Formation of a New Great Dark Spot on Neptune in 2018

For the first time, Hubble Space Telescope visible-wavelength imaging shows the formation history of a dark vortex on Neptune. A new northern Great Dark Spot (NDS-2018) was discovered in September and November 2018 images, spanning roughly 12° of latitude and 27° in longitude (11,000 × 5,000 km) at 23°N planetographic latitude. NDS-2018 is similar in size, shape, and longitudinal drift rate as the Voyager 2 Great Dark Spot near 22°S. NDS-2018 was prominent in blue-wavelength November 2018 images and drifted at 2.46°/hr westward. Yearly global maps demonstrate possibly related cloud activity in prior years. The drift rate and formation history of the dark vortex indicates a deep origin. We estimate that the north-south gradient in zonal wind speed is almost 4 times larger at the depth of the dark vortex, compared with cloud-tracked zonal winds.