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Jupiter鈥檚 auroral stratosphere as revealed by IRTF-TEXES spectroscopy

(2025)

Authors:

James Sinclair, Glenn Orton, Thomas Greathouse, Rohini Giles, Conor Nixon, Vincent Hue, Leigh Fletcher, Patrick Irwin

Abstract:

Jupiter has the strongest planetary magnetic field and the most volcanically active moon (Io) in the solar system.聽 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.聽 Energy is deposited as deep as the lower stratosphere, which drives atmospheric heating, dynamics and unique chemistry.聽 Jupiter provides a natural laboratory to study how the external space environment can modulate a planet鈥檚 atmosphere and context for the extreme space weather likely experienced by exoplanets orbiting close to their host star.聽 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鈥檚 IRTF (Infrared Telescope Facility).聽 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.聽 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.聽 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鈥檚 polar stratosphere.聽 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).聽 We will present these spectra to indicate whether these species have been detected.聽 聽Detected spectral features will be inverted to derive vertical and spatial variations in its abundance. 聽In the case of a non-detection, an upper limit would be derived. 聽The presence or absence of such hydrocarbon species would provide unique insight into how auroral processes modify the chemistry of Jupiter鈥檚 stratosphere.

Lunar Trailblazer: Improving Brightness Temperature Estimation Methods and Applications of Temperature Retrieval for Future Missions

(2025)

Authors:

Fiona Henderson, Namrah Habib, Katherine Shirley, Neil Bowles

Abstract:

Introduction: The Lunar Thermal Mapper (LTM) is a multispectral infrared radiometer, built by the 91探花 Physics Instrumentation Group for the Lunar Trailblazer mission; a small satellite launched in February 2025 under NASA鈥檚 Small Innovative Missions for Planetary Exploration (SIMPLEx). Trailblazer aims to advance our understanding of the lunar water cycle by mapping surface temperature, water abundance, distribution and form (OH, H2O, ice) and silicate lithology (i.e., Si-O Christiansen spectral feature). LTM was developed to improve upon existing infrared instrumentation in lunar orbit (e.g., Diviner Lunar Radiometer Experiment, hereafter referred to as Diviner) to provide higher resolution temperature estimations and refine interpretations of thermophysical properties at the surface [2, 3]. Accurately determining surface temperatures on airless bodies is essential for deriving emissivity spectral features (such as the Christiansen Feature and Restrahlen bands, which are diagnostic of silicate lithologies) that are representative of the surface. Temperature errors can affect spectral shape, resulting in the misidentification of surface composition [5, 8].聽聽Our team compared six methods for estimating LTM鈥檚 brightness temperature (BT), including the temperature retrieval approach used by Diviner, to (1) determine which method provides the most representative surface temperature and (2) assess how variations in BT estimation affect derived emissivity spectral shape. Despite challenges facing the Trailblazer mission, refining methods for BT estimation remains relevant to the planetary community, as future missions continue to depend on infrared instrumentation and accurate BT retrievals for remote compositional interpretation (e.g., LEAP, L-CIRiS, Europa Clipper). 聽聽Instrumentation: LTM is a 15-channel infrared imager that covers a range between 6 to 100鈥祄 [2,3]. LTM advances infrared compositional analysis by incorporating eleven narrowband compositional filters across the 6.25 to 10鈥祄 range. This expanded spectral coverage enables more precise characterization of key features, such as the Christiansen Feature, Reststrahlen bands, and transparency features, which are essential for identifying spectral endmembers (Table 1) [2,3]. 聽聽LTM builds upon Diviner, a nine-channel instrument that has a broad spectral range from 0.3 to 400鈥祄 (Table 1) [1]. Diviner鈥檚 three narrowband compositional channels, 7.55鈥8.05鈥祄 (Channel 3), 8.10鈥8.40鈥祄 (Channel 4), and 8.38鈥8.60鈥祄 (Channel 5), are specifically tuned to capture the Christiansen Feature (CF), an emissivity peak that is diagnostic of broad silicate mineralogy and sensitive to variations in silica content [1,4]. 聽Table 1: LTM and Diviner observational parameters.聽聽聽聽Methodology: To assess BT and emissivity retrieval techniques for LTM, we measured four lunar analog samples under controlled laboratory conditions to retrieve high-resolution emission spectra. These laboratory spectra were down-sampled to match LTM鈥檚 narrowband spectral resolution. Six BT estimation methods were tested to determine how effectively each method preserved laboratory spectral shape and temperature. The following section describes the laboratory setup and the BT estimation methods examined in this study.聽Laboratory: Using the PASCALE (Planetary Analogue Surface Chamber for Asteroids and Lunar Environments) in conjunction with a Bruker 70V Fourier Transform Infrared (FTIR) spectrometer, we conducted thermal infrared measurements of four volcanic lunar analogue samples; dunite (Twin Sisters -1 and -2), basalt (BIR-1) and rhyolite (RGM-1) under controlled ambient conditions (350 K, 1000 mbar, N2 atmosphere) [4]. The integration of PASCALE with FTIR allows for the acquisition of thermal emission spectra (as opposed to typical laboratory reflectance), offering a more representative analog of data collected by orbiting infrared instrumentation. Spectra were measured across ~6000 to 350鈥痗m鈦宦 at a resolution of 4鈥痗m鈦宦. Quality assurance and calibration procedures followed established protocols outlined in [6,7,8].聽聽BT Estimations: To evaluate BT performance at LTM鈥檚 spectral resolution, each sample鈥檚 measured radiance was convolved with LTM鈥檚 filter response to simulate instrument-resolution radiance. The resulting spectra were converted to BT using the Planck function. Seven distinct methods were applied to the LTM-resolution BT data to determine the maximum BT values for each sample (Table 2). Emissivity was subsequently derived as the ratio between the observed LTM-resolution radiance and an ideal blackbody at the retrieved maximum BT for each method across all samples. The accuracy of the BT estimation methods was assessed by comparing the resulting emissivity spectra and maximum BT values to the full laboratory reference data (350K and full resolution emissivity). Additionally, a focused comparison with Diviner鈥檚 BT retrieval method was conducted to identify method-specific discrepancies and evaluate cross-instrument consistency.Table 2: BT estimation methods聽Results & Discussion: Six BT estimation methods were applied to laboratory emissivity spectra of four lunar analogue samples (dunite, basalt, and rhyolite), as shown in Figure 2. The associated standard errors (SE) for each method are reported in Table 3. Among the tested approaches, four methods (3rd degree polynomial, quadratic, spline and narrowband maximum) showed close agreement with high-resolution laboratory spectra (Figure 2). Temperature variations across compositions were minor, with low SE values (Table 3). 聽Since the spline fit did not significantly outperform the simpler polynomial or narrowband methods, lower complexity approaches are preferred for LTM temperature retrievals, with a maximum SE of 3.42%.In contrast, due to limited spectral sampling, the Diviner method underestimates surface temperatures by up to 19 K (SE max: 5.55%) in the dunite (TS-2) sample. Expanding this analysis to include a broader range of lithologies or impacted processed samples would help assess whether the Diviner approach (and potential other methods with sparse spectral sampling) introduce systematic shifts in the Christiansen Feature (CF) position or affect the spectral shape relative to more spectrally resolved techniques. 聽Table 3: BT estimations and associated SE of temperature for dunite (TS-1, TS-2), basalt (BIR-1) and rhyolite (RGM-1).聽Fig 2: Six BT methods are fitted to laboratory emissivity spectra of four lunar analogues.聽Conclusion:Comparisons between BT estimation methods indicate the 3rd-degree polynomial, quadratic, and narrowband maximum methods offer the best agreement with laboratory data (SE max: 3.42%). Although Diviner鈥檚 method tends to underestimate surface temperatures (up to 19 K), it still preserves spectral shape and wavelength range, 91探花ing the reliability of compositional interpretations. Expanding the dataset to include a broader range of compositions could confirm whether different approaches result in systematic shifts in the Christiansen Feature across different lithologies. This work enhances the accuracy of remote compositional interpretation and 91探花s future exploration on airless bodies.

