The temporal brightening of Uranus’ northern polar hood from HST/WFC3 HST/STIS observations

Observations of Uranus since northern spring equinox (2007) have tracked the formation and evolution of a north polar ‘hood’ within its atmosphere. Hubble Space Telescope Wide-Field Camera (HST/WFC3) observations spanning 2015 to 2021, obtained from the Outer Planet Atmospheres Legacy (OPAL) programme, confirm a brightening of this large-scale latitudinal feature with time. The vertical aerosol model of Irwin et al. (2023), consisting of a deep haze layer based at ∼5 bar (aerosol-1), a 1 – 2 bar haze layer (aerosol-2) and an extended haze (aerosol-3) rising up from the 1 – 2 bar layer, was applied to retrievals on HST Space Telescope Imaging Spectrograph (HST/STIS) observations from 2012 2015 (Sromovsky et al., 2014, 2019). Retrieved cloud-top methane (CH4) volume mixing ratio (VMR) latitudinal profiles reveal a reduction in CH4 by an average of 0.19 ± 0.03% between 40 – 80◦N (i.e., a ∼10% reduction) between 2012 and 2015. Latitudinal retrievals on the HST/WFC3 dataset, again employing the IRW23 model, reveal a temporal thickening of the 1 – 2 bar haze layer to be the main cause of the polar hood brightening, finding an average increase of 1.2 ± 0.3 in the integrated opacity at 0.8 μm north of ∼45◦N between 2015 and 2021. A forward-model study carried out on the HST/STIS dataset showed this solution to be insufficient to account for the spectral fingerprint of the polar hood brightening. Subsequent HST/STIS retrievals show the same signature thickening of the 1 – 2 bar haze layer, finding the integrated opacity to increase by an average of 1.09 ± 0.08 at 0.8 μm north of ∼45◦N (i.e., a ∼34% increase), concurrent with a decrease in the imaginary refractive index spectrum of the 1 – 2 bar haze layer north of ∼40◦N and longwards of ∼0.7 μm, and between 60◦N and 80◦N at ∼0.5 μm. Small contributions to the brightening were also found from a thickening of the deep aerosol layer, with an average increase in the integrated opacity of 0.6 ± 0.1 north of 45◦N (i.e., a ∼56% increase) between 2012 and 2015, and from the aforementioned decrease in cloud-top CH4 abundance. Our results are consistent with the slowing of a meridional circulation, similar to that posited by Fletcher et al. (2020), exhibiting strong subsidence at the poles.