The Vertical Aerosol Structure of Jupiter’s Great Red Spot from JWST/MIRI 5 - 7 µm Spectroscopy

The Great Red Spot (GRS) is a remarkable feature of the Jovian atmosphere. Situated within the South Tropical Zone, it dominates the morphology of the southern hemisphere. Numerous spacecraft have visited Jupiter, but they have either lacked the mid-infrared instruments required to assess tropospheric structure and composition, or have been fly-by missions with insufficient time to probe such a specific part of the Jovian atmosphere. Ground-based observatories are capable of characterising this spectral region, often with higher spatial and spectral resolutions than space-based observatories. However, telluric absorption and the limited spectral coverage offered by narrow filters allows only a handful of discrete altitudes to be probed. As a result, there are still numerous unanswered questions regarding the aerosol formation processes, composition as well as the driving mechanism and longevity of the vortex.

JWST/MIRI observed the GRS in July and August 2022 as part of a Guaranteed-Time programme (Cycle 1 – GTO 1246). The Medium Resolution Spectrometer (MRS) was used, allowing the full 5 – 28 µm spectral region to be observed in the 6.6 x 7.7 arcsec field of view. This included the first ever mapping of the 5.5 – 7.7 µm range on Jupiter, a transitional region between thermal emission at long wavelengths and reflected sunlight at shorter wavelengths. This range required spectral models that considered multiple scattering of photons, which allowed us to probe the elevated aerosol structure dominating the GRS vortex in unprecedented detail. Firstly, the vertical temperature structure was constrained in the 7.3 – 10.8 µm range using the NEMESIS atmospheric retrieval software (Irwin et al., doi: 10.1016/j.jqsrt.2007.11.006). Secondly, these temperatures were used to map the 3D aerosol and gaseous distributions within the GRS in the 4.9 – 7.3 µm range. Various models were used to reproduce the observed spectra and the implications of these observations were assessed through comparison to theoretical models. A further comparison of the retrieved distribution of ammonia and water to this inferred aerosol abundance allowed us to speculate on the cloud formation processes taking place within the troposphere. Meanwhile, an analysis of the retrieved phosphine distribution enabled us to identify regions of convective upwelling outside the vortex. A considerable phosphine excess inferred above the GRS was linked to the higher aerosol opacity within the vortex. Potentially, this opacity shields phosphine from the UV light that would normally photolyse and remove it. Finally, the medium-resolution spectroscopy of the instrument was also used to search for spectral signatures of atmospheric species that cannot be detected from the ground. The 9.5 µm ammonia ice feature was not detected within the GRS wake on this occasion, possibly as a result of it not being present at the altitudes probed within this study, or due to it being rapidly coated by other species that obscure the spectral signature of the ammonia ice.

The MIRI data were also acquired alongside visible-light Hubble and near-infrared JWST/NIRCam observations taken close to the same date, probing the colourful upper aerosol layers. A comparison of this context data to the MIRI observations can be seen in Fig. 1. Further observations, courtesy of the VLT/VISIR instrument also provided additional mid-infrared data between the dates of the MIRI observations. This data visualised the whole Jovian disc and thus provided wider spatial context of the GRS’s interaction with the surrounding atmosphere. In this presentation we will: (i) introduce the retrieval methods used in this study; (ii) describe the inferred deep tropospheric aerosol structure and dynamics of the GRS; and (iii) using the spatial context observations, explore the interaction between the GRS aerosols and the surrounding atmosphere.

Figure 1: Comparison of False-colour JWST/MIRI data from channels 1 and 2 to visible context data from (a) Hubble (R=631 nm, G=502 nm, B=395 nm) and (b) Ground-Based amateur observations (Isao Miyazaki, 2024). The left-hand column displays data from 2022-07-28 while the right-hand column displays data from 2022-08-15. Differences in colour between the visible observations are due to different instruments and post-processing techniques being used. The size of the MIRI FOV is indicated by the blue and green lines for channels 1 and 2 respectively. (c) and (d) False-colour images of the ch1-short GRS MIRI data with the relevant visual context image as the background. For both MIRI images; B = 5.40 µm, G = 5.50 µm and R = 5.60 µm. The red band contains reflected sunlight from within a deep ammonia absorption band, indicating strong aerosol reflection rather than temperature. The green and blue bands are dominated by thermal emission, darker blue and green components outside the GRS therefore correspond to thick aerosol layers outside the GRS. (e) and (f) False-colour images of the ch2-short GRS MIRI data. For both; B = 8.62 µm, G = 8.57 µm and R = 8.56 µm. Beyond 7.30 µm, the Jovian spectrum is dominated by thermal emission, darker regions correspond to regions of high aerosol opacity and cooler temperatures. All frames have been shifted to be centred on the GRS longitudinal position in July and August. Subtle changes in colour between epochs are due to the different observing geometries of the observations.