Spatial and temporal variability in the Uranian atmosphere

Thesis

VLT/MUSE, HST/STIS and HST/WFC3 observations of Uranus in the visible/near-infrared were employed to probe spatial and temporal variability within its atmosphere. A Minnaert analysis of Uranus’ north polar hood reveals a temporal brightening of this feature, predomi- nantly caused by physical changes within the atmosphere, as opposed to any brightening effects caused by changes in viewing geometry as Uranus’ orbit progresses towards northern summer solstice in 2030. A reduction in the temporal rate of brightening was also provisionally ob- served. The holistic aerosol model [Irwin et al., 2022] was applied to latitudinal retrieval analyses on the three datasets, determining an increase in the integrated opacity (τ), and a re- duction in the imaginary refractive index spectrum (ni) longwards of 0.6 μm, of the 1 – 2-bar haze (aerosol-2) layer north of ∼40°N to be the predominant cause of the hood’s brightening. The single-scattering albedo of the aerosol-2 layer in the 60 – 70°N latitude band at 0.8 μm is observed to increase from ∼0.958 ± 0.002 in 2012, to ∼0.973 ± 0.002 in 2015, and to ∼0.9918 ± 0.0002 in 2021 from HST/STIS and VLT/MUSE retrieval analyses. Between the 2012 and 2015 STIS observations, the average increase in τ2 was 1.09 ± 0.08 at 0.8 μm, a ∼33% increase. An average increase in τ for the deep haze (aerosol-1) layer north of ∼45°N of 0.6 ± 0.1 at 0.8 μm (∼56% increase), and an average decrease in polar cloud-top methane (CH4) VMR from 40 – 80°N of 0.0019 ± 0.0003 (∼10% decrease) were also found between 2012 and 2015. A further reduction in polar cloud-top CH4 was found from analysis of the 2021 VLT/MUSE observation contrary to Sromovsky et al. [2019]’s conclusion of a general latitudinal stability. The 1 – 2-bar haze layer altitude is observed to be very stable, provid- ing evidence for it lying within a region of static stability [Irwin et al., 2022]. The results were found to be consistent with a temporal slowing of a stratospheric meridional circulation exhibiting downwelling at the poles.

Authors: James, A

• DOI: 10.5287/ora-kzevyxojj
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: atmospheric retrieval; radiative transfer; polar hood; Uranus; atmosphere; atmospheric physics
• Subjects: Atmospheric physics; Radiative transfer; Uranus (Planet)