Exoplanet atmospheres

Abstract:

The nature of the red colouration of Jupiter’s belts and some of its major anticyclones is still debated to this day. Sromovsky et al. (2017) proposed the existence of an “universal chromophore” by fitting Cassini/VIMS-V observations. Baines et al. (2019) concluded that this chromophore should be located in a thin layer above the ammonia clouds, giving rise to the so called “Crème Brûlée” model. Both of these works had as a basis the red compound that formed through the reaction of photolyzed ammonia with acetylene as obtained in the laboratory by Carlson et al. (2016).However,  both Pérez-Hoyos et al. (2020) and Braude et al. (2020) found that a less blue and more vertically extended chromophore layer would fit better their HST/ WFC3 North Temperate Belt disturbance observations for the former and latitudinal swath from MUSE/VLT observations for the later, without fully discarding the possible existence of an “universal chromophore”. Recently, analysis of HST/WFC3 images of Jupiter’s Great Red Spot, its surroundings, and, Oval BA by Anguiano-Arteaga et al. (2021,2023) suggest the presence of two distinct colouring aerosols. The first being similar to the “universal chromophore” and the second one being a new UV-absorbing species below the main chromophore layer at tropospheric levels. This highlights the uncertainties on the vertical distribution of aerosols, their properties and their variability.To address this uncertainty, we used new Jupiter spectra obtained with CARMENES (The Calar Alto High-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs) in 2019. This instrument consists of two separated spectrographs with spectral resolutions R = 80,000-100,000, covering the wavelength ranges of 0.52 to 0.96 μm and of 0.96 to 1.71 μm. The original purpose of these observations was to measure winds through the Doppler velocimetry method. We used a downgraded resolution version (R = 173-570) so the observations match the available spectral data for methane, as this resolution is enough for constraining aerosol properties. Due to the original nature of the observations, no calibration star was recorded. In order to achieve flux calibration, we used  2017 observations of Saturn with CARMENES. We employed Saturn’s B ring to obtain the response function of the instrument, since no other sources of calibration are available at the desired resolution or epoch.We used the reflectivity (I/F) spectrum obtained with Cassini/VIMS (Cuzzi et al., 2009) at phase angles less than 3º. We applied the response function to the centre of disc spectrum of Saturn and compared the obtained reflectivity spectrum with results from Clark and McCord (1979) and Mendikoa, et al. (2017). Lastly, we applied the flux calibration to the Jupiter observations and compared them results from Mendikoa, et al., (2017) and Irwin et al. (2018) (Figure 1). All calibrations agree within 10% with MUSE calibration.We were able to perform a Minnaert Limb-darkening approximation and produce 2 synthetic spectra (zenith angle = 0º/61.45º) for five distinct sample areas (EZ (Figure 2), SEB, NEB, transition region from EZ to SEB, and from NEB to NTrZ). We performed retrievals using the same a priori atmospheric parameterization as presented in Braude et al. (2020), Pérez-Hoyos et al. (2020) and Anguiano-Arteaga et al. (2021), comparing the retrieved results of each in order to constrain the uncertainties in the Jovian aerosol scheme. To achieve this, we used the NEMESIS (Nonlinear Optimal Estimator for MultivariatE Spectral analySIS) radiative transfer suite (Irwin et al., 2008). We present here the results of this analysis.Figure 1: Comparison of centre of disk Jupiter spectrum after flux calibration with EZ spectrum from Irwin et al. (2018) and 0º latitude spectrum from Mendikoa et al. (2017).Figure 2: Observation spectra compared to the obtained synthetic spectra after retrieving the atmospheric parameters for the EZ using Braude et al. (2020) model. Top row corresponds to nadir (incidence and emission angle = 0º) and bottom row to limb (incidence and emission angle = 61.45º). Figure 3: Comparison between the a priori aerosol vertical profiles and the retrieved profiles for every region for the model from Braude et al. (2020).  References:Carlson, R. W., et al. (2016). Chromophores from photolyzed ammonia reacting with acetylene: Application to Jupiter's Great Red Spot. Icarus, 274, 106–115. Sromovsky, L. A., et al. (2017). A possibly universal red chromophore for modeling color variations on Jupiter. Icarus, 291, 232–244. Baines, K. H., et al. (2019). The visual spectrum of Jupiter's Great Red Spot accurately modelled with aerosols produced by photolyzed ammonia reacting with acetylene. Icarus, 330, 217–229. Pérez-Hoyos, S., et al. (2020). Color and aerosol changes in Jupiter after a North temperate belt disturbance. Icarus, 132, 114021. Braude, A. S., et al. (2020). Colour and tropospheric cloud structure of Jupiter from MUSE/VLT: Retrieving a universal chromophore. Icarus, 338, 113589. Anguiano-Arteaga, A., et al. (2021). Vertical Distribution of Aerosols and Hazes Over Jupiter's Great Red Spot and Its Surroundings in 2016 From HST/WFC3 Imaging. Journal of Geophysical Research: Planets, 126, e2021JE006996. Anguiano-Arteaga, A., et al. (2023). Temporal variations in vertical cloud structure of Jupiter's Great Red Spot, its surroundings and Oval BA from HST/WFC3 imaging. Journal of Geophysical Research: Planets, 128, e2022JE007427. Karkoschka, E. (1994). Spectrophotometry of the Jovian Planets and Titan at 300- to 1000-nm Wavelength: The Methane Spectrum. Icarus, 111, 1, 174–192. Irwin, P., et al. (2008). The NEMESIS planetary atmosphere radiative transfer and retrieval tool. J. Quant. Spectrosc. Radiat. Transf., 109, 1136–1150. Rodgers CD. (2000). Inverse methods for atmospheric sounding: theory and practice. Singapore: World Scientific. Cuzzi, J., et al., 2009. Ring Particle Composition and Size Distribution. Springer Netherlands, Dordrecht. pp. 459–509. Clark, R.N., McCord, T.B., 1979. Jupiter and Saturn: Near-infrared spectral albedos. Icarus 40, 180–188. Mendikoa, I., et al., 2017. Temporal and spatial variations of the absolute reflectivity of Jupiter and Saturn from 0.38 to 1.7 𝜇m with planetcam-upv/ehu. A&A 607, A72. Irwin, P.G., et al., 2018. Analysis of gaseous ammonia (NH3) absorption in the visible spectrum of Jupiter. Icarus 302, 426–436

