The temporal brightening of Uranus’ northern polar hood from HST/WFC3 &
HST/STIS observations
Abstract
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 ~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 (HST/STIS) observations (Sromovsky et al., 2014, 2019}
revealing a reduction in cloud-top CH4 by an average of 0.19 {plus
minus} 0.03% between 40 - 80{degree sign}N between 2012 and 2015. A
combination of latitudinal retrievals on the HST/WFC3 & HST/STIS
datasets, 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 1.09 {plus minus} 0.08 at
0.8 μm north of ~45{degree sign}N, concurrent with a
decrease in the imaginary refractive index spectrum of the 1 - 2 bar
haze layer north of ~40{degree sign}N and longwards of
~0.7 μm, and between 60{degree sign}N and 80{degree
sign}N at ~0.5 μm. 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 {plus minus} 0.1
north of 45{degree sign}N 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, exhibiting
strong subsidence at the poles.