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Giants through Time: Towards a Comprehensive Giant Planet Climatology

Periodic Reporting for period 3 - GIANTCLIMES (Giants through Time: Towards a Comprehensive Giant Planet Climatology)

Reporting period: 2020-04-01 to 2021-09-30

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 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 that 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.
The GIANTCLIMES team has made substantial progress in delivering on the aims of the ERC-funded grant, summarised as follows:

WP1: Jupiter's Climate
The first work package 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. Using more than three decades of ground-based observations, we have succeeded in identifying periodic cycles of atmospheric variability, both in the deep clouds, in the stratosphere, and now in the tropospheric temperatures. The interrelation between these climate cycles, and the forces driving them, will be a key topic for the remainder of the grant. As part of the Juno team and atmospheres working group, the GIANTCLIMES team have exploited microwave and infrared observations to explore waves, plumes, giant vortices, and the structure of the famous belts and zones as a function of depth below Jupiter's clouds. We also extended our spectral analysis toolkit to enable ultraviolet studies from Cassini, Juno, and Hubble. The Juno work has been supported by an extensive ground-based observing campaign, primarily using the VLT in Chile and the IRTF in Hawaii. In the coming years, we will be analysing the first infrared observations of Jupiter from JWST, alongside development work for our role on ESA's Jupiter Icy Moons Explorer.

WP2: Saturn's Seasonal Stratosphere
Cassini completed its unprecedented exploration of Saturn in 2017, allowing the GIANTCLIMES team to publish new insights into the long term seasonal variations in Saturn's atmosphere, including the effects of an enormous springtime storm, and the discovery that the famous hexagon extends well into Saturn's stratosphere. We have supplemented this work using the long-term ground-based infrared campaign, which extends both before and after Cassini's mission, allowing us to track a full seasonal cycle of the giant planet for the first time, and allowing us to search for inter-annual variability. These projects will culminate with the first JWST maps of Saturn's northern summer hemisphere in 2022.

WP3: Ice Giants
Uranus and Neptune remain the least explored planets in our solar system. GIANTCLIMES has enabled the first maps of Uranus' stratosphere and revealed a surprisingly lack of thermal variability, whereas a comprehensive assessment of Neptune's stratosphere over two decades has revealed surprising sub-seasonal variability for the first time. This has been coupled with an analysis of Spitzer infrared observations of both worlds, providing a comprehensive assessment of their vertical temperatures and compositional structures. These long-term studies pave the way for the first global maps of the Ice Giant atmospheres from JWST in 2022, where the GIANTCLIMES team leads observations that span reflected sunlight, ionospheric emission, and the thermal structure of the atmosphere.
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 and the Ice Giants. 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, and is under development to explore microwave observations by the Juno spacecraft. 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. 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, and the JWST observations of all four giant planets, expected in 2022.
The giant planets in infrared light, as observed by the Very Large Telescope in Chile.