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We present first results from the James Webb Space Telescope (JWST) observations of Uranus, which provide the first spatially resolved, infrared spectra of the planet’s atmosphere spanning from 1.66 to 28.6 µm. We evaluate these unprecedented JWST NIRSpec (1.66–3.05 µm, 2.87–5.14 µm) and MIRI (4.9-28.6 μm) spectra in the context of existing observations and questions concerning Uranus’ stratospheric chemistry and thermal structure [1].Owing to its frigid atmospheric temperatures, Uranus’ infrared spectrum is extremely weak. Much of the spectrum has never been spatially resolved before, while some had never been clearly observed at all.From the ground, spatially resolved observations of Uranus’ mid-infrared emission are limited to imaging observations targeting the brighter regions of the infrared spectrum (i.e. ~13 µm emission from stratospheric acetylene, and 17-25 µm from the H2 continuum). Images from the Very Large Telescope VISIR instrument at 13 µm show a stratospheric structure distinct to Uranus, with elevated radiance at high latitudes. The physical nature of this structure–-whether produced by chemical or thermal gradients–-is unclear given previously available data [1]. From space, the Spitzer Space Telescope observed Uranus' mid-infrared spectrum between ~7 and 36 µm, but it lacked the spatial resolution necessary to resolve potential thermal and chemical structure across the disk [2].Now, with its exceptional sensitivity and outstanding spatial and spectral resolution, JWST reveals Uranus’ stratospheric temperature and chemistry with exquisite new detail, placing new constraints on hydrocarbon abundances and temperature structure across the disk.With a projected lifetime of over a decade, JWST promises to continue providing exciting new insights into the atmospheric structure, composition, and variability of the ice giants for years to come.[1] Roman, M.T, et al. "Uranus in northern..." AJ 159.2 (2020): 45.[2] Rowe-Gurney, N., et al. "Longitudinal variations..." Icarus 365 (2021): 114506.

In this talk, we present highlights from our recent analyses of mid-infrared observations of Uranus and Neptune, and we look ahead to anticipated discoveries from the James Webb Space Telescope. Drawing from a combination of archival and recent ground-based imaging and spectroscopy, we examine the spatial structure and trends of mid-infrared emission from the ice giant atmospheres. We report on surprising temporal variability in the atmosphere of Neptune (see Figure 1) with an unexpected decline in stratospheric temperatures since at least 2003. Recent VLT-VISIR imaging and spectroscopy are presented, revealing how this trend has progressed. In contrast, we show that no evidence yet exists of long-term thermal changes in Uranus’ stratosphere, but mid-IR observations of Uranus are still extremely limited. The observed spatial structure of Uranus’ mid-infrared emission is intriguing (Figure 2), as it is inconsistent with simple models of the atmospheric circulation and/or chemistry and its physical interpretation remains unclear. We share recent observations from VLT-VISIR and express the need for continued ground-based imaging for both Uranus and Neptune. Finally, we discuss how the James Webb Space Telescope MIRI observations will help greatly advance our understanding of the ice giants in the years ahead. In particular, given its supreme sensitivity compared to ground-based observations, JWST-MIRI observations will resolve the nature of Uranus stratospheric thermal/chemical structure and reveal how temperature and chemical abundances change with the seasons in the atmospheres of both ice giants. Figure 1: Observed changes in Neptune’s thermal-infrared brightness, a measure of temperature in Neptune’s atmosphere. The plot shows the relative change in the thermal-infrared brightness from Neptune’s stratosphere with time for all existing images taken by ground-based telescopes. Brighter images are interpreted as warmer. Corresponding thermal-infrared images (top) at wavelengths of ~12 µm show Neptune’s appearance in 2006, 2009, 2018 (observed by the European Southern Observatory’s Very Large Telescope VISIR instrument), and 2020 (observed by Subaru’s COMICS instrument). The south pole appears to have become dramatically warmer in just the past few years. Figure 2: Uranus' 13-micron images from VLT-VISIR in 2009 and 2018, with enhanced emission from high latitudes. From current ground-based observations alone, it is unclear whether the enhanced radiances are due to greater temperatures or enhance acetylene abundances, but JWST will solve this mystery.

We present the primary results from our recent analyses of mid-infrared observations of Neptune and Uranus from ground-based telescopes, including VLT-VISIR, Subaru-COMICS, and Gemini-TEXES. We discuss our recent discovery that Neptune’s stratospheric temperatures appear to be changing dramatically in just the past few years, following decades of cooling. In contrast, we show that no evidence yet exists of long-term thermal changes in Uranus’ stratosphere, but mid-IR observations of Uranus are still extremely limited. We share new observations from VLT-VISIR, express the need for continued ground-based imaging, and discuss how the James Webb Space Telescope MIRI observations will help greatly advance our understanding of the Ice Giants in the years ahead.

