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The Rise and Fall of a Mid-West Tilt: Seasonal Evolution of Titan’s Stratospheric Tilt Axis

(2025)

Authors:

Lucy Wright, Nicholas Teanby, Patrick Irwin, Conor Nixon, Nicholas Lombardo, Juan Lora, Daniel Mitchell

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.
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The bolometric Bond albedo and energy balance of Uranus

(2025)

Authors:

Patrick Irwin, Daniel Wenkert, Amy Simon, Emma Dahl, Heidi Hammel

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
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Using SOFIA’s EXES to improve the upper limits for C6H2 and C4N2 in Titan’s atmosphere

(2025)

Authors:

Zachary McQueen, Curtis DeWitt, Antoine Jolly, Juan Alday, Nicholas Teanby, Véronique Vuitton, Panayotis Lavvas, Joseph Penn, Patrick Irwin, Conor Nixon

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
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VIVA (Venus' Interior, Volcanism and Atmosphere): a Venus mission to reveal unknown interior structure, thermosphere dynamics and meteoroid flux from atmospheric response to seismic waves, volcanic events and external forcings

(2025)

Authors:

Raphael Garcia, Matthias Grott, Neil Bowles, Jim Cutts, Elizabeth Klioner, Marouchka Froment, Gabriella Gilli, Lauriane Soret, Apostolos Christou

Abstract:

Despite being often described as Earth’s sister planet due to a similar distance to the Sun and comparable size, Venus’s internal structure and geodynamic regime, together with its upper atmosphere dynamics and asteroid entry rates, are poorly constrained. Whereas Venus is a prime candidate for being a tectonically active planet and presents a very dynamic atmosphere, future missions will not constrain high frequency phenomena such as seismic waves, meteoroid impacts, and high frequency gravity waves. These short duration events can be used to infer Venus' seismicity, internal structure, upper atmosphere dynamics and the small Solar System bodies population [1].We present a mission concept that targets high rate observations of upper atmosphere airglow emissions on both the day and night side of Venus, as well as thermal imaging in the visible. These observations will allow us to image the propagation of acoustic waves generated by seismic waves, enabling us to investigate quake locations and magnitudes, as well as to determine the structure of the crust and upper mantle. Volcanic events will also be studied through the associated increase in surface and atmosphere temperature. In addition, variations in airglow emissions will constrain the transfer of mechanical energy from the lower atmosphere to the thermosphere, as well as atmosphere dynamics (winds) and composition, and its response to solar forcing. Finally, the observation of fireballs produced by asteroid entries will constrain the asteroid population that crosses Venus’s orbit.The instruments required to perform these high rate observations are presented. They are based on a strong heritage relying on previous implementations in planetary missions.The mission concept and spacecraft demand new capabilities in terms of on-board attitude determination and data processing capabilities. In particular, a dedicated on-board data processing unit capable of autonomously detecting different event types with advanced algorithms, including machine learning methods, has been identified as a key component of the mission. This unit will also be used to average out phenomena over different temporal and spatial scales. To maximise science return, the mission will adopt an operational concept involving the capability to download high rate event data from a first quicklook information, similar to the one implemented on InSight NASA mission.The feasibility of the mission, already partly demonstrated by VAMOS JPL/NASA mission concept study [2,3], is validated through a dedicated mission analysis study.References[1] Christou A.A., Gritsevich M. 2024. Feasibility of meteor surveying from a Venus orbiter, Icarus, 417, 15 July 2024, 116116, DOI 10.1016/j.icarus.2024.116116[2] Sutin, B.M. et al. In Space Telescopes and Instrumentation 2018: Optical, Infrared, and Millimeter Wave, volume 10698. SPIE, 2018. doi:10.1117/12.2309439.[3] Didion, A. et al. In 2018 IEEE aerospace conference. IEEE, 2018.
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What goes on inside the Mars north polar vortex?