Abstract:

Jupiter has the strongest planetary magnetic field and the most volcanically active moon (Io) in the solar system.  Magnetospheric dynamics and interactions with the solar wind ultimately drive ions and electrons deep into its neutral atmosphere producing auroral emissions over a large range of the electromagnetic spectrum.  Energy is deposited as deep as the lower stratosphere, which drives atmospheric heating, dynamics and unique chemistry.  Jupiter provides a natural laboratory to study how the external space environment can modulate a planet’s atmosphere and context for the extreme space weather likely experienced by exoplanets orbiting close to their host star.  In this work, we present an analysis of high-resolution mid-infrared spectra recorded in March 2025 by the TEXES (Texas Echelon Cross Echelle Spectrograph, Lacy et al. 2002, PASP 114, 153) instrument on NASA’s IRTF (Infrared Telescope Facility).  As part of a long-term program, spectral scans were performed across high-northern and high-southern latitudes in settings centered at 8.0, 10.53, 12.21 and 13.70 micron in order to target the stratospheric emissions of CH4 (methane), C2H4 (ethylene), C2H6 (ethane) and C2H2 (acetylene), respectively.  Such spectra are inverted using the NEMESIS radiative transfer software (Irwin et al., 2008, JQSRT 109, 1136) to derive spatial variations in the vertical profiles of temperature, C2H2, C2H4 and C2H6 and the vertical location of the hydrocarbon homopause.  We will present these results, in addition to those derived from previous measurements, in order to highlight the thermal, chemical and dynamical evolution of Jupiter’s polar stratosphere.  As part of a new project, TEXES spectra were also recorded in settings centered at 10.95, 11.83 and 13.37 with the goal of detecting CH2CCH2 (allene), C3H6 (propene) and C3H8 (propane).  We will present these spectra to indicate whether these species have been detected.   Detected spectral features will be inverted to derive vertical and spatial variations in its abundance.  In the case of a non-detection, an upper limit would be derived.  The presence or absence of such hydrocarbon species would provide unique insight into how auroral processes modify the chemistry of Jupiter’s stratosphere.

Abstract:

