Presentation #115.01 in the session “Ice Giant Atmospheres”.
Permanent bright polar caps on Uranus were suggested to explain its long-term variation in visual brightness, as they would increase the average brightness of the planet as one or the other came into direct view during solstices (Hammel and Lockwood 1991, Icarus 186, 291-301). The south polar cap revealed by 1986 Voyager observations (near solstice) was also seen in 1997 HST/NICMOS H-band images and in 2002 HST/STIS observations. In 2002 a major contribution to the south polar brightness was low methane absorption at high latitudes due to a low upper tropospheric methane mixing ratio relative that at middle to low latitudes. The existence of that latitudinal methane gradient was established by Karkoschka and Tomasko (2009, Icarus 202, 780-797) using their 2002 spatially resolved STIS spectra. From similar STIS observations in 2012, Sromovsky et al. (2014, Icarus 238, 137-155) showed that methane was also low by similar amounts in the north polar region, where a bright cap had begun to form following equinox, while the southern cap faded. Further STIS observations in 2015 revealed a brightness increase in the northern cap between 2012 and 2015, which was largely due to increased aerosol scattering (Sromovsky et al. 2019, Icarus 317, 266-306). However, the northern cap continued to brighten substantially after 2015, raising new questions about its origin: did methane become more depleted in the north polar region after 2015, or did aerosol scattering continue to increase? A comparison of near-IR Keck imaging at 1.29 microns (dominated by methane absorption) and at 1.09 microns (more sensitive to hydrogen absorption) indicates that increased aerosol scattering is again likely to explain most of the temporal change. Aerosol scattering increases with time are also indicated by HST/WFC3 imaging with the F467M filter, which samples wavelengths without detectable methane absorption. Whether there is any room at all for temporal change in the upper tropospheric polar methane abundance will be more accurately determined by Cycle 29 HST/STIS spatially resolved spectral observations planned for early November 2021. This research was supported by NASA SSO Grant 80NSSC21K092 and JPL Contracts 1608059 and 1633473. The authors especially thank those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests and whose generous hospitality made possible our Keck observations.