., Jordan, S., Nicholls, H., Lichtenberg, T., & Bower, D. J. (2024). Distinguishing oceans of water from magma on mini-Neptune K2-18b. The Astrophysical Journal Letters, 962(1), L8. Vazan, A., Sari, R. E., & Kessel, R. (2022). A new perspective on the interiors of ice-rich planets: ice–rock mixture instead of ice on top of rock. The Astrophysical Journal, 926(2), 150. Wogan, N. F., Batalha, N. E., Zahnle, K. J., Krissansen-Totton, J., Tsai, S. M., & Hu, R. (2024). JWST observations of K2-18b can be explained by a gas-rich mini-Neptune with no habitable surface. The Astrophysical Journal Letters, 963(1), L7.
The aim of this project is to i) tabulate existing expressions (equations of state; EoS) for non-ideal mixtures among the major atmosphere-forming gases (e.g., in the C-O-H system), ii) quantify/explain differences between these EoS and compare them with experimental data and iii) use optimised expressions for the EoS to better interpret existing JWST spectral data for atmospheres of sub-Neptunes. The candidate should therefore have an interest in at least one of the following topics; thermodynamics, planetary science; astrophysics.
The aim of this project is to i) tabulate existing expressions (equations of state; EoS) for non-ideal mixtures among the major atmosphere-forming gases (e.g., in the C-O-H system), ii) quantify/explain differences between these EoS and compare them with experimental data and iii) use optimised expressions for the EoS to better interpret existing JWST spectral data for atmospheres of sub-Neptunes. The candidate should therefore have an interest in at least one of the following topics; thermodynamics, planetary science; astrophysics.
Prof. Paolo Sossi (psossi@ethz.ch)
Dr. Dan Bower (dbower@ethz.ch)
