Exoplanet atmospheres

Abstract:

Jupiter, the largest planet in our solar system, is a vital reference point for understanding gaseous exoplanets and their atmospheres. While we know its upper tropospheric chemical composition well, the nature and structure of its clouds remain puzzling. We, therefore, rely on theoretical models and remote sensing data to address this.While traditional equilibrium chemistry condensation models (ECCM) are sensitive to input parameters, advanced models [1] offer more realistic cloud property predictions. Remote sensing data can help determine cloud properties and test theoretical predictions thanks to the application of multiple scattering atmospheric retrieval. Still, the process is highly degenerate and, therefore, computationally demanding. The predicted tropospheric layers are upper ammonia ice (∼0.7 bar) and ammonium hydrosulfide (∼2 bar) clouds [2], but their spectral detection has been limited to small, dynamically active regions (

Abstract:

To aid the search for atmospheres on rocky exoplanets, we should know what to look for. An unofficial paradigm is to anticipate CO2 present in these atmospheres, through analogy to the solar system and through theoretical modelling. This CO2 would be outgassed from molten silicate rock produced in the planet’s mostly-solid interior—an ongoing self-cooling mechanism that should proceed, in general, so long as the planet has sufficient internal heat to lose.Outgassing of CO2 requires relatively oxidising conditions. Previous work has noted the importance of how oxidising the planet interior is (the oxygen fugacity), which depends strongly on its rock composition. Current models presume that redox reactions between iron species control oxygen fugacity. However, iron alone need not be the sole dictator of how oxidising a planet is. Indeed, carbon itself is a powerful redox element, with great potential to feed back upon the mantle redox state as it melts. Whilst Earth is carbon-poor, even a slightly-higher volatile endowment could trigger carbon-powered geochemistry.We offer a new framework for how carbon is transported from solid planetary interior to atmosphere. The model incorporates realistic carbon geochemistry constrained by recent experiments on CO2 solubility in molten silicate, as well as redox couplings between carbon and iron that have never before been applied to exoplanets. We also incorporate a coupled 1D energy- and mass-balance model to provide first-order predictions of the rate of volcanism.We show that carbon-iron redox coupling maintains interior oxygen fugacity in a narrow range: more reducing than Earth magma, but not reducing enough to destabilise CO2 gas. We predict that most secondary atmospheres, if present, should contain CO2, although the total pressure could be low. An atmospheric non-detection may indicate a planet either born astonishingly dry, or having shut off its internal heat engine.

Abstract:

Titan is renowned for its complex atmosphere, where ongoing photochemistry leads to a rich mixture of organic molecules. Beginning with the splitting of methane by sunlight and other energetic particles, multi-carbon molecules are built up by successive addition of CxHy radicals and ions to one another. This process leads to the formation of ever-larger  molecules and eventually particulates, that sediment out on the surface. Our experimental knowledge of the molecular inventory comes from two techniques: direct sampling mass spectrometry, and remote sensing.  While the former has shown the presence of species at a very wide range of masses from 1-100+ Da, their structure and even stoichiometry is poorly known. In this respect, remote sensing spectroscopy is more robust, providing definitive detections of individual molecular types via unique patterns of IR and sub-millimeter energy transitions, however for a more limited range of species. Currently, 25 species have been definitively identified by remote sensing, ranging in size from H2 to benzene (C6H6). These include 12 hydrocarbons, with the rest a mixture of diatomics, nitriles and small oxygen compounds (H2O, CO, CO2). With direct sampling currently impossible before the Dragonfly mission returns a spacecraft to Titan in 2034, astronomers have been pushing forward with chemical identifications using a range of ground and space-based observatories. We report here on recent attempts to identify new C3 and C4 hydrocarbons in Titan’s atmosphere using the high-resolution (R~100000) TEXES spectrometer at the Infrared Telescope Facility (IRTF) – see examples in Fig. 1. Associated laboratory spectroscopy work is ongoing at the Jet Propulsion Laboratory (JPL) using a Bruker FTS spectrometer to identify the positions and intensities of the strongest gas bands, to assist with targeting the telescope searches, and interpretation of the data.  Identifications of new, heavy molecular species are urgently needed to constrain photochemical and dynamical models, and make advances in our understanding of the workings of Titan’s atmosphere, and its potential for astrobiology. Such work is also important for planning data collection and analysis from the upcoming NASA Dragonfly mission, where a sensitive mass spectrometer will assess the composition of surface materials and their relation to the atmospheric constituents, as well as Titan atmospheric data from other telescopes such as ALMA and JWST.Figure 1: Examples of currently undetected molecules in Titan's atmosphere: isomers of C4H8 and C4H10. We report on ongoing searches for these species with IRTF/TEXES.