Saturn’s largest moon Titan has a nitrogen-methane atmosphere and a rich organic photochemistry. Dissociation of Titan’s molecular methane and nitrogen into N and methyl (CH3) radicals forms the basis of this photochemistry and results in a vast array of hydrocarbon and nitrile species. The abundance of CH3 is thus of critical importance to understanding Titan’s atmospheric chemistry. CH3 is predicted by photochemical models and must be present to explain Titan’s trace gas composition, but has never been directly observed. Cassini’s mass spectrometer was unable to make a detection as the extreme reactivity of radicals results in reactions on the instrument wall (e.g. recombination with H) before detection is possible. Emission features in the infra-red are also very weak, so detection from remote-sensing spectroscopy has previously not been possible. Here we use the very high sensitivity of the James Webb Space Telescope’s (JWST) Mid-InfraRed Instrument (MIRI) to detect emission from CH3 at 16.5 microns. We have used this to validate model predictions that underpin Titan’s rich atmospheric chemistry.JWST/MIRI observations were taken in Medium Resolution Spectroscopy (MRS) mode on 11th July 2023 as part of Guaranteed Time Observation programme 1251 [Nixon et al., 2025]. Observations were reduced using the standard pipeline and combined to give a disc-averaged spectrum (Fig 1). The observed spectrum was compared to a forward model generated with a reference Titan atmosphere using the NEMESIS radiative transfer suite [Irwin et al., 2008]. The reference atmospheric temperature profile was based on observation from Cassini half a Titan year previous, augmented with ground-based measurements from ALMA and in-situ measurements from the Huygens probe (Fig 2a). A baseline atmospheric composition was compiled from Cassini/Huygens measurements [Teanby et al., 2019]. For the CH3 profile, in the absence of measurements, we used the predicted abundance from a photochemical model [Vuitton et al., 2019] (Fig 2a).The abundance profile of CH3 is expected to be extremely steep with very high fractional abundances in the thermosphere (100 ppm at 1000km) and much lower abundances in the stratosphere and mesosphere (1 ppb at 300km). Peak emission under conditions of local thermodynamic equilibrium should originate from the mid-thermosphere at an altitude of ~800km (Fig 2b). However, our analysis shows that non-local thermodynamic equilibrium (non-LTE) emission is expected due to very low thermospheric pressures [Nixon et al., 2025]. This supresses emission below that expected from the Planck function and reduces infra-red emission from thermospheric CH3 to negligible levels. When non-LTE effects are considered, we find that the emission instead originates from the stratopause region (~300km) where CH3 abundances are predicted to be around 1 ppb (Fig 2c).Agreement between forward modelled non-LTE emission using the photochemical model profile and the JWST/MIRI observation match very well (Fig 1) – confirming the model predicted abundances are consistent with conditions in Titan’s middle atmosphere. Our initial results were presented in Nixon et al., (2025). Here we present an updated analysis using improved pipeline processing, more in-depth treatment of the disc-averaged nature of the observation, and provide formal limits on the CH3 abundance profiles. The consistency of our results with predictions from photochemical models gives confidence to current chemical schemes for Titan’s low-order chemistry, which provides a sound basis for a deeper analysis of Titan’s more exotic species such as high-order hydrocarbons and poly-aromatic hydrocarbons.ReferencesIrwin, P.G.J., et al., 2008. The NEMESIS planetary atmosphere radiative transfer and retrieval tool. Journal of Quantitative Spectroscopy and Radiative Transfer 109, 1136–1150.Nixon, C.A., et al., 2025., Titan’s Atmosphere in Late Northern Summer from JWST and Keck Observations. Nature Astronomy, in press.Teanby, N.A., et al., 2019. Seasonal Evolution of Titan’s Stratosphere During the Cassini Mission. Geophysical Research Letters 46, 3079–3089.Vuitton, V., et al., 2019. Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model. Icarus 324, 120–197.Fig 1: JWST/MIRI disc-average spectrum compared with forward models with and without CH3. The model including CH3 provides a much better fit to the observations.Fig 2: (a) Titan’s atmospheric temperature structure and uncertainty envelope from Nixon et al. (2025), along with photochemical model prediction of the CH3 profile from Vuitton et al. (2019). (b) Contribution functions for LTE case with nominal temperature profile (green), hot temperature limit (red) and cold temperature limit (blue). For LTE, peak emission would be from the thermosphere at ~800km, but this is not realistic. (c) Contribution functions for a more realistic non-LTE emission case peak at ~300km around the mesopause as non-LTE effects suppress emission at very low pressures. Our observations are thus most sensitive to abundances around the stratopause. 

Abstract:

