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Abstract:

Clouds have broad significance in understanding the evolution and climate of planetary atmospheres. Moreover, the presence of clouds in the atmospheres of hot Jupiter exoplanets is 91̽��ed both by direct spectral detections (Grant et al. 2023, Inglis et al. 2024), and observational trends, such as nightside brightness temperature (Beatty et al. 2019) and phase curve hot spot offsets (Bell et al. 2024), suggesting that an accurate understanding of clouds is needed, not only to understand the atmospheres of these planets, but to properly interpret observations. However, the properties of clouds are impacted by inherently coupled effects of circulation, radiation, and cloud microphysics. Full coupling of these processes remains computationally expensive, and as a result, current modeling schemes implement simplified cloud parametrizations that neglect one or more of these effects. Within this work, we implement a one-way indirect coupling of the cloud microphysical model 1D CARMA and MITgcm/DISORT, a general circulation model including double-grey radiative transfer, through including a novel particle size distribution that better represents the output of CARMA. We use pre-existing CARMA data for ultra-hot Jupiter TOI-1431b from Gao & Powell (2021), which has particle size distributions that are not well described by a log-normal distribution, with corundum in particular displaying distinctly bimodal behavior. We hypothesize the smaller particle size mode corresponds to nucleation, whereas the larger particle size has formed through condensational growth and coagulation. We present a particle size distribution function that can account for this wide range of distribution variability using two log-normals and two log-exponentials. We implement this particle size distribution for corundum within MITgcm/DISORT for ultra-hot Jupiter TOI-1431b, and compare this work to that of Komacek et al. (2022a), which includes a log-normal roughly corresponding with the larger particle size mode in our distribution. We present the results of this comparison, and discuss the impact of particle size distribution on properties of ultra-hot Jupiters.

Abstract:

Titan’s substantial atmosphere is primarily composed of molecular nitrogen (N2) and methane (CH4), which are dissociated by solar UV photons and subsequently generate a vast chemical network of trace gases. The composition of Titan’s atmosphere is markedly different than that of Saturn, including both the complex molecular inventory and the hitherto measured isotopic ratios – including that of nitrogen (14N/15N). Atmospheric and interior evolution models (e.g., Mandt et al., 2014) indicate that the atmospheres of Saturn and Titan did not form in the same manner or from the same constituents, and that Titan’s atmospheric N2 may have originated from its interior as NH3. The evolution of 14N/15N in Titan’s atmosphere over time does not result in a value comparable to that measured on Saturn and instead is closer to cometary values; this indicates that the origin of Titan’s atmosphere appears to be from protosolar planetesimals enriched in ammonia and not from the sub-Saturnian nebula. However, selective isotopic fractionation of molecular species in Titan’s atmosphere complicates this picture, as the isotopic ratios may vary as a function of altitude (Figure 1). To further constrain the evolution of Titan’s atmosphere – and indeed, its origin – isotopic ratios must be measured throughout its atmosphere, instead of being interpreted from bulk values likely only representative of the stratosphere.While the measurement of Titan’s 14N/15N in N2 (167.7; Niemann et al. 2010) places it firmly below the lower limit derived for Saturn (~350; Fletcher et al., 2014), Titan’s atmospheric nitriles (e.g., HCN, HC3N, CH3CN) are further enriched in 15N, resulting in ratios closer to 70 (Molter et al., 2016; Cordiner et al., 2018; Nosowitz et al., 2025). The variation in nitrogen isotopic ratios between the nitriles and N2 is thought to be the result of higher photolytic efficiency of 15N14N compared to N2 in the upper atmosphere (~900 km), resulting in increased 15N incorporated into nitrogen-bearing species (Liang et al., 2007; Dobrijevic & Loison, 2018; Vuitton et al., 2019). As these species are advected to lower altitudes, the nitrogen isotope ratio may vary vertically (Figure 1, red and black profiles), but previous measurements have only presented bulk atmospheric isotope ratios primarily representing Titan’s stratosphere (Figure 1, blue lines).Recent observations with the Atacama Large Millimeter/submillimeter Array (ALMA) have allowed for the derivation of vertical abundance profiles of Titan’s trace atmospheric species and measurements of N, D, and O-bearing isotopologues (Molter et al., 2016; Serigano et al., 2016; Cordiner et al., 2018; Thelen et al., 2019; Nosowitz et al., 2025). However, vertical isotopic ratio profiles have yet to be derived. Here, we utilize observations acquired with ALMA in July 2022 containing high sensitivity measurements of the HC15N J=4–3 transition at 344.2 GHz (~ 0.87 mm) to investigate vertical variations in the 14N/15N of Titan’s HCN. We compare the results of the vertical 14N/15N profile to those predicted by photochemical models to determine the impact of the isotopic-selective photodissociation of nitrogen-bearing molecular species in Titan’s atmosphere, and the impact of the Saturnian and space environments that vary between model implementations.Figure 1. 14N/15N profile for HCN predicted by photochemical models from Vuitton et al. (2019; black line) and Dobrijevic & Loison (2018; red line). Blue colored bars in the lower atmosphere represent previous HCN nitrogen isotope ratios from Cassini, Herschel, and ground-based (sub)millimeter observations (see Molter et al., 2016, and references therein). Measurements are offset vertically for clarity, and all refer to HC14N/HC15N measurements for the bulk stratosphere.References:Cordiner et al., 2018, The Astrophysical Journal Letters, 859, L15.Dobrijevic & Loison, 2018, Icarus, 307, 371.Fletcher et al., 2014, Icarus, 238, 170.Liang et al., 2007, The Astrophysical Journal Letters, 644, L115.Mandt et al. 2014, The Astrophysical Journal Letters, 788, L24.Molter et al., 2016, The Astronomical Journal, 152, 42.Niemann et al., 2010, Journal of Geophysical Research, 115, E12006.Nosowitz et al., 2025, The Planetary Science Journal, 6, 107.Serigano et al., 2016, The Astrophysical Journal Letters, 821, L8.Thelen et al., 2019, The Astronomical Journal, 157, 219.Vuitton et al., 2019, Icarus, 324, 120.

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.