Abstract:

Introduction: Recent advances in observation with the JWST and high-resolution ground-based instruments have enabled the study of exoplanets to progress towards atmospheric characterisation, as opposed to merely detection. Hot and ultra-hot Jupiters remain among the best characterised and studied class of exoplanet, due to their large sizes and close orbits, however how the internal heating and resulting radius inflation of bloated ultra-hot Jupiters and related coupling to the internal magnetic field impacts their atmospheric circulation remains an open question. Moreover, the impact of atmospheric dynamics on observable properties can now be studied in detail. This study investigates the effect of varying both atmospheric drag and internal heat flux on the observable properties of WASP-76b, with comparisons made to JWST NIRSpec white-light phase curves. In addition, we perform a broader parameter sweep using the SPARC/MITgcm to investigate the influence of internal heating and inflated radii on the observable properties of hot and ultra-hot Jupiters.Methods: A suite of general circulation models are run, which solve the primitive equations of meteorology coupled to non-grey correlated-k radiative transfer with the SPARC/MITgcm [1]. The effect of Lorentz forces is represented by changing a spatially constant drag timescale , and for WASP-76b we consider two different internal heat fluxes for comparison, across the range of predicted values for hot and ultra-hot Jupiters [2]. We then will perform a broader parameter sweep, exploring the space of inflated-radii hot and ultra-hot Jupiters by covering a range of irradiation levels from zero-albedo full-redistribution equilibrium temperatures of 1000 – 3200K, again using the SPARC/MITgcm. This parameter space is inclusive of most inflated gas-giant planets, excluding KELT-9b, and will allow us to study the impact of internal heating on atmospheric circulation, with interior heating and evolution modelled using MESA [3]. We then use the gCMCRT radiative transfer code [4] to post-process the GCM results to produce simulated phase curves.Results: The key result from this study is shown in Figure 1, with simulated phase curves shown in comparison to Spitzer telescope data [5] at 3.6mm. We make the comparison to Spitzer data here as a placeholder for the comparison to JWST NIRSpec data, as the JWST data is not yet published. Figure 3 shows the impact of the interior heat flux on the internal temperature structure of WASP-76b. There is no observable difference between the interior heat flux scenarios. Figures 2 and 4 show characteristics of the atmospheric dynamics and temperature structure. Strong drag acts to suppress all winds throughout the atmosphere, as is expected, while intermediate drag removes the offset of the hot spot due to the suppression of the deep equatorial jet. There is a strong equatorial jet within the deep atmosphere, and the T-P profile implies that cloud species Al2O4 and Mg2SiO4 can form on the night-side and terminators of WASP-76b, and within its deep atmosphere.Conclusions: Spitzer data is best matched by a strong () drag case. There is no potentially observable difference between the hot interior flux and cold interior flux. The comparisons of these simulated phase curve to JWST NIRSpec white-light phase curves will help further constrain drag in the ultra-hot regime, which will be a useful point of comparison to other ultra-hot Jupiters. Other ultra-hot Jupiters with Spitzer phase-curves (WASP-18b [6], WASP-103b [7], WASP-121b [8]) also show high dayside-nightside temperature differences. This may imply that the drag mechanisms are similar in each planet in the ultra-hot regime (~2000-2500 K). New JWST NIRSpec/G395H phase-curve data (JWST GO proposal 5268) will also constrain metallicity, breaking the drag/metallicity degeneracy. The similarity in deep-atmosphere temperature shown by Figure 3 motivates the need for a parameter sweep where the temperature at the bottom boundary is varied, as opposed to an interior heat flux, in order to speed up convergence. Likewise, the T-P profile in Figure 4 motivates the need for longer simulation runs, as the model has not reached equilibrium within the deep atmosphere.References:[1] Showman, A.P. et al. (2009), The Astrophysical Journal, 699(1), pp. 564–584.[2] Thorngren, D. et al. (2019), ApJL (Vol. 884, Issue 1)[3] Jermyn, A.S. et al. (2023), The Astrophysical Journal Supplement Series, 265, p. 15.[4] Lee, E.K. et al. (2022), The Astrophysical Journal, 929(2), p. 180[5] May, E.M. et al. (2021), The Astronomical Journal, 162(4), p. 158.[6] Maxted, P.F. et al. (2012), Monthly Notices of the Royal Astronomical Society, 428(3), pp. 2645–2660[7] Kreidberg, L. et al. (2018), The Astronomical Journal, 156(1), p. 17[8] Davenport, B. et al. (2025),  Available at: https://arxiv.org/abs/2503.12521 (Accessed: 20 March 2025).