We present an analysis of all currently available ground-based imaging of Neptune in the mid-infrared. Dating between 2003 and 2020, the images reveal changes in Neptune’s mid-infrared (∼8–25 μm) emission over time in the years surrounding Neptune’s 2005 southern summer solstice. Images sensitive to stratospheric ethane (∼12 μm), methane (∼8 μm), and CH3D (∼9 μm) display significant subseasonal temporal variation on regional and global scales. Comparison with H2 S(1) hydrogen quadrupole (∼17.035 μm) spectra suggests that these changes are primarily related to stratospheric temperature changes. The stratosphere appears to have cooled between 2003 and 2009 across multiple filtered wavelengths, followed by a dramatic warming of the south pole between 2018 and 2020. Conversely, upper-tropospheric temperatures—inferred from ∼17 to 25 μm imaging—appear invariant during this period, except for the south pole, which appeared warmest between 2003 and 2006. We discuss the observed variability in the context of seasonal forcing, tropospheric meteorology, and the solar cycle. Collectively, these data provide the strongest evidence to date that processes produce subseasonal variation on both global and regional scales in Neptune’s stratosphere.

1. Overview The James Webb Space Telescope (JWST) will be the premier observatory for infrared planetary science over the coming decade, revolutionising our views of cold, distant targets that have remained out of reach to ground-based facilities. For planetary atmospheres, JWST offers an unprecedented opportunity to explore the weather and climate in three dimensions, from the turbulent cloud layers to the chemically-rich stratospheres, via spatially-resolved spectroscopic mapping in the 1-30 µm range. This presentation will summarise the JWST Giant Planet Atmospheres programme during the first year of science operations (Cycle 1, 2022-23), which is set to observe all four giants. These observations have been made possible by a combination of Guaranteed-Time Observing (GTO), Early Release Science (ERS), and General Observing (GO), and their combination permits comparative planetology of the Gas and Ice Giants. This presentation will: (i) summarise the science goals at each planet; (ii) discuss the available toolkits for data reduction, mapping, and spectral interpretation; and (iii) advocate for a ground-based campaign to support the JWST observations. 2. Giant Planet Observations Ice Giants: Uranus and Neptune Comparing the atmospheric dynamics and chemistry of Uranus and Neptune will provide new insights into the processes shaping intermediate-sized and chemically-enriched worlds. Voyager’s infrared observations were largely limited to the far-IR, and despite recent advances in ground-based observations (Fletcher et al., doi:10.1016/j.icarus.2013.11.035, Roman et al., doi:10.3847/1538-3881/ab5dc7) and disc-integrated spectroscopy (Orton et al., doi:10.1016/j.icarus.2014.07.010; Rowe-Gurney et al., doi:10.1016/j.icarus.2021.114506), Ice Giant stratospheres have remained largely inaccessible. JWST MIRI 5-28 µm spectroscopic maps of Uranus (GTO1248) and Neptune (GTO1249) will reveal spatial gradients of temperature and chemicals, to diagnose the circulation and chemistry of Ice Giant stratospheres for the first time (Moses et al., doi:10.1016/j.icarus.2018.02.004), and link it to meteorological activity in the troposphere. Simultaneous NIRSPEC 2-5 µm spectroscopy of Uranus (GTO1248) will reveal aerosol structures in the troposphere, and H3+ emission in the ionosphere. Both Uranus and Neptune will be observed several times over a full rotation to construct global maps. MIRI observations of Neptune will be repeated when the Ice Giant returns to the Field of Regard (FoR), to search for the sources of temporal variability of mid-IR emission (Roman et al., EPSC2021) over timescales of months (GO1604). Expected dates: Uranus window 1 (2022-08-05 to 2022-09-26), Uranus window 2 (2022-12-22 to 2023-02-09), Neptune window 1 (2022-06-11 to 2022-08-02), Neptune window 2 (2022-10-31 to 2022-12-19). Gas Giants: Jupiter and Saturn The Gas Giants provide a crucial test of JWST’s capabilities to explore extended, rotating, moving, and bright targets. Indeed, the angular sizes of Jupiter and Saturn means that mosaics are required to map atmospheric regions of interest, and the high surface brightness means that MIRI observations are expected to saturate beyond 11 µm (Jupiter) or 16 µm (Saturn). On Jupiter, we have selected two scientific targets: the Great Red Spot will be targeted via mosaics with MIRI (5-11 µm, GTO1246) and NIRSpec (1-5 µm, ERS1373), to understand how the circulation of this archetypal anticyclone influences its temperatures, composition (e.g., ammonia as a cloud-forming volatile, phosphine as a tracer of vertical mixing), and aerosols. Secondly, Jupiter’s south pole will be observed with NIRSpec and MIRI (ERS1373) to understand how energy propagates from the aurora (via H3+ emission) to the stratosphere (via methane emission). ERS1373 will also use NIRCAM imaging (1-5 µm) to measure atmospheric motions at multiple levels in Jupiter’s clouds and tropospheric hazes. For Saturn’s atmosphere, MIRI 5-16 µm spectroscopy (GTO1247) offers the opportunity to revisit the seasonally-evolving giant five years after the demise of the Cassini mission. Saturn’s summertime hemisphere will be mapped from the pole to the equator, capturing the demise of the polar stratospheric vortex since summer solstice in 2017 (Fletcher et al., doi:10.1038/s41467-018-06017-3). The atmospheric observations are part of wider programmes to capture MIRI, NIRSpec, and NIRCAM imaging of Jupiter and Saturn’s satellite and ring systems (GTO1247/ERS1373). Finally, a target-of-opportunity programme (GO1424) will respond within 2 weeks to unforeseen extreme events on either Jupiter or Saturn, providing NIRSpec and MIRI spectroscopic maps of cometary or asteroid impact sites, or the eruptions of significant new storms. In the event of a trigger, this would be repeated twice to capture the evolution of the feature. Expected dates: Jupiter window 1 (2022-06-23 to 2022-08-15), Jupiter window 2 (2022-11-06 to 2022-12-27), Saturn window 1 (2022-05-10 to 2022-07-01), Saturn window 2 (2022-09-27 to 2022-11-16). 3. Data Analysis Toolkit We will present the current status of data analysis tools: Forward-Modelled MIRI spectra: the suite of radiative transfer and spectral analysis tools (NEMESIS, Irwin et al., doi:10.1016/j.jqsrt.2007.11.006) have been adapted to simulate MIRI 5-28 µm spectra in all 12 sub-bands (disperser, detector, and grating combinations), generating idealised synthetic cubes from temperature, composition, and aerosol maps from existing data and/or photochemical models (e.g., Moses et al., doi:10.1016/j.icarus.2018.02.004). MIRI simulations: Synthetic spectra are passed to the MIRISim package to generate MIRI detector images, testing assumptions about exposure times, dither patterns, and background subtraction. Detector images are them passed to the MIRI calibration pipeline, reconstructing MIRI cubes as if they were taken by the observatory. Data processing: MIRI and NIRSpec spectral cubes are then passed through an image navigation pipeline to construct maps, so that multiple dithers/exposures can be coadded. Spectral inversion: NEMESIS will then be used to assess retrievability of the original temperature, composition, and aerosol inputs, preparing the techniques required for the real data in 2022. 4. Ground-Based Support Giant planet atmospheres evolve significantly with time, and spatial, temporal, and spectral context information will be required to fully exploit the JWST observations. For example: observations of Jupiter’s Great Red Spot will rely on precise positioning information in early 2022; spatial variations in thermal emission related to storms/vortices on each world will require visible-light context imaging; and candidates for impact events and significant storm eruptions will need to be rapidly assessed to determine the use of triggered observations. We advocate for a programme of ground-based support, both from professional and amateur observers, in the times preceding the FoR windows specified above. Amateur observers should continue to make use of the PVOL database (http://pvol2.ehu.eus/pvol2/) - see presentation by Hueso et al. (EPSC2021-80) for further details.

