Skip to main content

Giants through Time: Towards a Comprehensive Giant Planet Climatology

Periodic Reporting for period 2 - GIANTCLIMES (Giants through Time: Towards a Comprehensive Giant Planet Climatology)

Reporting period: 2018-10-01 to 2020-03-31

Planetary science stands at a unique threshold – the discovery of thousands of new worlds beyond the confines of our own Solar System is bringing a fascinating new perspective on the forces shaping both the architecture of our planetary system and the fragile climate of our home planet. As exoplanetary studies move from a discovery phase into an era of atmospheric characterisation, our own solar system offers an extreme test of our understanding of how planetary climate, dynamics and chemistry differ from world to world, and whets our appetite for the myriad possibilities that remain to be discovered. The next five years will likely be transformational in our understanding of outer solar system environments and the role of giant planet formation on planetary system evolution, with the culmination of the Juno mission at Jupiter and the Cassini mission at Saturn, along with new exploration of Uranus and Neptune by the James Webb Space Telescope (JWST).

But these missions offer only fleeting glimpses of these dynamic, ever-changing worlds when compared to their long orbital periods (10 years for Jupiter, 165 years for Neptune). How does the climate, circulation and chemistry of these atmospheres evolve over these large spans of time? What are the natural cycles of climate variability, and how do they differ from world to world? What can the giant planets reveal about the governing forces shaping planetary atmospheres in all their guises? The aim of GIANTCLIMES is to assemble the first comprehensive climatology of all four giant planets by combining an unprecedented multi-decade archive of ground-based infrared imaging with new spectroscopic observations from space telescopes and visiting spacecraft. This wealth of observational data will be studied via a sophisticated suite of spectral analysis software to understand the spatial and temporal variability in the redistribution of energy and material on the four giants. The GIANTCLIMES programme at the University of Leicester consists of three objectives will provide something that no single space mission has ever done before: the ability to explore the giants over large spans of time.

1. Climate Cycles on Jupiter: What drives the spectacular transformations of Jupiter’s banded structure and triggers storms? Can these events be predicted? We are searching for natural atmospheric cycles on this archetypal giant by constructing a comprehensive climate record (temperature, composition, clouds) over three Jovian years to set the environmental context for short-lived spacecraft missions (e.g. Juno).

2. Gas Giant Stratospheres: How does the circulation and chemistry of planetary stratospheres change with time as a result of the different conditions on each world? We are exploring the changing stratospheres of seasonal Saturn and non-seasonal Jupiter over long timescales to develop a new paradigm for the processes shaping these poorly-understood atmospheric regimes.

3. Ice Giants: How do circulation patterns and climate cycles differ between the Ice and Gas Giants? We are combining state-of-the-art observations from space observatories (Spitzer, Herschel, and JWST) with simulations of their dynamics and chemistry to reveal the climate of these distant worlds like never before, providing the first 3D maps of temperature, composition and circulation within an ice giant atmosphere.
Theme 1: Jupiter's Climate Variability

The first work package of the GIANTCLIMES programme explores the atmospheric variability of Jupiter over long spans of time, combining data from Earth-based observatories with new observations from NASA’s Juno spacecraft. During the course of the ERC programme, the PI (Fletcher) was invited to serve as a Participating Scientist on the Juno mission, developing microwave capabilities for the NEMESIS software to explore the jovian atmosphere beneath the clouds. The Jupiter theme is supported by PDRAs Arrate Antunano (2017-Present) and Henrik Melin (2019-Present), alongside STFC-supported PhD student Padraig Donnelly. Notable projects as part of this work package include:

• Exploration of cyclic variations in Jupiter’s troposphere: this included characterisation of the moist convective plumes responsible for the revival of Jupiter's South Equatorial Belt in 2009-2011 (Fletcher et al., 2017, http://dx.doi.org/10.1016/j.icarus.2017.01.001) the 3-5 year cycle of expansion activity within Jupiter's North Equatorial Belt (Fletcher et al., 2017, http://dx.doi.org/10.1002/2017GL073383) a review of cyclic activity across all of Jupiter's main belts (Fletcher et al., 2017, http://dx.doi.org/10.1002/2017GL073806); the discovery of a cycle of equatorial disturbances responsible for periodically removing the clouds every 6-7 years (Antunano et al., 2018, https://doi.org/10.1029/2018GL080382); and a long-term study of Jupiter’s atmospheric variability from 1983 to the present day (Antunano et al., 2019, https://doi.org/10.3847/1538-3881/ab2cd6). Ongoing work involves the extension of these studies to other wavelength ranges, and further characterisation of the newly-discovered equatorial disturbance. In 2018, the team produced a series of papers (https://doi.org/10.3847/1538-3881/aace02 https://doi.org/10.3847/1538-3881/aacaf5) using Juno, Hubble, and ground-based data to characterise newly-discovered waves in the atmosphere, which could be linked to these long-term cycles and the formation of cyclones. Fletcher recently submitted a large-scale review paper (https://arxiv.org/abs/1907.01822) on Jupiter’s banded structure to Space Science Reviews, asking how much we really understand about the belts and zones of giant planets.

