Exploring clouds and gaseous abundances in the atmospheres of Uranus and Neptune

Uranus and Neptune, known as the "Ice Giants" are amongst the most mysterious and poorly understood planets in our solar system. The poles of Uranus are tipped over by an extraordinary 98 degrees (compared with an obliquity of 23.5 degrees for the Earth) leading to enormous annual variations in solar forcing, with the poles annually receiving more sunlight per unit area than the equator! In contrast, Neptune's obliquity of 29 degrees appears much less anomalous. The Voyager 2 fly-bys in 1986 and 1989 provided our only close-up views of these worlds and revealed that Uranus is in almost perfect radiative balance with the Sun, while Neptune emits thermally more than 2.5 times the solar radiation it receives! Perhaps as a result of this imbalance, the atmospheric circulation of Uranus was found to be rather quiescent, while that of Neptune was extraordinarily dynamic and active.

More than a quarter of a century later, the spatial resolution of ground-based telescopes has been transformed by the development of adaptive optics. The activity of Uranus's atmosphere has been seen to increase dramatically through its equinox in 2007, while Neptune's atmosphere shows enormous changes in cloud activity. We have been monitoring and these developments with an extensive programme of near-infrared ground-based observations at the Gemini-North Telescope in Hawai'i and ESO's Very Large Telescope in Chile. Near infrared reflectance spectroscopy enables us to determine the vertical and horizontal distribution of cloud and gaseous abundances in these atmospheres, using our world-leading NEMESIS retrieval code. Recent highlights include: 1) the first positive detection of hydrogen sulphide in the atmosphere of Uranus (Irwin et al., 2018; https://doi.org/10.1038/s41550-018-0432-1) and probable detection in Neptune's atmosphere (Irwin et al., 2019; https://doi.org/10.1016/j.icarus.2018.12.014); 2) the first ground-based detection of methane latitudinal variability in Neptune's atmosphere (Irwin et al., 2021; https://doi.org/10.1016/j.icarus.2020.114277); and most recently 3) a 'holistic' cloud model that matches the observed reflectivity spectra of both Uranus and Neptune from 0.3 to 2.5 microns (Irwin et al., 2022; https://doi.org/10.1029/2022JE007189), and can explain the difference in colour between these two worlds.

The aim of this project is to develop our ground-based visible and near-infrared observing programme and combine existing observations with recent observations made with the James Webb Space Telescope, and also propose and conduct future observations. This project is particularly timely given that a dedicated space mission to one of the Ice Giants was recommended by the 2022 NASA Decadal Survey in Planetary Science.

The work will use the newly-developed Minnaert analysis scheme of Irwin et al. (2019) (https://doi.org/10.1016/j.icarus.2018.12.014), which enables efficient fitting of both the mean spectra at target latitudes and their limb-darkening properties, which constrains the particle scattering properties much more strongly. In addition, the work will use a newly-developed image deconvolution scheme (Irwin et al., 2022; https://doi.org/10.1029/2022JE007189) which improves the spatial resolution and allows better discrimination between different latitudes and also the search for discrete atmospheric features. Finally, there is a possibility in the project to implement a more Bayesian analysis of the observations via transcription of the scattering code (currently written in Fortran) to python and using a nested sampling approach that is already used by NEMESIS for exoplanet retrievals.