Introduction: NASA’s Spitzer Infrared Spectrometer (IRS) acquired mid-infrared (5 - 37 micron) disc-averaged spectra of Neptune in May 2004, November 2004, November 2005, and May 2006. Meadows et al., (2008, doi: 10.1016/j.icarus.2008.05.023) discovered Neptune's complex hydrocarbons methylacetylene and diacetylene and derived their abundances using the May 2004 data. The rest of the Neptune data has yet to be published. The data have all been reduced using the same methodology as Rowe-Gurney et al., (2021, doi: 10.1016/j.icarus.2021.114506) used for Uranus, so that each year can be reliably compared. We detect the same hydrocarbons seen in Meadows et al., (2008). This includes the strongest bands of methane (CH4), acetylene (C2H2) and ethane (C2H6) as-well-as weaker but still clearly recognisable features of ethylene (C2H4), carbon dioxide, methyl (CH3), methylacetylene (C3H4) and diacetylene (C4H2). At Uranus, there was a considerable longitudinal variation in stratospheric emission detected in the Spitzer data for multiple epochs (Rowe-Gurney et al., 2021). A variation is not present at Neptune in 2005 or late 2004, when all the separate longitudes displayed the same brightness temperature. In May 2004 a stratospheric variation is present, although it is tentative due to the deviation only appearing at a single longitude and because there are larger uncertainties on this early dataset. If the variation is real then it could be caused by stratospheric methane injection associated with convective clouds or perturbations to the location of the south polar warm vortex (Orton et al., 2012, doi: 10.1016/j.pss.2011.06.013). Optimal Estimation Retrievals: The data from 2005 have optimised exposure times, multiple observed longitudes, and therefore the lowest noise. It is this data we are using to derive the vertical structure of the temperature and composition in the stratosphere and upper troposphere (between around 1 nanobar and 2 bars of pressure). We present full optimal estimation inversions (using the NEMESIS retrieval algorithm, Irwin et al., 2008, doi: 10.1016/j.jqsrt.2007.11.006) of the globally averaged November 2005 data with the aim of constraining the temperature profile and the abundances of the stratospheric hydrocarbons. We fit both the low-resolution (R~120) and high-resolution (R~600) module data, testing multiple temperature priors derived from chemical models (Moses et al., 2018, doi: 10.1016/j.icarus.2018.02.004) and observations from AKARI (Fletcher et al., 2010, doi: 10.1051/0004-6361/200913358). Initial findings show that we are sensitive to stratospheric D/H ratio (derived from the relative abundances of CH4 and CH3D) and therefore we will attempt to constrain this value by finding the best fit for our model. Conclusion: Full spectrum mid-infrared data from Neptune in 2005 taken by the Spitzer Infrared Spectrometer is to be analysed using optimal estimation retrievals for the first time. The globally-averaged stratospheric temperature structure and the abundances of stratospheric hydrocarbons will be determined along with the ratio of D/H. The disc-averaged thermal and chemical structure from Spitzer will likely be our best characterisation of Neptune’s thermal structure until JWST/MIRI acquired spatially-resolved mid-infrared spectroscopy in 2022.