Methyl Radical Detected on Titan with JWST/MIRI

(2025)

Authors:

Nicholas Teanby, Conor Nixon, Manuel L贸pez-Puertas, Brandon Coy, V茅ronique Vuitton, Panayotis Lavvas, Lucy Wright, Joshua Ford, Patrick Irwin

Abstract:

Saturn鈥檚 largest moon Titan has a nitrogen-methane atmosphere and a rich organic photochemistry. Dissociation of Titan鈥檚 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鈥檚 atmospheric chemistry. CH3 is predicted by photochemical models and must be present to explain Titan鈥檚 trace gas composition, but has never been directly observed. Cassini鈥檚 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鈥檚 (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鈥檚 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) 鈥 confirming the model predicted abundances are consistent with conditions in Titan鈥檚 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鈥檚 low-order chemistry, which provides a sound basis for a deeper analysis of Titan鈥檚 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鈥1150.Nixon, C.A., et al., 2025., Titan鈥檚 Atmosphere in Late Northern Summer from JWST and Keck Observations. Nature Astronomy, in press.Teanby, N.A., et al., 2019. Seasonal Evolution of Titan鈥檚 Stratosphere During the Cassini Mission. Geophysical Research Letters 46, 3079鈥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鈥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鈥檚 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.聽

Microphysical Modeling of Hydrogen Sulfide Clouds in the Atmospheres of the Ice Giants

(2025)

Authors:

Daniel Toledo, Pascal Rannou, Patrick Irwin, Bruno de Batz de Trenquell茅on, Michael Roman, No茅 Cl茅ment, Gwenael Milcareck, Victor Apestigue, Ignacio Arruego, Margarita Yela

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 (H鈧係) gas abovethis cloud layer on both planets (3,4) suggests that H鈧係 ice is the most likely main constituent. Thisinterpretation is further 91探花ed by the expectation that methane (CH鈧) clouds condense at higheraltitudes (5). However, due to their depth and observational limitations, our understanding of theproperties of H鈧係 clouds on these planets remains very limited.To investigate the properties of H鈧係 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 CH鈧 cloud microphysics in the Ice Giants (5,8,9).Figure 1 shows, as an example, simulated H鈧係 ice profiles for Uranus using this microphysical model.The vertical transport of H鈧係 gas is simulated using an eddy diffusion coefficient (Kedd蕪), which controlsthe supply of vapor for cloud nucleation and particle growth. We employed the Kedd蕪 profiles derivedin [10] for H鈧係 abundances of 10脳 and 30脳 solar. Since several cloud microphysical parameters for H鈧係remain 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脳 solar abundance and 6.4 bar for30脳 solar. Near the cloud base, particle mean radii range from 40 to 55 渭m, depending on the assumedcontact parameter and abundance. At higher altitudes, particle sizes decrease; for instance, at ~3 bar,mean radii are around 20 渭m. In general, H鈧係 cloud simulations produce higher opacities than CH鈧刢louds.In this work, we will present a series of cloud microphysical simulations of H鈧係 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/m鲁) for Uranus, simulated for different values of the cloudcontact parameter and deep H2S abundances. These simulations employ the Kedd蕪 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).聽

Modelling the Influence of Oxidative Chemistry on Trace Gases in Mars' Atmosphere.

(2025)

Authors:

Bethan Gregory, Kevin Olsen, Ehouarn Millour, Megan Brown

Abstract:

