Modelling the seasonal cycle of Uranus's colourãnd magnitude,ãnd comparison with Neptune

Abstract

We presentã quantitativeãnalysis of the seasonal record of Uranus's disc-averaged colourãnd photometric magnitude in Strömgren bãnd y filters (centredãt 467ãnd 551 nm, respectively), recordedãt the Lowell Observatory from 1950 to 2016,ãnd supplemented with HST /WFC3 observations from 2016 to 2022. We find that the seasonal variations of magnitude can be explained by the lowerãbundance of methaneãt polar latitudes combined withã time-dependent increase of the reflectivity of theãerosol particles in layer near the methane condensation levelãt 1 -2 bar. This increase in reflectivity is consistent with theãddition of conservãti vely scattering particles to this layer, for which the modelled background haze particlesãre stronglyãbsorbingãt both blueãnd red wavelengths. We suggest that thisãdditional component may come fromã higher proportion of methane ice particles. We suggest that the increase in reflectivity of Uranus in both filters between the equinoxes in 1966ãnd 2007, noted by previousãuthors, might be related to Uranus's distance from the Sunãnd the production rate of dark photochemical haze products. Finally, we find thatãlthough the visible colour of Uranus is less blue than Neptune, due to the increasedãerosol thickness on Uranus,ãnd this difference is greatestãt Uranus's solstices, it is much less significant than is commonly believed due toã long-standing misperception of Neptune's 'true' colour. We describe how filter-imaging observations, suchãs those from Voyager-2/ISSãnd HST /WFC3, should be processed to yieldãccurate true colour representations.