The Temporal Brightening of Uranus' Northern Polar Hood From HST/WFC3 and HST/STIS Observations

Hubble Space Telescope Wide-Field Camera 3 (HST/WFC3) observations spanning 2015 to 2021 confirm a brightening of Uranus' north polar hood feature with time. The vertical aerosol model of Irwin et al. (2023, ) (IRW23), consisting of a deep haze layer based at & SIM;5 bar, a 1-2 bar haze layer, and an extended haze rising up from the 1-2 bar layer, was applied to retrievals on HST Space Telescope Imaging Spectrograph (STIS) (HST/STIS) observations (Sromovsky et al., 2014, 2019, , ) revealing a reduction in cloud-top CH4 volume mixing ratio (VMR) (i.e., above the deep & SIM;5 bar haze) by an average of 0.0019 & PLUSMN; 0.0003 between 40-80 degrees N (& SIM;10% average reduction) from 2012 to 2015. A combination of latitudinal retrievals on the HST/WFC3 and HST/STIS data sets, 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 in integrated opacity of 1.09 & PLUSMN; 0.08 (& SIM;33% increase) at 0.8 & mu;m north of & SIM;45 & DEG;N, concurrent with a decrease in the imaginary refractive index spectrum of the 1-2 bar haze layer north of & SIM;40 & DEG;N and longwards of & SIM;0.7 & mu;m. Small contributions to the brightening were found from a thickening of the deep aerosol layer, with an average increase in integrated opacity of 0.6 & PLUSMN; 0.1 (58% increase) north of 45 & DEG;N between 2012 and 2015, and from the aforementioned decrease in CH4 VMR. Our results are consistent with the slowing of a stratospheric meridional circulation, exhibiting subsidence at the poles. Uranus' north polar hood-a bright cap-like feature encircling the northern polar region within its atmosphere-is observed to be brightening over time. Using several observations of Uranus captured between 2012 and 2021 by the Hubble Space Telescope, this study aims to pinpoint, for the first time, the specific changes occurring within the atmosphere leading to this evolution. Analysis of the observations confirmed the predominant cause of the hood's brightening to be changes in the scattering properties of the atmosphere's aerosol layers. A vertical aerosol model consisting of 3 distinct haze layers was employed to investigate these changes. We find that the hood's brightening mainly stems from changes in the middle haze layer in the model (centered at 1-2 bar), finding a thickening of this layer concurrent with an increase in the reflectivity of its aerosols over time at latitudes coincident with the north polar hood (& SIM;45-90 & DEG;N). Small contributions to the temporal brightening were also found from a & SIM;10% reduction in cloud-top methane and a thickening of the deepest haze layer in the model (centered at & SIM;5 bar) at north polar hood latitudes. We confirm that the brightening of Uranus' north polar hood is predominantly due to changes in aerosol scatteringA temporal thickening and increase in aerosol reflectivity of Irwin et al. (2023, )'s 1-2 bar haze is the main cause of the brighteningWe find a further reduction in polar cloud-top methane over time from retrievals carried out on Hubble Space Telescope's/Space Telescope Imaging Spectrograph observations