In this presentation, we will show efforts made to include accurate photochemical modelling of hydrogen chloride (HCl) and ozone (O3) in the Mars Planetary Climate Model in order to reconcile recent observations.The ExoMars Trace Gas Orbiter (TGO) has detected and characterised trace gases in the Martian atmosphere over several Mars years. With its data, upper limits of potential constituents have been constrained, the accuracy of species鈥 concentration measurements has been improved, and seasonal and spatial variations in the atmosphere have been observed. The wealth of data obtained has addressed several open questions about the nature of Mars鈥 atmosphere, while other measurements have revealed much that remains poorly understood. For example, models continue to struggle to reproduce ozone distributions, both spatially and temporally, as well as seasonal variations in atmospheric oxygen (O2), suggesting that some key photochemical interactions may be being overlooked. As another example, despite seven years of dedicated observations producing very low upper limits on atmospheric methane levels, there remains no unifying hypothesis that simultaneously explains the detections reported by other Mars assets at Gale Crater [e.g., 1-4].Hydrogen chloride鈥攖he first new gas detected by TGO [5,6]鈥攈as been investigated recently using the mid-infrared channel on TGO鈥檚 Atmospheric Chemistry Suite (ACS MIR) [7,8]. Observations show a strong seasonal dependence of HCl in the atmosphere, with almost all detections occurring during the latter half of the year between the start of dust activity and the southern hemisphere autumnal equinox. There are also unusual measurements of HCl, localised in both time and space, during the aphelion season. Chlorine-bearing species such as HCl are important to understand in the Mars atmosphere because on Earth they are involved in numerous processes throughout the planetary system, including volcanism, from which HCl on Earth ultimately originates. Further, chlorine species play a key role in atmospheric chemistry: they influence oxidative chemistry and variations in the aforementioned O2 and O3 concentrations (e.g., by catalysing the destruction of ozone), and by extension, potential CH4 in the Martian atmosphere [9]. However, much remains unknown about original source and sinks of HCl, as well as the factors controlling its distribution and variation.Here, we use the Mars Planetary Climate Model鈥攁 3-D global climate model that includes a photochemical network鈥攖o investigate potential mechanisms accounting for patterns in ozone and HCl detections and interactions between them. We begin with the role of heterogeneous chemistry involving ice and dust aerosols, by implementing modelling developed for the Open University Mars Global Climate Model [10] and building on existing chlorine photochemical model networks [11,12,13]. Heterogeneous chemistry affects the abundances of聽oxidative species such as OH and HO2, and by extension, O and O3. In addition, we investigate how such processes can potentially serve as a mechanism for direct release and sequestration of HCl from the atmosphere. We also explore potential mechanisms behind the annual occurrence of spatially-constrained aphelion HCl, including volcanic sources, and we investigate the interplay between chlorine-bearing species and OH, HO2,O, and O3. Figure 1 shows the way that HCl appears during spring and summer in the southern hemisphere (solar longitudes 180-360掳) when water vapour is present in the Martian atmosphere. Ozone behaves in the opposite manner and is present when water vapour abundances are low. As shown, these species are anti-correlated; we explore the important chemical pathways connecting them.Understanding the role of oxidative chemistry on HCl and other trace gases is key to achieving a more complete picture of processes occurring in the present-day Mars atmosphere, as well as processes that have shaped its evolution and habitability.Figure 1: Observations of CO, O2, O3 and HCl seasonally and across multiple Mars Years. Upper panel: CO and O2 observations from Curiosity鈥檚 Sample Analysis at Mars (SAM) instrument (stars; [14]) and the Mars Climate Database (lines; [15]). Lower panel: O3 and HCl observations from TGO鈥檚 ACS instrument [8]. MY=Mars Year; NH/SH=northern/southern hemisphere. Figure from Kevin Olsen.References:[1] Giuranna, M.,聽et al.聽(2019). Nat. Geosci. 12, 326鈥332. [2] Korablev, O.聽et al.聽(2019). Nature聽568, 517鈥520. [3] Montmessin, F. et al. (2021). Astron. Astrophys. 650, A140. [4] Webster, C. R.聽et al. (2015). Science 347, 415-417. [5] Korablev O. I. et al. (2021). Sci. Adv., 7, eabe4386. [6] Olsen K. S. et al. (2021). Astron. Astrophys., 647, A161. [7] Olsen K. S. et al. (2024a). JGR, 129, e2024JE008350. [8] Olsen K. S. et al. (2024b). JGR, 129, e2024JE008351. [9] Taysum, B. M. et al. (2024). Astron. Astrophys., 687, A191. [10] Brown M. A. J. et al. (2022). JGR, 127, e2022JE007346. [11] Rajendran, K. et al. (2025). JGR: Planets 130(3), p.e2024JE008537. [12] Streeter, P. M. et al. (2025). GRL 52(6), p.e2024GL111059. [13] Benne, B. et al. (2024). EPSC, pEPSC2024-1037. [14] Trainer, M. G. et al. (2019). JGR 124, 3000. [15] Millour, E. et al. (2022). Mars Atmosphere: Modelling and Observations, p. 1103.