Thermal-infrared imaging observations spanning more than three decades from the VISIR instrument on the VLT, COMICS on Subaru and archived observations from NASA IRTF are used to characterise Saturn’s seasonal changes. Radiative transfer modelling (using NEMESIS [8]) provides the northern hemisphere temperature progression of the atmosphere over 15 years (2005-2020), both during and beyond the Cassini mission. Comparisons with a record of temperature variability from Cassini/CIRS measurements [10] establish the ability to replicate ground-based observations with CIRS temperature and composition inputs to NEMESIS. Imaging observations taken one Saturn year apart (1989-2018) show the limited extent of the interannual variability of Saturn’s northern hemisphere climate. Further comparisons from VISIR observations in 2017 and COMICS in 2020 show the seasonal progression of Saturn since the demise of Cassini. We characterize the seasonal atmospheric temperature progression of Saturn over the course of a full Saturnian year for the first time. Introduction With the culmination of Cassini's unprecedented 13-year exploration of the Saturn system in September 2017, and with no future missions currently scheduled to visit the ringed world, the requirement to build on Cassini's discoveries now falls upon Earth-based observatories. Mid-infrared observations have been used to characterise features such as the extreme temperatures within an enormous storm system in 2011 [1,6], the cyclic variations in temperatures and winds associated with the 'Quasi-Periodic Oscillation' (QPO) in the equatorial stratosphere [2] and the onset of a seasonal warm polar vortex over the northern summer pole [3]. Saturn's axial tilt of 27º subjects its atmosphere to seasonal shifts in insolation [4], the effects of which are most significant at the gas giant's poles. The north pole emerged from northern spring equinox in 2009 (planetocentric solar longitude Ls=0º), and northern summer solstice in May 2017 (Ls=90º), providing Earth-based observers with their best visibility of the north polar region since 1987, with its warm central cyclone and long-lived hexagonal wave [5,6]. Studying these interconnected phenomena within Saturn's atmosphere (particularly those that evolve with time in a cyclic fashion) requires regular temporal sampling throughout Saturn's long 29.5-year orbit. We present here a showcase of research from the wealth of archived observations from VLT/VISIR and Subaru/COMICS since 2005, as well as infrared imagery obtained from NASA/IRTF throughout the 1980s. 1.1 Temperature progression Methane (CH4) is used to determine stratospheric temperatures due to its even distribution across the planet. Figure 1 shows the changes in the stratospheric and tropospheric conditions as seen by VISIR over 3 years; these images are representative of a range of filters between 7-20 µm. We probe changes in the atmospheric 2D temperature brightness distribution across the planet disc in VISIR and COMICS observations taken from April 2005 (Ls=303.6º) to July 2020 (Ls=124.6º); thereby discerning the spatial variability as well as temporal (figure 2). VISIR observations concurrent with the Cassini/CIRS observations are used to cross-check the time-series from Cassini, which can be extended beyond the end-of-mission with the newer VISIR and COMICS observations. These profiles provide a new measure of long-term temperature variability in the context of an established model. 1.2 Interannual Variability VISIR imaging from September 2017 have provided a unique opportunity, as they were acquired nearly one Saturn year apart from the 1989 observations of Gezari et al, (1989) [7], which were the first images of Saturn in the mid-IR to be taken with a 2D detector, rather than raster scanning. Examining the differences in brightness temperatures and composition indicate limited interannual variation for Saturn’s northern hemisphere (Figure 3). This also provides unique insight into the timescale of Saturn’s equatorial stratospheric oscillation which will be contrasted with a previously suggested biennial cycle [2]. The seasonal temperature progression measured in Section 1.1 also enables us to place this interannual variability in a wider context and provides further opportunity for insightful comparison with the comparatively shorter-term temperature variability. Figure 1: VISIR observations from P95-102 sensing the troposphere (right) and stratosphere (left). Polar warming is evident in the stratosphere; but is considerably smaller than that seen during southern summer and in the historical record of the 1980s. The warm polar hexagon is seen at the north pole, the first such observation from the ground. Work to remove residual striping is ongoing and has been successfully applied to the 2017 images. 2015-16 images have been published by Fletcher et al., 2017 [2]. (a) (b) Figure 2: (a) The latitudinal brightness temperature progression of COMICS and VISIR ground-based observations from 2005 (dark blue) to 2017 (dark red), (b) the latitudinal brightness temperature progression of CIRS observations (from Fletcher et al, 2009) from 2005 (dark blue) to 2017 (dark red). Figure 3: A comparison of a latitudinal brightness temperature profile sampled from a 12.4µm observation (of Gezari et al. 1989) in red and a latitudinal bright temperature profile from a synthetic image generated using the temperatures and composition from CIRS observations in 2017 shown in black. The contrast between these profiles shows the degree of interannual variability at 12.4µm. Acknowledgements This research is funded by a European Research Council consolidated grant under the European Union’s Horizon 2020 research and innovation program, grant agreement 723890. We would like to thank co-author Mael Es-Sayeh for his significant contribution to this research. References [1] Fletcher et al., 2012, Icarus 221, p560-586 [2] Fletcher et al., 2017, Nature Astronomy, 1, p765-770 [3] Fletcher et al. 2015, Icarus. 251, 131-153 [4] Fletcher et al., 2015, https://arxiv.org/abs/1510.05690 [5] Fletcher et al., 2008, Science. 319, 79-81 [6] Fouchet et al., 2016, Icarus, 277, p196-214 [7] Gezari et al., 1989, Nature, 342, 777–780 [8] Irwin et al. 2008, JQSRT 109:1136-1150 [9] Orton et al., 2008, Nature 453, p198 [10] Fletcher et al. 2009, Icarus 200, 154-175

