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

Presentation #409.06 in the session Uranus and Neptune Systems (Oral Presentation)

Published onOct 23, 2023
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 over 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 revealing a reduction in cloud-top CH4 between 40 - 80°N from 2012 to 2015. Uranus (along with its ice giant counterpart Neptune) is observed to exhibit a gradient in cloud-top CH4 with the value increasing by ~2x when moving from the poles to the equator. This result demonstrates the first signs of interseasonal variability in the cloud-top CH4, which has previously been assumed to be generally stable.

A combination of latitudinal retrievals on the HST/WFC3 and 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 ± 0.08 at 0.8 μm north of ~45°N, 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 (i.e., more reflective aerosols). Small contributions to the brightening were also found from a thickening of the deep ~5 bar aerosol layer, with an average increase in integrated opacity of 0.6 ± 0.1 at 0.8 μm north of 45°N between 2012 and 2015, and from the aforementioned decrease in cloud-top CH4 abundance. These results are consistent with the slowing of a meridional circulation, exhibiting strong subsidence at the poles.

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