• Exploration of atmospheric cycles in Jupiter’s stratosphere: PDRA Melin published a study using the mid-infrared TEXES spectrometer on NASA's Infrared Telescope Facility, expanding on our previous work mapping Jupiter's temperatures, clouds and composition on a single date (http://dx.doi.org/10.1016/j.icarus.2016.06.008) to understand the variations of stratospheric hydrocarbons over half a jovian year (https://doi.org/10.1016/j.icarus.2017.12.041). This same instrument was used by Dr. Richard Cosentino (Collaborator at Goddard Spaceflight Center) to understand Jupiter's Quasi-Quadrennial Oscillation (https://dx.doi.org/10.1002/2017JE005342) and we are presently following up this study to explore whether the QQO is in fact variable with time, responding to forcing from the deeper atmosphere. We are also extending this study into the ultraviolet: PDRA Melin has added the necessary capabilities to our NEMESIS tool (a test of the new UV modelling procedure has been applied to Cassini/UVIS data and submitted for publication in October 2019), and will begin applying this to long-term records of UV observations of Jupiter.

• Ground-Based Observing Campaign: As part of Fletcher’s role as Juno Participating Scientist, and as members of the supporting team for Juno, the team continues to observe Jupiter’s atmosphere using the Very Large Telescope (VLT) in Chile and NASA’s Infrared Telescope Facility (IRTF) in Hawaii. Fletcher and Donnelly had a successful visitor-mode run at the VLT in May 2018 that coincided perfectly with Juno's 13th perijove. High-resolution thermal maps of Jupiter were presented at the DPS meeting in 2018 and are being prepared for publication as part of the PhD thesis of Donnelly. Antunano has visited the IRTF for training for TEXES observations. In order to support the connection between Juno and the Earth-based community, Fletcher co-organised (with Dr. John Rogers of the British Astronomical Association) a Europlanet-funded workshop at the Royal Astronomical Society in London to bring together members of the Juno team with professional and amateur astronomers exploring atmospheric variability on Jupiter. Finally, the GIANTCLIMES team have been collaborating with Imke de Pater and colleagues in Berkeley, using both the VLA (https://doi.org/10.1016/j.icarus.2018.11.024) and ALMA (https://doi.org/10.3847/1538-3881/ab3643) arrays to probe ammonia at high pressures in the microwave regime, as a precursor to Fletcher's work with Juno data.

• Space-Based Observations: Guaranteed-time observations of Jupiter's Great Red Spot (programme ID 1246) using the James Webb Space Telescope (JWST) have been awarded and prepared for execution in the first year of operations. Fletcher and Melin are also part of an Early-Release Science team for observations of Jupiter’s Great Red Spot and polar auroras, with key new discoveries expected in 2021-22.


Theme 2: Saturn's Seasonal Stratosphere

The second work package of GIANTCLIMES seeks to complete the analysis of Cassini observations of Saturn’s seasonal atmosphere, and to extend the exploration over longer timespans using ground-based observations. This work is supported by Fletcher and by PDRA James Blake, and notable developments so far include:

• Completion of Cassini: The Cassini mission came to an end in September 2017, and the ERC team at Leicester were involved in a number of media activities associated with the end of mission. Two publications have been prepared by Dr. Fletcher that deal with Cassini infrared spectroscopy spanning the entire mission, 2004 to 2017: the first investigates the evolution of Saturn's tropical oscillations in the equatorial stratosphere, and the substantial perturbation caused by the Great Northern Storm of 2010-2013 (Fletcher et al., 2017, Nature Astronomy; http://dx.doi.org/10.1038/s41550-017-0271-5); the second looks at the changes occurring in the polar regions as Saturn's northern summer approached and discovered that Saturn’s famous hexagon extends high into the stratosphere (Fletcher et al., 2018, Nature Communications, https://doi.org/10.1038/s41467-018-06017-3). With these two papers published, the ERC contribution to the Cassini analysis is now complete. In addition, PDRA Antunano published a detailed study of the dynamics of Saturn's polar vortices (https://doi.org/10.1029/2018JE005764) Dr. Richard Achterberg (Collaborator) has used the high-resolution infrared observations from Cassini's final orbits to understand the properties of Saturn's polar cyclones (https://doi.org/10.1029/2018GL078157); and Dr. Sandrine Guerlet (Collaborator) used Cassini observations of Saturn's limb to further understand the equatorial oscillations (https://dx.doi.org/10.1002/2017JE005419).

• Long-term variability on Saturn: PDRA Blake is working to construct a database of Saturn’s global temperatures over a full Saturnian year (30 Earth-years), using data from the IRTF, VLT, and the Japanese Subaru telescope. A comparison of observations taken 30 years apart is showing hints of interannual variability (work presented at DPS in 2019). A record of VLT infrared imaging observations from 2008 to 2019 is being directly compared to the close-up measurements of the Cassini mission. This long-term record will be a key output of GIANTCLIMES.