(2025)

Authors:

Kevin Olsen, Bethan Gregory, Franck Montmessin, Lucio Baggio, Franck Lefèvre, Oleg Korablev, Alexander Trokhimovsky, Anna Fedorova, Denis Belyaev, Juan Alday, Armin Kleinböhl

Abstract:

Mars has an axial tilt of 25.2°, comparable to that on Earth of 23.4°. This gives rise to very similar seasons, and even leads to our definition of Martian time, aligning the solar longitudes (Ls) such that Ls 0° and 180° occur at the equinoxes. In the northern hemisphere, between the equinoxes, the north polar region experiences polar days without darkness in spring and summer, and days of total darkness in the fall and winter. The dark polar winters give rise to a polar vortex that encircles the polar region and encircles an atmosphere of very cold and dry air bound within (1-3).The Atmospheric Chemistry Suite (ACS) mid-infrared channel (MIR) on the ExoMars Trace Gas Orbiter (TGO; 4) operates in solar occultation mode in which the Sun is used as a light source when the atmosphere lies between the Sun and TGO. The tangent point locations of ACS MIR observation necessarily lie on the solar terminator on Mars. At the poles when either polar night or polar day are experienced, there is no terminator, and solar occultations are restricted to outside such a region. The latitudinal distribution of ACS MIR solar occultations during the north polar fall and winter over four Mars years (MYs) is shown in Fig. 1. The furthest northern extent of observations occurs at the equinoxes, and falling northern boundary is seen between, as the north pole points further away from the Sun (similarly in the south, where it is polar day).While direct observations of the north polar vortex are forbidden with solar occultations, the polar vortex is not perfectly circular (1-3) and occasionally, descends into the illuminated region where we are making observations. The characteristic signs that we are sampling the polar vortex are a sudden drop in temperature below 20 km, the almost complete reduction in water vapour volume mixing ratio (VMR) and an enhancement in ozone VMR, the latter of which is extremely rare (5).To measure the extent of the polar vortex, we use temperature measurements from the Mars Climate Sounder (MCS; 6, 7) on Mars Reconnaissance Orbiter (MRO). We define the polar vortex as the average temperature over 10-20 km being within a boundary of 170 K (30). We introduce a novel technique to determine this boundary during a 1° Ls period using an alpha hull. We show that we can accurately measure the area of the polar vortex and achieve similar results to (3). The impact of the southern summer and dust activity is clearly visible in the time series of the northern polar vortex extent, leading to maxima occurring at the equinoxes, and shrinking toward perihelion. The impact of global dust storms and the late season dust storms are also pronounced.We will show the vertical structure of water vapour and ozone VMRs inside and outside the north polar vortex, the results of a search for polar vortex temperatures from the near-infrared channel (NIR) of ACS (along the dark blue dots in Fig. 1), and show whether these results agree with the polar vortex extent measurements using MCS.       Figure 1: The latitudes of ACS MIR solar occultation as a function of time (solar longitude Ls) during northern fall (Ls 180-270°) and winter (Ls 270-360°). Data from Mars years (MYs) 34-37 are indicated with colours. The region of interest in searching for polar vortex excursions is highlighted in blue.References:(1) Streeter, P. M. et al. J. Geophys. Res. 126, e2020JE006774 (2021).(2) Streeter, P. M., Lewis, S. R., Patel, M. R., Holmes, J. A., & Rajendran, K. Icarus 409, 115864 (2024).(3) Alsaeed, N.R., Hayne, P. O. & Concepcion, V. J. Geophys. Res. 129, e2024JE008397 (2024).(4) Korablev, O. et al. Space Sci. Rev. 214, 7 (2018).(5) Olsen, K. S., et al. J. Geophys. Res. 127, e2022JE007213 (2022).(6) Kleinböhl, A., et al. J. Geophys. Res., 114, E10006 (2009).(7) Kleinböhl, A., Friedson, A. J., & Schofield, J. T. J. Quant. Spectrosc. Radiat. Transfer. 187, 511-522 (2017).
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