Exoplanet atmospheres

Abstract:

Introduction: Due to Uranus’ weak thermal radiance, the thermal and compositional structures of its atmosphere have remained poorly characterised. Here, using the unprecedented sensitivity of JWST's MIRI and NIRSpec instruments, we present an analysis of Uranus' spatially resolved spectrum spanning the near- and mid-infrared, revealing how temperatures, composition, and clouds vary across the planet's northern hemisphere.Observations: JWST observed Uranus on 8--9 January 2023 (program1248) as part of the Solar System Guaranteed Time Observations (GTO). Integral field spectroscopy (IFS) with the Near-Infrared Spectrograph (NIRSpec) and the Mid-Infrared Instrument/Medium Resolution Spectrometer (MIRI/MRS) were combined to provide nearly simultaneous and continuous spatial and spectral data between 1.66 and 28.70 microns.Temperatures: The nearly continuous spectral coverage offered by the combination of NIRSpec and MIRI provide constraints on the temperature structure from the stratosphere down to several bars. The average temperature-pressure vertical profile is largely consistent with that determined from Spitzer [1], but the spatially resolved JWST reveal how these temperatures vary with latitude in the stratosphere and cloud layer for the first time [2]. They also suggest the possibility of a sub-adiabatic cloud layer.Chemistry: Our radiative transfer analysis of MIRI-MRS spectra 1) provide new constraints on minor species in Uranus’ stratosphere and 2) reveals how various hydrocarbons vary as a function of latitude. The observed distributions are indicative of a combination of seasonal photochemistry [3] and dynamical processes, as we will briefly discuss.Clouds and hazes: Finally, we briefly examine the vertical cloud structure and its latitudinal variation as sensed by NIRSpec data. The data reveal the opacity of Uranus clouds and hazes spanning the transition from scattered sunlight to thermal emission for the first time. The overall vertical structure suggested by these new data largely agrees with that of prior work [3,4,5], but a comparison between observed and model spectra reveal interesting discrepancies and possibly a need for additional sources of opacity. [1] Orton, G.S., Fletcher, L.N., Moses, J.I., Mainzer, A.K., Hines, D., Hammel, H.B., Martin-Torres, F.J., Burgdorf, M., Merlet, C., Line, M.R.: Mid-infrared spectroscopy of uranus from the spitzer infrared spectrometer: 1. determination of the mean temperature structure of the upper troposphere and stratosphere. Icarus 243, 494–513 (2014)[2] Roman, M.T., Fletcher, L.N., Orton, G.S., Rowe-Gurney, N., Irwin, P.G.: Uranus in northern midspring: persistent atmospheric temperatures and circulations inferred from thermal imaging. The Astronomical Journal 159(2), 45 (2020)[3] Moses, J.I., Fletcher, L.N., Greathouse, T.K., Orton, G.S., Hue, V.: Seasonal stratospheric photochemistry on uranus and neptune. Icarus 307, 124–145 (2018)[4] Sromovsky, L.A., Karkoschka, E., Fry, P.M., Pater, I., Hammel, H.B.: The methane distribution and polar brightening on uranus based on hst/stis, keck-nirc2, and irtf/spex observations through 2015. Icarus 317, 266–306 (2019)189[5] Irwin, P.G., Teanby, N.A., Fletcher, L.N., Toledo, D., Orton, G.S., Wong, M.H.,Roman, M.T., Perez-Hoyos, S., James, A., Dobinson, J.: Hazy blue worlds:A holistic aerosol model for uranus and neptune, including dark spots[6] Roman, M.T., Banfield, D., Gierasch, P.J.: Aerosols and methane in the ice giant atmospheres inferred from spatially resolved, near-infrared spectra: I. uranus, 2001–2007. Icarus 310, 54–76 (2018)

Abstract:

Titan’s entire stratosphere is in superrotation (Flasar et al. 2005) and appears to rotate about an axis offset from its solid body rotation axis by around 4o (Achterberg et al. 2008). The stratospheric tilt axis has been estimated previously through temperature measurements (Achterberg et al. 2011; 2008), composition retrievals (Sharkey et al. 2020; Teanby 2010), and by analysis of stratospheric haze (Kutsop et al. 2022; Roman et al. 2009; Snell and Banfield 2024; Vashist et al. 2023) and a polar cloud (West et al. 2016). Despite this, the mechanism causing the tilt is not well understood. This challenge is further heightened as Titan General Circulation Models (GCMs) are yet to resolve a tilt consistent with observations (e.g., Lombardo and Lora (2023a; 2023b)).Understanding the cause of Titan’s stratospheric tilt may provide insight into the underlying dynamics that drive superrotation in Titan’s atmosphere and the behaviour of superrotating atmospheres in general. Furthermore, due to the strength of Titan’s zonal winds, the offset of the stratospheric rotation axis may have a significant effect on the atmospheric descent of the upcoming Dragonfly mission to Titan. Thus, improved constraints on the tilt axis may better inform the landing site calculations for Dragonfly.We determine the evolution of Titan’s stratospheric tilt axis over 13 years (Ls = 293—93o), which spans almost half a Titan year. The tilt was determined by inspecting zonal symmetry in the (i) thermal and (ii) composition structure of Titan’s stratosphere. These two independent methods probe different latitude regions. We use infrared observations acquired by the Composite Infrared Spectrometer (CIRS) (Flasar et al. 2004; Jennings et al. 2017; Nixon et al. 2019) instrument onboard the Cassini spacecraft, which toured the Saturn system from 2004 to 2017. We use nadir CIRS observations acquired at a low apodised spectral resolution (FWHM∼13.5–15.5 cm−1). This data set provides excellent spatial coverage of Titan’s middle atmosphere throughout the Cassini mission and achieves the best horizontal spatial resolution of any of the CIRS observations. Despite the subtle and often blended spectral features in these data, Wright et al. (2024) show that they can be reliably forward modelled. Vertical profiles of temperature and gas volume mixing ratios (VMRs) are estimated from CIRS FP3/4 spectra using the Non-linear Optimal Estimator for MultivariatE Spectral AnalySIS (NEMESIS) radiative transfer and retrieval code (Irwin et al. 2008). The observations probe pressure levels of ~10—10-3 mbar in Titan’s atmosphere, with peak contributions at around 1 mbar. These data enable us to reveal Titan’s stratospheric thermal and composition structure in the highest meridional resolution to date and facilitate an independent study of the tilt offset of Titan’s stratosphere.We find that the tilt axis in the mid-latitudes (from (i)) and the equatorial region (from (ii)) are in good agreement, which supports the theory that Titan’s entire stratosphere is tilted relative to its solid body (Achterberg et al. 2008). In addition to this, we present the best evidence yet that the pointing direction of Titan’s stratospheric tilt axis is constant in the inertial reference frame (Wright et al. in press), consistent with previous studies (Achterberg et al. 2011; Kutsop et al. 2022; Sharkey et al. 2020; Snell and Banfield 2024). The tilt azimuth is determined to be 121± 7o West of the sub-solar point at Titan’s northern spring equinox (Ls = 0o). Put another way, the pointing direction of the tilt axis would appear constant to an observer looking down on the Solar System.In addition, we present new evidence that the magnitude of Titan’s stratospheric tilt axis may have a seasonal dependence, oscillating between values of approximately 2o to 10o with a period similar in length to half a Titan year. If this pattern is real, it suggests that the tilt of Titan’s stratosphere is impacted by seasonal forcing, even though the direction of the tilt remains constant.Fig 1: Schematic showing the direction of Titan’s stratospheric tilt axis from Wright et al. (in press). Titan and Saturn are shown at some example times in their orbit. The tilt direction is determined to be approximately constant in the inertial reference frame, that is, fixed with respect to the Titan-Sun vector at northern spring equinox (Ls = 0◦). The approximate size of the tilt magnitude, β, is indicated by font size. References:Achterberg, R. K., et al. 2008. Icarus 197 (2): 549–55. https://doi.org/10.1016/j.icarus.2008.05.014.Achterberg, R. K., et al. 2011. Icarus 211 (1): 686–98. https://doi.org/10.1016/j.icarus.2010.08.009.Flasar, F. M., et al. 2005. Science 308 (5724): 975–78. https://doi.org/10.1126/science.1111150.Flasar, F. M., et al. 2004. Space Science Reviews 115 (1–4): 169–297. https://doi.org/10.1007/s11214-004-1454-9.Irwin, P.G.J., et al. 2008. Journal of Quantitative Spectroscopy and Radiative Transfer 109 (6): 1136–50. https://doi.org/10.1016/j.jqsrt.2007.11.006.Jennings, D. E., et al. 2017. Applied Optics 56 (18): 5274. https://doi.org/10.1364/AO.56.005274.Kutsop, N. W., et al. 2022. The Planetary Science Journal 3 (5): 114. https://doi.org/10.3847/PSJ/ac582d.Lombardo, N. A., and J. M. Lora. 2023a. Journal of Geophysical Research: Planets 128 (12): e2023JE008061. https://doi.org/10.1029/2023JE008061.Lombardo, N. A., and Juan M. Lora. 2023b. Icarus 390 (January):115291. https://doi.org/10.1016/j.icarus.2022.115291.Nixon, C. A., et al. 2019. The Astrophysical Journal Supplement Series 244 (1): 14. https://doi.org/10.3847/1538-4365/ab3799.Roman, M. T., et al. 2009. Icarus 203 (1): 242–49. https://doi.org/10.1016/j.icarus.2009.04.021.Sharkey, J., et al. 2020. Icarus 337 (February):113441. https://doi.org/10.1016/j.icarus.2019.113441.Snell, C., and D. Banfield. 2024. The Planetary Science Journal 5 (1): 12. https://doi.org/10.3847/PSJ/ad0bec.Teanby, N. A. 2010. Faraday Discussions 147:51. https://doi.org/10.1039/c001690j.Vashist, Aadvik S, et al. 2023. The Planetary Science Journal 4 (6): 118. https://doi.org/10.3847/PSJ/acdd05.West, R. A., et al. 2016. Icarus 270 (May):399–408. https://doi.org/10.1016/j.icarus.2014.11.038.Wright, L., et al. 2024. Experimental Astronomy 57 (2): 15. https://doi.org/10.1007/s10686-024-09934-y.Wright, L., et al. in press. The Planetary Science Journal. https://doi.org/10.3847/PSJ/adcab3.