• New Spectroscopic Observations: In order to move beyond Cassini’s spectral capabilities, Blake is using high-resolution spectroscopy acquired by the TEXES spectrometer on the IRTF to study Saturn’s atmospheric chemistry. A paper on Saturn’s deuterium-to-hydrogen ratio has been submitted (Blake et al., 2020), with implications for the origin of Saturn’s atmosphere. Blake has been trained on the TEXES instrument in Hawaii, and proposals for new IRTF and VLT data have been accepted. Most importantly, Guaranteed-time observations of Saturn's north polar atmosphere and rings (programme ID 1247) using the James Webb Space Telescope (JWST) have been awarded and prepared for execution in the first year of operations.


Theme 3: Ice Giants

The final GIANTCLIMES work package extends our understanding of planetary atmospheres out to the distant and under-explored Ice Giants, Uranus and Neptune. This theme will use existing ground- and space-based observations of the ice giants to prepare for observations with the James Webb Space Telescope (JWST), where guaranteed time observations of both worlds (programme IDs 1248 and 1249) have been awarded and prepared by Fletcher. Fletcher is supported by ERC-funded PhD student Naomi Rowe-Gurney; and two PDRAs – Michael Roman (for observations) and Jan Vatant d’Ollone (for numerical simulation). Progress to date includes:

• Ground-based characterisation: PDRA Roman has lead the effort to characterise the atmospheres of Uranus and Neptune using existing Very Large Telescope (VLT) observations in the mid-infrared. This led to a serendipitous discovery of the Uranian rings in thermal emission (Molter et al., 2019, https://doi.org/10.3847/1538-3881/ab258c) and has allowed us to characterise Uranus’ stratospheric circulation pattern for the first time (Roman et al., submitted to AJ). Roman is currently applying similar techniques to new observations of Neptune (a preliminary paper on Neptune has been submitted by our collaborator, James Sinclair, exploring stratospheric conditions in 2008). In 2019 we observed Neptune from the Gemini telescope (full spectroscopy using the TEXES instrument), and we have proposals accepted by the Spanish GranteCan observatory and the VLT for future observations of both targets.

• Space-based characterisation: PhD student Rowe-Gurney is mining an archive of Spitzer Space Telescope observations of Uranus and Neptune, taken between 2004 and 2007. She has developed the techniques to model the disc-averaged spectra, and has a paper in preparation exploring the longitudinal variability of Uranus’ stratosphere, due for submission in 2020. In addition, Fletcher used Spitzer and Cassini observations to discover hydrogen dimer features in the spectra of all four giants, most noticeably the ice giants (https://doi.org/10.3847/1538-4365/aaa07a).

• Numerical Modelling: Working with Dr. Julianne Moses (Collaborator), we produced a seasonal photochemical model of both ice giants in the absence of dynamics, which was used to predict the appearance of both worlds using JWST (http://doi.org/10.1016/j.icarus.2018.02.004). Following this effort, we have formed a new collaboration with A. Spiga and S. Guerlet at the Laboratoire de Météorologie Dynamique (LMD) in Paris, through the hire of PDRA d’Ollone, to develop a numerical simulation of Ice Giant circulation, based upon the existing DYNAMICO model. This will be the first simulation of its type, and work on a preliminary publication characterising Ice Giant radiative balance is currently underway.

• Winds and Clouds: Working with international collaborators, the ERC team have also explored shorter wavelength observations: Joshua Tollefson (Collaborator) used our temperature inversions to explore thermal windshear in Neptune's atmosphere (https://doi.org/10.1016/j.icarus.2018.04.009); and Prof. Patrick Irwin (Collaborator) presented the first detections of hydrogen sulphide above the clouds of Uranus (https://doi.org/10.1038/s41550-018-0432-1) and Neptune (https://doi.org/10.1016/j.icarus.2018.12.014) using observations from Gemini.

• Future Missions: The ERC team plays an active role in advocacy for future missions to Uranus and Neptune. Fletcher is the lead organiser of an Ice Giants Meeting at the Royal Society in London (January 2020), and led the submission of white papers to ensure that Ice Giant missions are part of ESA’s Voyage2050 programme. Fletcher is also contributing to mission studies and white papers associated with the next US decadal survey in planetary science.
The ERC GIANTCLIMES programme has provided the resources required to extend giant planet science beyond mere snapshots, isolated in time. By providing personnel to support these efforts, we have already seen the benefits of exploiting the long-term Jupiter dataset (new discoveries of atmospheric cycles), and we are in the process of doing the same for Saturn (PDRA Blake) and ultimately the Ice Giants (PDRA Roman). From a modelling perspective, our general-purpose spectral retrieval code (NEMESIS, Irwin et al., 2008) has been enhanced to cover ultraviolet reflectance spectroscopy for the first time (PDRA Melin), and is under development to explore microwave observations by the Juno spacecraft (Fletcher). Finally, the personnel afforded by this grant has allowed the ERC team to expand in a new direction, hiring expertise in numerical simulations of giant planets via a new collaboration with the Laboratoire de Météorologie Dynamique (LMD) in Paris. This new direction will potentially lead to significant improvements in our methods for interpreting the giant planet observations.

The most significant progress beyond the state of the art comes from new data from spacecraft – namely the Juno observations that will be analysed by Fletcher and Melin, and the JWST observations of all four giant planets, expected in 2021-22 (right at the end of this grant).