https://www.cnet.com/news/nasa-spots-mysterious-dark-vortex-on-neptune-hubble-uranus/

Scientists aren't sure what causes the dark vortices to form on the solar system's fourth-biggest planet. However, NASA notes that in 2016 there were more clouds in the region than is usually observed right before the vortex appeared. The vortices may arise from deeper within Neptune -- which is mostly composed of hydrogen and helium -- and then become visible to Hubble (and us!) when they rise to high enough altitudes.

Wide Field Camera 3 photo of Uranus revealing a cloud cap on the north pole.