Observations of thermal emission from Uranus and Neptune have been made over a broad wavelength range from ground-based platforms, airborne observatories, Earth-proximal spacecraft and from the Voyager-2 flybys in the 1980s. Observations since the Voyager flybys have included long-wavelength observations of disk-averaged radiances from the Infrared Space Observatory and the Herschel Space Observatory covering the far-infrared to millimeter range. We present recent airborne spectra from SOFIA covering 17-35 µm, together with Akari and Spitzer spectroscopy at wavelengths extending down to 7 µm, below which contributions from reflected sunlight and potential auroral emissions may confuse the signature of thermal emission. We also show how these disk-averaged spectra are complemented by ground-based filtered imaging and spectroscopy at 8-10m telescopes, which have enabled spatially resolved measurements, complementing those of Voyager IRIS from several decades ago. The critical insights into the structure, chemistry and dynamics of the atmospheres of these Ice Giants attest to the need for significant parts of this spectral region to be included in the instrument complement to be assigned to spacecraft sent to these planets. A vigorous program of Earth-based observations in the accessible spectral range should accompany the spacecraft capability in order to track potential seasonal and non-seasonal variability of these planets, as is evident in the atmospheres of both Jupiter and Saturn. The latter would include mid-infrared observations from the James Webb Space Telescope.

NASA's Spitzer Infrared Spectrometer (IRS) acquired mid-infrared (5-37 microns) disc-averaged spectra of Uranus very near to its equinox in December 2007. A mean spectrum was constructed from observations of multiple central meridian longitudes, spaced equally around the planet, which has provided the opportunity for the most comprehensive globally-averaged characterisation of Uranus' temperature and composition ever obtained (Orton et al., 2014 a [arXiv:1407.2120], b [arXiv:1407.2118]). In this work we analyse the disc-averaged spectra at four separate central meridian longitudes to reveal significant longitudinal variability in thermal emission occurring in Uranus' stratosphere during the 2007 equinox. We detect a variability of up to 15% at wavelengths sensitive to stratospheric methane, ethane and acetylene at the ~0.1-mbar level. The tropospheric hydrogen-helium continuum and deuterated methane absorption exhibit a negligible variation (less than 2%), constraining the phenomenon to the stratosphere. Building on the forward-modelling analysis of the global average study, we present full optimal estimation inversions (using the NEMESIS retrieval algorithm, Irwin et al., 2008 [10.1016/j.jqsrt.2007.11.006]) of the Uranus-2007 spectra at each longitude to distinguish between thermal and compositional variability. We found that the variations can be explained by a temperature change of less than 3 K in the stratosphere. Near-infrared observations from Keck II NIRC2 in December 2007 (Sromovsky et al., 2009 [arXiv:1503.01957], de Pater et al., 2011 [10.1016/j.icarus.2011.06.022]), and mid-infrared observations from VLT/VISIR in 2009 (Roman et al., 2020 [arXiv:1911.12830]), help to localise the potential sources to either large scale uplift or stratospheric wave phenomena., 26 pages, 21 figures, to be published in Icarus (accepted 2021-04-27)

