Ice Giants Telescope Observations

Recent reconstructions of the true colours of Uranus and Neptune (credit: NASA / JPL-Caltech / Björn Jónsson, https://www.planetary.org/articles/uranus-neptune-color-difference ).

I have been interested in studying the reflectance spectra of the 'Ice Giants' (Uranus and Neptune) since the early 2000s and have been engaged in ground-based observations since 2006.

In 2018 I led a paper announcing the discovery of hydrogen sulphide (H₂S), from Gemini/NIFS observations made in 2009/2010 and published in Irwin et al. Nature Astronomy, 2, 420-427, 2018, doi: 10.1038/s41550-018-0432-1. This resulted in widespread media coverage (e.g., here).

This paper was followed up by a probable detection of H₂S in Neptune's atmosphere, published in Irwin et al., Icarus, 321, 550-563,2019, doi: 10.1016/j.icarus.2018.12.014.

Recently, I have been working on improved modelling the reflectance spectrum of these planets using the Minnaert limb-darkening approximation. We presented our initial work on Neptune at the EPSC 2020 meeting, and a copy of the presentation may be found here for here. This work has now been published as Irwin et al., Icarus, 357, 114277, 2021, doi: 10.1016/j.icarus.2020.114277.

Following on from this we published our 'holistic' model of the atmospheres in these planets, Irwin et al., J. Geophys. Res: Planets, 127, e07189, 2022, doi: 10.1029/2022JE007189. This paper was accompanied by a fantastic press release from the NOIRLab and received quite a bit of press attention (e.g., here).

This diagram shows three layers of aerosols in the atmospheres of Uranus and Neptune, as modelled in our study. The height scale on the diagram represents the pressure above 10 bar. The deepest layer (the Aerosol-1 layer) is thick and composed of a mixture of hydrogen sulphide ice and particles produced by the interaction of the planets' atmospheres with sunlight. The key layer that affects the colors is the middle layer, which is a layer of haze particles (referred to in the paper as the Aerosol-2 layer) that is thicker on Uranus than on Neptune. The team suspects that, on both planets, methane ice condenses onto the particles in this layer, pulling the particles deeper into the atmosphere in a shower of methane snow. Because Neptune has a more active, turbulent atmosphere than Uranus does, the team believes Neptune's atmosphere is more efficient at churning up methane particles into the haze layer and producing this snow. This removes more of the haze and keeps Neptune's haze layer thinner than it is on Uranus, meaning the blue color of Neptune looks stronger. Above both of these layers is an extended layer of haze (the Aerosol-3 layer) similar to the layer below it but more tenuous. On Neptune, large methane ice particles also form above this layer.

Most recently, we have made the first ever ground-based detection of a dark spot in the atmosphere of Neptune, and the first-ever measurement of the visible reflectance spectrum of such a spot, using the Multi Unit Spectroscopic Explorer (MUSE) instrument at the European Southern Observatory (ESO) Very Large Teslescope (VLT) in Chile. This was reported in Irwin et al., Nature Astronomy, 7, 1198-1207, 2023, doi: 10.1038/s41550-023-02047-0, which was accompanied by a fabulous press release from ESO, and resulted in lots of media coverage (e.g., here).

The image to the right combines all colours captured by MUSE into a 'true' colour view of Neptune, where a dark spot can be seen to the upper-right. Then we see images at specific wavelengths: 551 nanometres (blue), 831 nm (green), and 848 nm (red); note that the colours are only indicative, for display purposes.

A subsequent paper, following up on the details of this discovery was published in Irwin et al., J. Geophys. Res: Planets, 128, e07980, 2023, doi: 10.1029/2023JE007980.

Panels a and b show Voyager 2/ISS images of Uranus and Neptune released shortly after the Voyager 2 flybys in 1986 and 1989, respectively. Panels c and d show a reprocessing of the individual filter images in this study to determine the best estimate of the true colours of these planets.

A further paper on the colouration of Uranus and Neptune and modelling the seasonal cycle of Uranus's colour was published at the start of 2024 (Irwin et al., MNRAS, 527, 11521 - 11538, 2024, doi: 10.1093/mnras/stad3761) and received a lot of press coverage. Supporting images and videos may be found here. A video showing the changing appearance of Uranus during its 84-year orbit about the Sun can be found on here on YouTube.

Changing appearance of Uranus from 2014 - 2022 as viewed by HST/WFC3 with 'true' colour reconstructed as we describe in our paper. Latitude lines at the equator and 35°N have been added for clarity. As the north pole swings towards the Earth and Sun, the polar hood can be seen to brighten.