Radiative transfer analyses of spectra obtained from Uranus and Neptune have revealed the presence ofa cloud layer at pressures greater than ~2 bar (1,2). The detection of hydrogen sulfide (H₂S) gas abovethis cloud layer on both planets (3,4) suggests that H₂S ice is the most likely main constituent. Thisinterpretation is further supported by the expectation that methane (CH₄) clouds condense at higheraltitudes (5). However, due to their depth and observational limitations, our understanding of theproperties of H₂S clouds on these planets remains very limited.To investigate the properties of H₂S clouds in the atmospheres of Uranus and Neptune, we employed aone-dimensional cloud microphysics model originally developed for Titan and Mars (6,7). The modelincludes nucleation, condensation, evaporation, coagulation, and precipitation processes, and haspreviously been used to simulate haze and CH₄ cloud microphysics in the Ice Giants (5,8,9).Figure 1 shows, as an example, simulated H₂S ice profiles for Uranus using this microphysical model.The vertical transport of H₂S gas is simulated using an eddy diffusion coefficient (Keddʏ), which controlsthe supply of vapor for cloud nucleation and particle growth. We employed the Keddʏ profiles derivedin [10] for H₂S abundances of 10× and 30× solar. Since several cloud microphysical parameters for H₂Sremain uncertain (e.g., the contact parameter), different values are tested in the simulations. In theexample shown, the model indicates cloud bases near 5.3 bar for 10× solar abundance and 6.4 bar for30× solar. Near the cloud base, particle mean radii range from 40 to 55 μm, depending on the assumedcontact parameter and abundance. At higher altitudes, particle sizes decrease; for instance, at ~3 bar,mean radii are around 20 μm. In general, H₂S cloud simulations produce higher opacities than CH₄clouds.In this work, we will present a series of cloud microphysical simulations of H₂S clouds in the Ice Giants.Various cloud properties, such as particle size distributions and precipitation rates, will be constrained.We will also discuss the implications of our results for the atmospheric circulation of these planets andfor the future exploration of Uranus.Figure 1. Vertical distributions of H2S ice (g/m³) for Uranus, simulated for different values of the cloudcontact parameter and deep H2S abundances. These simulations employ the Keddʏ profiles calculated in[10] for the corresponding H2S abundances.References: [1] P. G. Irwin, et al., JGR: Planets, 127, e2022JE007189. [2] L. Sromovsky, et al., Icarus,Volume 317, (2019) [3] P. G. Irwin, et al., Nature Astronomy 2, 420 (2018). [4] P. G. Irwin, et al.,Icarus 321, 550 (2019). [5] D. Toledo, et al., A&A, 694, A81 (2025). [6] P. Rannou, et al., Science 311,201 (2006). [7] F. Montmessin, et al., JGR: Planets 107, 4 (2002). [8] D. Toledo, et al., Icarus, 333, 1-11, (2019). [9] D. Toledo, et al., Icarus, Volume 350, (2020). [10] H. Ge, et al., The Planetary ScienceJournal,5, 101(2024). 

Abstract:

In 2018, analysis of Gemini-NIFS near-infrared observations revealed the probable presence of H2S above the main cloud deck on Neptune [1]. The spectral signature of the gas was found to be much stronger at Neptune's south pole compared to regions nearer the equator.Conversely, analysis of Neptune's microwave emission with ALMA suggested strongly enhanced H2S abundances at midlatitudes [2], with much less at the south pole. Determining the true variation of H2S with latitude is crucial for understanding the tropospheric circulation of Neptune.We present our analysis of observations of Neptune from VLT-SINFONI in 2018. Using a limb-darkening approximation, we are able to fit the reflected solar radiance from multiple zenith angles, which allows us to discriminate between gas and aerosol opacity. Despite the lower spectral resolution of this instrument compared to Gemini-NIFS, we are able to detect the H2S spectral signature. With our radiative transfer retrieval code, archNEMESIS [3], we use nested sampling to fit a parameterised cloud model (similar to that of [4]) to these observations over a range of latitudes. We prescribe a latitudinally varying deep methane abundance derived from recent VLT-MUSE observations [5], which enables us to constrain the depth of the cloud top.Our retrieved results are in agreement with the results derived from ALMA [2] - we find a significant enhancement of deep H2S at Neptune's southern midlatitudes, decreasing towards the equator and the pole. Our results show a much deeper cloud top towards the pole, resulting in the increased cloud top column abundance of H2S observed here in the previous near-infrared analysis [1].Figure 1: A comparison of fits to a spectrum extracted from the 50°S to 60°S latitude band, with a model including H2S (blue) and a model without H2S (red). Note the significant discrepancy around 1.58 microns. The models are fitted to spectra at two zenith angles simultaneously.[1] Irwin, P. G., Toledo, D., Garland, R., Teanby, N. A., Fletcher, L. N., Orton, G. S., & Bézard, B. (2019). Probable detection of hydrogen sulphide (H2S) in Neptune’s atmosphere. Icarus, 321, 550-563.[2] Tollefson, J., de Pater, I., Luszcz-Cook, S., & DeBoer, D. (2019). Neptune's latitudinal variations as viewed with ALMA. The Astronomical Journal, 157(6), 251.[3] Alday, J., Penn, J., Irwin, P. G., Mason, J. P., & Yang, J. (2025). archNEMESIS: an open-source Python package for analysis of planetary atmospheric spectra. arXiv preprint arXiv:2501.16452.[4] Irwin, P. G., Teanby, N. A., Fletcher, L. N., Toledo, D., Orton, G. S., Wong, M. H., ... & Dobinson, J. (2022). Hazy blue worlds: a holistic aerosol model for Uranus and Neptune, including dark spots. Journal of Geophysical Research: Planets, 127(6), e2022JE007189.[5] Irwin, P. G., Dobinson, J., James, A., Wong, M. H., Fletcher, L. N., Roman, M. T., ... & de Pater, I. (2023). Latitudinal variations in methane abundance, aerosol opacity and aerosol scattering efficiency in Neptune's atmosphere determined from VLT/MUSE. Journal of Geophysical Research: Planets, 128(11), e2023JE007980.