Unveiling Uranus: A Difference in Dark Spots Compared to Neptune

Abstract

We searched for discrete dark features on Uranus at wavelengths near 467 nm and 547 nm, using Hubble Space Telescope data spanning 2014–2022 from the OPAL program (Simon et al. 2015). We unveil subtle cloud features by subtracting a rotationally-averaged mean image from each individual exposure to remove limb-darkening effects. In these wavelength bands, we can rule out the presence of any dark spots with contrasts darker than 1.4% at 467 nm and 1.2% at 547 nm. Dark spots with contrast similar to those seen before on Neptune (Wong et al. 2022) and Uranus (Hammel et al. 2009) would easily have been detected using our approach. Overall, our results suggest that the occurrence rate of dark spots on Uranus is less than one per 11 years, or at least 2—3 times less frequent than on Neptune (Hsu et al. 2019).