Abstract:

The radiative heat balance of Uranus has long been a mystery amongst the solar system giant planets. Jupiter, Saturn and Neptune all emit much more power thermally (Pout) than they absorb from the Sun (Pin) with Pout/Pin having values of 1.7 to 2.6. This shows that all three planets retain a considerable amount of heat left over from formation, which they are still slowly radiating away into space. In stark contrast, Uranus appears to be unexpectedly cold. Measurements made by Voyager-2 determined a radiative heat balance ratio of only Pout/Pin = 1.06 ± 0.08 (Pearl et al. 1990), which is consistent (to within error) with Uranus being in thermal equilibrium with the Sun and thus, perhaps, having no heat of formation left over at all. Meanwhile, Voyager-2 determined a radiative heat balance ratio for Neptune of Pout/Pin = 2.61 ± 0.28 (Pearl and Conrath, 1991), which is the largest ratio determined for any of the giant planets.How can the radiative heat balance ratios of Uranus and Neptune, the solar system’s ‘Ice Giants’ be so different? And is Uranus really in thermal equilibrium with the Sun, with no internal heat of formation left over? To answer this last question, we have performed a modelling study (Irwin et al., 2025) using our NEMESIS radiative transfer tool (Irwin et al., 2008) and a newly developed ‘holistic’ atmospheric model of the aerosol structure in Uranus’s atmosphere, based upon observations made by HST/STIS, Gemini/NIFS and IRTF/SpeX from 2000 – 2009 (Irwin et al., 2022). Taking our fitted aerosol structure and extrapolating our calculations to all wavelengths, we have made a new estimate of the bolometric geometric albedo of Uranus during the period 2002 – 2009 of p* = 0.249. The bolometric geometric albedo is the fraction of sunlight reflected by the planet back towards an observer in line with the Sun, but to determine heat balance we need to calculate the bolometric Bond Albedo, which is the fraction of sunlight incident on the planet that is scattered into all directions. With our holistic aerosol model and NEMESIS, we can calculate the appearance of Uranus to an observer at any phase angle from the Sun, and integrating these modelled curves over all phase angles we can calculate the phase integral, q, which relates the geometric albedo, p, to the Bond albedo, A, through the relation A = pq.From this modelling we determine a bolometric (i.e., integrated over all wavelengths) phase integral of 𝑞∗ = 1.36, and thus a bolometric Bond albedo of 𝐴∗ = 0.338 for the period 2002 – 2009. However, to determine the overall radiative heat balance of Uranus, we first need to account for the seasonal variation in 𝐴∗, which changes significantly during Uranus’s year due to the formation of a polar ‘hood’ of haze over the summer pole, which becomes thicker and more observable near the solstices. In addition, in terms of energy balance, we also need to account for the fact that the incident sunlight at Uranus varies significantly during its eccentric (e = 0.046) orbit about the Sun by ±10%. Also, since Uranus is significantly oblate and has high polar inclination, there is a small, but significant difference in its projected area towards the Sun between solstice and equinox, which affects the total power of sunlight received by the planet.To estimate the orbital-average bolometric Bond albedo and radiative heat balance we used a simple seasonal model, developed by Irwin et al. (2024) to be consistent with the disc-integrated blue and green magnitude data from the Lowell Observatory from 1950 – 2016 (Lockwood, 2019). Taking all hood thickness/visibility, distance and projected area effects into account, we model how Uranus’s reflectivity and heat budget vary during its orbit and determine a new orbital-mean average value for the bolometric Bond albedo of 𝐴∗ = 0.349 ± 0.016 and estimate the orbital-average mean absorbed solar flux to be  𝑃in = 0.604 ± 0.027 W m−2. Assuming the outgoing thermal flux to be 𝑃out = 0.693 ± 0.013 W m−2, previously determined from Voyager 2 observations, we arrive at a new estimate of Uranus’s average heat flux budget of Pout/Pin = 1.15 ± 0.06. We find, however, that there is considerable variation of the radiative heat balance with time due mainly to Uranus’s orbital eccentricity, which leads Pout/Pin to vary from 1.03 near perihelion, to 1.24 near aphelion. We conclude that although Pout/Pin is still considerably smaller than for the other giant planets, Uranus is not in thermal equilibrium with the Sun.References. Irwin et al. (2008) DOI:10.1016/j.jqsrt.2007.11.006; Irwin et al. (2022) DOI: 10.1029/2022JE007189; Irwin et al. (2024) DOI: 10.1093/mnras/stad3761;Irwin et al. (2025) DOI: 10.48550/arXiv.2502.18971; Lockwood (2019) DOI: 10.1016/j.icarus.2019.01.024; Pearl et al. (1990) DOI:  10.1016/0019-1035(90)90155-3; Pearl and Conrath (1991) DOI: 10.1029/91JA01087; Wenkert (2023) DOI: 10.17189/T2R8-RK88