In its 16 years of scientific measurements, the Spitzer Space Telescope performed a number of groundbreaking and key infrared measurements of Solar System objects near and far. In this second of two Review Articles, we describe results from Spitzer observations of asteroids, dust rings and planets that provide new insight into the formation and evolution of our Solar System. The key Spitzer results presented here can be grouped into three broad classes: characterizing the physical properties of asteroids, notably including a large survey of near-Earth objects; detection and characterization of several dust/debris disks in the Solar System; and comprehensive characterization of ice giant (Uranus and Neptune) atmospheres. Many of these observations provide critical foundations for future infrared space-based observations. In the Spitzer Space Telescope’s 16 years of operation, it observed many Solar System objects and environments. In this second Review Article of a pair, Spitzer’s insight into asteroids, dust clouds and rings and the ice giant planets are summarized.

AbstractObservations from the Texas Echelon Cross Echelle Spectrograph (TEXES) on NASA’s IRTF and the VISIR instrument on the VLT are used to characterize the Saturn’s seasonal changes. Radiative transfer modelling (using NEMESIS [8]) provides the northern hemisphere temperature progression of the atmosphere over 10 years, both during and beyond the Cassini mission. Comparisons between imaging observations taken one Saturn year apart (1989-2018) show the extent of the interannual variability of Saturn’s northern hemisphere climate for the first time.1. IntroductionWith the culmination of Cassini's unprecedented 13-year exploration of the Saturn system in September 2017, and with no future missions currently scheduled to visit the ringed world, the requirement to build upon Cassini's discoveries now falls upon Earth-based observatories. Mid-infrared observations have been used to characterise features such as the extreme temperatures within an enormous storm system in 2011 [1,6], the cyclic variations in temperatures and winds associated with the 'Quasi-Periodic Oscillation' (QPO) in the equatorial stratosphere [2] and the onset of a seasonal warm polar vortex over the northern summer pole [3].Saturn's axial tilt of 27º subjects its atmosphere to seasonal shifts in insolation [4], the effects of which are most significant at the gas giant's poles. The north pole emerged from northern spring equinox in 2009 (planetocentric solar longitude Ls=0º), and northern summer solstice in May 2017 (Ls=90º), providing Earth-based observers with their best visibility of the north polar region since 1987, with its warm central cyclone and long-lived hexagonal wave [5,6].Studying these interconnected phenomena within Saturn's atmosphere (particularly those that evolve with time in a cyclic fashion) requires regular temporal sampling throughout Saturn's long 29.5-year orbit. We present here a showcase of research from the wealth of archived observations from both TEXES and VISIR obtained over the past decade.1.1 Temperature progressionMethane (CH4) is used to determine stratospheric temperatures due to its even distribution across the planet, as well as its well understood emissive behavior. Figure 1 shows the visible changes in the stratospheric and tropospheric conditions as seen by VISIR over 3 years; these images are representative of a range of filters between 7-20 µm, which can be stacked and inverted to derive the 3D temperature distribution in the upper troposphere and stratosphere. Using this technique, we probe changes in the atmospheric 3D temperature distribution across the planet disc in VISIR observations taken from April 2008 (Ls=343º) to July 2018 (Ls=102º); thereby discerning the spatial variability as well as temporal. VISIR observations concurrent with the Cassini/CIRS observations will be used to cross-check the time-series from Cassini, which can be extended beyond the end-of-mission with the newer VISIR observations. These profiles will provide a new measure of long-term temperature variability in the context of an established model.1.2 Interannual VariabilitySpectroscopic maps of the northern summer hemisphere from TEXES instrument on the IRTF collected in September 2018 have provided a unique opportunity, as they were acquired exactly one Saturn year apart from the 1989 observations of Gezari et al, (1989) [7], which were the first ever 2D images of Saturn in the mid-IR. Examining the differences in brightness temperatures and composition will indicate the extent of any interannual variation for Saturn’s northern hemisphere. This study will also provide unique insight into the timescale of the QPO which will be contrasted with a previously suggested biennial cycle [2]. The seasonal temperature progression measured in Section 1.1 also enables us to place this interannual variability in a wider context and provides further opportunity for insightful comparison with the comparatively shorter-term temperature variability.Figure 1: VISIR observations from P95-102 sensing the troposphere (right) and stratosphere (left). Polar warming is evident in the stratosphere; but is considerably smaller than that seen during southern summer and in the historical record of the 1980s. The warm polar hexagon is seen at the north pole, the first such observation from the ground. Work to remove residual striping is ongoing and has been successfully applied to the 2017 images. 2015-16 images have been published by Fletcher et al., 2017 [2].AcknowledgementsThis research is funded by a European Research Council consolidated grant under the European Union’s Horizon 2020 research and innovation program, grant agreement 723890. We would like to thank co-author Mael Es-Sayeh for his significant contribution to this research.References[1] Fletcher et al., 2012, Icarus 221, p560-586[2] Fletcher et al., 2017, Nature Astronomy, 1, p765-770[3] Fletcher et al. 2015, Icarus. 251, 131-153[4] Fletcher et al., 2015, https://arxiv.org/abs/1510.05690[5] Fletcher et al., 2008, Science. 319, 79-81[6] Fouchet et al., 2016, Icarus, 277, p196-214[7] Gezari et al., 1989, Nature, 342, 777–780[8] Irwin et al. 2008, JQSRT 109:1136-1150[9] Orton et al., 2008, Nature 453, p198