Abstract:

Over four decades have elapsed since the last in situ spectrophotometric observations of the Venusian atmosphere, specifically from the Venera 11 (1978) and Venera 13 and 14 (1982) missions. These missions recorded spectral data during their descent from approximately 62 km to the surface. Unfortunately, the original data were lost; however, a portion has been reconstructed by digitising the graphical outputs that were generated during the initial data processing phase of each of the three missions [1]. This reconstructed data is crucial as it remains the sole set of in situ spectrophotometric observations of Venus’s atmosphere and is likely to be so for the foreseeable future.While re-analysing the reconstructed Venera datasets, we identified several artefacts, errors and sources of noise, necessitating the implementation of some corrections and validation checks to isolate the most unaffected part of the reconstructed data. Then, using NEMESIS, a radiative transfer and retrieval tool [2], we conducted a series of retrievals to simultaneously fit the downward-going spectra at all altitudes. During this process, several parameters were retrieved. The first set of retrievals focused on the structure of the main cloud deck (MCD), which includes the cloud base altitude and abundance profiles of all four cloud modes. Previous corrections that were used to account for the effect of the unknown UV absorber did not result in good fits with the spectra shortward of 0.6 µm. Hence, we derived a new correction by retrieving the imaginary refractive index spectra of the Mode 1 particles.In the next phase, the MCD retrievals were used to update the model atmospheres for each of the missions. Then, the H2O volume mixing ratio profiles were retrieved and compared with the previous retrievals using the same data by [1] along with other remote sensing observations. The final retrieval phase concentrated on characterising particulate matter in the deep atmosphere. In [3], we outlined a methodology for retrieving a near-surface particulate layer using the reconstructed Venera 13 dataset. In this new work, we apply this methodology to encompass the Venera 11 and 14 datasets and compare the retrievals from the three datasets.This research thus provides a comprehensive overview of three distinct retrievals: 1) main cloud deck, 2) H2O, and 3) near-surface particulates using the reconstructed spectrophotometric data of Venera 11, 13, and 14.References: [1] Ignatiev, N. I., Moroz, V. I., Moshkin, B. E., Ekonomov, A. P., Gnedykh, V. I., Grigor’ev, A. V., and Khatuntsev, I. V. Cosmic Research 35(1), 1–14 (1997).[2] Irwin, P. G., Teanby, N. A., de Kok, R., Fletcher, L. N., Howett, C. J., Tsang, C. C., Wilson, C. F., Calcutt, S. B., Nixon, C. A., and Parrish, P. D. Journal of Quantitative Spectroscopy and Radiative Transfer 109(6), 1136–1150 (2008).[3] Kulkarni, S. V., Irwin, P. G. J., Wilson, C. F., & Ignatiev, N. I. Journal of Geophysical Research: Planets, 130, e2024JE008728, (2025).