Abstract:

Recent JWST transmission and emission spectroscopic observation of hot Jupiters have demonstrated that mineral clouds are likely common in hot Jupiter atmospheres. These mineral clouds have long been predicted to form and persist on the nightside and western dayside of hot Jupiters by cloud microphysical models and 3D General Circulation Models. Given the capability of JWST and recent advancements in modeling techniques, the time is right to determine the prevalence and distribution of mineral clouds across the parameter regime of hot Jupiters in order to provide a detailed test of our theoretical understanding of cloud nucleation, transport and growth processes, and the radiative feedback of clouds on the atmospheric circulation and climate of hot Jupiters. In this work, we develop an indirectly coupled cloud microphysics and atmospheric dynamics framework in order to present theoretical expectations for the 3D distribution of mineral clouds across hot Jupiter planetary parameter space. To do so, we develop a fundamental understanding of the cloud speciation alongside particle size and spatial distribution by feeding the results of 3D cloud-free GCMs into 1D CARMA cloud microphysics simulations. We then use these results to drive 3D MITgcm simulations of hot Jupiters with cloud-radiative feedback. We use a grid of GCM simulations to assess the radiative impact on clouds of the climates of hot and ultra-hot Jupiters. Notably, we find that mineral cloud particle size distributions are not ubiquitously unimodal and log-normal, leading to potentially stark differences between the 3D cloud distributions in our work compared to previous work that assumed a single log-normal cloud particle size distribution. Finally, we will discuss paths forward toward coupling the cloud microphysics and atmospheric dynamics of hot Jupiters using a modeling hierarchy encompassing multi-dimensional cloud microphysics and atmospheric dynamics.

Abstract:

IntroductionSaturn’s largest moon, Titan, has a dense atmosphere comprised mostly of nitrogen and methane. The photolysis and ionization of these major componentsleads to complex chemical reactions, which create substantial diversity among Titan’s minor atmospheric constituents. Remote sensing and molecular  pectroscopy historically have been a critical tool for detecting trace gases in Titan’s atmosphere and help corroborate predictions of Titan’s atmospheric composition from photochemical models. Following the Voyager and Cassini missions, which provided a wealth of spectroscopic studies of Titan’s  atmosphere, ground-based measurements have been useful for detecting elusive trace gases. The Echelon-Cross-Echelle Spectrometer (EXES) is a high-resolution (R ∼ 90, 000) mid-infrared spectrometer that was previously operated aboard NASA’s Stratospheric Observatory For Infrared Astronomy (SOFIA)(1 ). EXES benefited from the high altitude flights during the SOFIA mission to make observations above the bulk of the atmosphere to avoid strong telluric absorption lines that inhibit ground based mid-IR spectrometers such as its sister instrument TEXES.Here we present EXES observations of Titan which were made in an attempt to detect two trace gases, triacetylene (C6H2) and dicyanoacetylene (C4N2). C6H2 is an important polyyne and is predicted to form readily from the addition of the ethynyl (C2H) radical with diacetylene (C4H2). It remains yet tobe detected, though, and the previous upper limit study was limited by the lower spectral resolution of Voyager’s IRIS (R ∼ 145)(2 ). Delpech et al. 1994 derived an upper-limit of 6 × 10−11 which would be detectable by EXES.Gas-phase C4N2 formation is primarily completed through C3N addition to HCN or, alternatively, CN addition to HC3N(3 ). The ice-phase C4N2, which is formed through solid-state photochemical reactions on the surface of HC3N ice grains, has been detected in spectra measured by Voyager’s IRIS and CIRSduring the Cassini mission (4, 5 ), yet C4N2 in the gas-phase remains elusive to spectroscopic detections. Again, previous studies of the gas-phase upper limits (3σ = 1.53 × 10−9) were performed using spectra collected by CIRS (R ∼ 1240) which has a resolving power significantly lower than EXES(6 ). The high-resolution of EXES will help improve on the upper limits of both of these species and allow for an updated comparison to photochemical model predictions of their vertical profiles in Titan’s atmosphere.Observations and ModelingMid-infrared observations of Titan were made in June of 2021, using EXES. These observations aim to detect the ν11 out-of-plane bending mode of C6H2 at 621 cm−1 and the perpendicular ν9 stretch of the gasphase C4N2 at 472 cm−1. Figure 1 shows a small portion of the EXES spectrum measured at the 621 cm−1 spectral setting. In this region there are strong emission features from diacetylene (C4H2) and propyne (C3H4) which must be fit before analyzing the C6H2 upper limits. Highlighted in the blue box is the region where the ν11 vibrational mode for C6H2 should be present.To model the collected spectra, we use the arch-NEMESIS radiative transfer package which is a new Python implementation of the NEMESIS radiative transfer code (7, 8 ). The radiative transfer modeling of the measured spectra occurs in two steps. The initial step is to retrieve the atmospheric profiles of the aerosols and known gases using the archNEMESIS optimal estimation algorithm. For the 621 cm−1 spectral setting, the vertical profiles of C4H2, C3H4, and aerosol continuum are retrieved, however, at the 472 cm−1 region, there are no emission features to fit and just the continuum level is retrieved by adjusting the aerosol profile. For both spectral regions, we use a temperature profile and initial gas profiles defined in Vuitton et al. 2019 photochemical model (3 ). The quality of each retrieval is determined by a goodnessof-fit metric (χ2) which compares the residual of the modeled spectrum to the noise of the measurement. Following the retrieval, we derive the upper limits by building forward models of the spectral regions where the abundance of each target species is iteratively increased and a subsequent χ2 is determined. We then take the difference, Δχ2, between the retrieved and updated forward model χ2 to find where the abundance causes significant deviation from the retrieved spectrum. Step-profiles, which have a cutoff altitude and constant abundance above this cutoff, were used to determine the upper-limits for each species. This method has been applied for many different upper limits studies of gases predicted in Titan’s atmosphere (9, 10 ).ResultsBased on these observations, C6H2 and gas-phase C4N2 remain undetected and therefore, we derive the upper limits to their atmospheric abundance. We improve upon the upper limits of C6H2 and C4N2 by an order of magnitude for both species. Figure 2 shows Δχ2 increase sharply with increased abundance for both C6H2 and C4N2. For C6H2 the 3σ upper limit (Δχ2 = 9) is on the order of 10−11 and for C4N2, 10−10. These new upper limits improve on the previously derived upper limits by an order of magnitude for each target species. More work is still being done to precisely determine the upper limits and compare these values to the current photochemical model predictions of their abundance. The values of the 1σ, 2σ, and 3σ upper limits for each species will be reported in the presentation. The upper limits derived improved upon the previous upper limits by an order of magnitude and we are currently working on comparing these upper limits to photochemical models of Titan’s atmospheric composition to build a better understanding of the chemical pathways in Titan’s atmosphere which will also be discussed in the presentation. AcknowledgmentsThe material is based upon work supported by NASA under award number 80GSFC24M0006.References1. Richter et al., 20182. Delpech et al., 19943. Vuitton et al., 20194. Samuelson et al, 19975. Anderson et al, 20166. Jolly et al., 20157. Alday et al, 20258. Irwin et al., 20089. Nixon et al., 201010. Teanby et al., 2013