Above the dynamic clouds in Neptune’s atmosphere, the temperatures and photochemical composition within Neptune’s stratosphere are expected to respond to slowly modulating insolation over decades [1]. With Neptune’s seasons stretching over more than forty years, the entire history of reliable infrared observations resolving the planet’s disk comprises a mere fraction of a season. Yet, as we shall show, within this relatively brief window, observations appear to show changes in emission that would imply that sub-seasonal processes are significantly modifying the chemical and/or thermal structure in Neptune's stratosphere. We present an analysis of the mid-infrared images and spectra of Neptune acquired using the VISIR instrument at the Very Large Telescope (VLT) between 2006 and 2018. Images in Q-band (17.65, 18.72, 19.50 µm) are used to infer upper-tropospheric temperatures (~200 mbar), while Neptune’s stratosphere (~1 mbar) is analyzed using a combination of imaging and spectra sensitive to emission from ethane (12.2 µm), methane (7.9 µm), and the S(1) hydrogen quadrupole (17.03 µm). While we find that temperatures in the upper troposphere show no significant changes in time (remaining consistent with temperatures inferred from 1989 Voyager-IRIS measurements [2, 3]), the stratospheric emission appears to have changed unexpectedly over the past decade. Absolute radiances are uncertain, but relative trends in the spatially resolved emission are robust and show an asymmetric change in brightness temperature of up to ~3 K between and 2006 and 2008. The 2008 trend in emission continued throughout 2009 observations before returning to a relatively more symmetric pattern in 2018 (see Figure 1). Combined with the 2006 H2 S(1) quadrupole data, the observations indicate that the distribution of stratospheric emission observed in 2006 can likely be explained by the stratospheric temperature structure. However, no contemporaneous VLT-VISIR observations of the quadrupole emission were made in subsequent years, and so the nature of the observed changes between 2006 and 2008-2009 are ambiguous–they can potentially be explained by changes in the stratospheric temperatures, changes in the ethane abundances, or changes in both temperature and chemistry. The timing of these changes coincides with an extended maximum in Neptune’s photometric brightness following the 2005 southern summer solstice [4] and a decline in the discrete cloud coverage apparent in HST imaging [5]. Altogether, these observations suggest that Neptune’s stratosphere experiences intra-seasonal changes that may be coupled to dynamical forcing or variable mixing from the troposphere on timescales of years or less. Figure 1. Relative variation in the longitudinally averaged brightness temperatures inferred from 12.2 µm emission (ethane at ~1 mbar) as a function of latitude for observations from 2006 (light green), 2008 (blue), 2009 (dark green) and 2018 (orange). Example images corresponding to these years are shown in the inset. Absolute calibrations are uncertain and show some spread, so relative trends have been normalized to roughly coincide (arbitrarily) at southern latitudes and the planet limbs, with zero here chosen by the mean. These trends show that the stratospheric emission in 2008 and 2009 differed in shape from those observed in 2006 and 2018, possibly indicating a change in stratospheric temperatures or ethane. The red curve shows the trend in stratospheric hydrogen quadrupole emission for 2006 and suggests that the observed trend in 12.2 µm emission in 2006 was likely primarily determined by the temperature structure. [1] Moses, J. I., Fletcher, L. N., Greathouse, T. K., Orton, G. S., & Hue, V. 2018, Icar, 307, 124. [2] Conrath, B.J., Flasar, F.M., Gierasch, P.J., 1991. Thermal structure and dynamics of Neptune’s atmosphere from Voyager measurements. J. Geophys. Res. 96, 18931–18939. [3] Fletcher, Leigh N., et al. "Neptune at summer solstice: zonal mean temperatures from ground-based observations, 2003–2007." Icarus 231 (2014): 146-167. [4] Lockwood, G. W. "Final compilation of photometry of Uranus and Neptune, 1972–2016." Icarus 324 (2019): 77-85. [5] Karkoschka, Erich. "Neptune’s cloud and haze variations 1994–2008 from 500 HST–WFPC2 images." Icarus 215.2 (2011): 759-773.

We present results from mid-infrared imaging of Uranus at wavelengths of 13.0 micron and 18.7 micron, sensing emission from the stratosphere and upper troposphere, acquired using the VISIR instrument at the Very Large Telescope (VLT), September 4-October 20, 2018. Using a combination of inverse and forward modeling, we analyze these northern mid-spring (L_s~46) images and compare them to archival data to assess seasonal changes since the 1986 southern solstice and subsequent equinox. We find the data are consistent with little change (< 0.3 K) in the upper tropospheric temperature structure, extending the previous conclusions of Orton et al (2015) well past equinox, with only a subtle increase in temperature at the emerging north pole. Additionally, spatial-temporal variations in 13 micron stratospheric emission are investigated for the first time, revealing meridional variation and a hemispheric asymmetry not predicted by models. Finally, we investigate the nature of the stratospheric emission and demonstrate that the observed distribution appears related and potentially coupled to the underlying tropospheric emission six scale heights below. The observations are consistent with either mid-latitude heating or an enhanced abundance of acetylene. Considering potential mechanisms and additional observations, we favor a model of acetylene enrichment at mid-latitudes resulting from an extension of the upper-tropospheric circulation, which appears capable of transporting methane from the troposphere, through the cold trap, and into the stratosphere for subsequent photolysis to acetylene., 22 pages, 17 figures; Accepted for publication in The Astronomical Journal

This white paper, written in support of NASA's 2023-2032 Planetary Decadal Survey, outlines 10 major questions that focus on the origin, evolution, and current processes that shape the atmospheres of Uranus and Neptune. Prioritizing these questions over the next decade will greatly improve our understanding of this unique class of planets, which have remained largely unexplored since the Voyager flybys. Studying the atmospheres of the Ice Giants will greatly inform our understanding of the origin and evolution of the solar system as a whole, in addition to the growing number of exoplanetary systems that contain Neptune-mass planets., Comment: 8 pages, 0 figures, White Paper submitted to the Astrobiology and Planetary Science Decadal Survey

The abundance of deuterium in giant planet atmospheres provides constraints on the reservoirs of ices incorporated into these worlds during their formation and evolution. Motivated by discrepancies in the measured deuterium-hydrogen ratio (D/H) on Jupiter and Saturn, we present a new measurement of the D/H ratio in methane for Saturn from ground-based measurements. We analysed a spectral cube (covering 1151-1160 cm$^{-1}$ from 6 February 2013) from the Texas Echelon Cross Echelle Spectrograph (TEXES) on NASA's Infrared Telescope Facility (IRTF) where emission lines from both methane and deuterated methane are well resolved. Our estimate of the D/H ratio in stratospheric methane, $1.65_{-0.21}^{+0.27} \times 10^{-5}$ is in agreement with results derived from Cassini CIRS and ISO/SWS observations, confirming the unexpectedly low CH$_{3}$D abundance. Assuming a fractionation factor of $1.34 \pm 0.19$ we derive a hydrogen D/H of $1.23_{-0.23}^{+0.27} \times 10^{-5}$. This value remains lower than previous tropospheric hydrogen D/H measurements of (i) Saturn $2.10 (\pm 0.13) \times 10^{-5}$, (ii) Jupiter $2.6 (\pm 0.7) \times 10^{-5}$ and (iii) the proto-solar hydrogen D/H of $2.1 (\pm 0.5) \times 10^{-5}$, suggesting that the fractionation factor may not be appropriate for stratospheric methane, or that the D/H ratio in Saturn's stratosphere is not representative of the bulk of the planet., 9 pages, 6 figures, 1 table

We present a comparison of VLT-VISIR images and Keck-NIRC2 images of Neptune, which highlight the coupling between its troposphere and stratosphere. VLT-VISIR images were obtained on September 16th 2008 (UT) at 7.90 μ m and 12.27 μ m, which are primarily sensitive to 1-mbar CH 4 and C 2 H 6 emission, respectively. NIRC2 images in the H band were obtained on October 5th, 6th and 9th 2008 (UT) and sense clouds and haze in the upper troposphere and lower stratosphere (from approximately 600 to 20 mbar). At 7.90 μ m, we observe enhancements of CH 4 emission in latitude bands centered at approximately 25 ∘ S and 48 ∘ S (planetocentric). Within these zonal bands, tentative detections ( 2 σ ) of discrete hotspots of CH 4 emission are also evident at 24 ∘ S, 181 ∘ W and 42 ∘ S, 170 ∘ W. The longitudinal-mean enhancements in the CH 4 emission are also latitudinally-coincident with bands of bright (presumably CH 4 ice) clouds in the upper troposphere and lower stratosphere evidenced in the H-band images. This suggests the Neptunian troposphere and stratosphere are coupled in these specific regions. This could be in the form of (1) ‘overshoot’ of strong, upwelling plumes and advection of CH 4 ice into the lower stratosphere, which subsequently sublimates into CH 4 gas and/or (2) generation of waves by plumes impinging from the tropopause below, which impart their energy and heat the lower stratosphere. We favor the former process since there is no evidence of similar smaller-scale morphology in the C 2 H 6 emission, which probes a similar atmospheric level. However, we cannot exclude temperature variations as the source of the morphology observed in CH 4 emission. Future, near-infrared imaging of Neptune performed near-simultaneously with future mid-infrared spectral observations of Neptune by the James Webb Space Telescope would allow the coupling of Neptune’s troposphere and stratosphere to be confirmed and studied in greater detail.