The dissipation phase of dark spot NDS-2018 on Neptune

Neptune’s dark spot NDS-2018 — which is the sixth large, persistent dark spot seen in the planet’s atmosphere — is the first dark vortex with observations spanning its full lifetime. Neptune’s dark spots are thought to be anticyclonic vortices. Although internal flows have never been directly measured to confirm their rotation, observations effectively constrain the long-term evolution of these features. We observed NDS-2018 using WFC3 on the Hubble Space Telescope and MUSE at the Very Large Telescope.

Previous reports covered the formation and evolution of NDS-2018 as a mature vortex, while here we report that the contrast of the dark spot weakened rapidly over the 2021 to 2022 period, while it also drifted from 14 deg N to 6 deg N. The persistence of the spot so close to the equator in the final year is surprising, because only a very weak vortex could be geostrophically balanced at low latitudes. In the final year of the vortex before complete dissipation, its area decreased only modestly, and the HST imaging suggests extended arms or filaments potentially similar to features in Voyager’s observations of the Great Dark Spot. Some aspects of the evolution of NDS-2018 are easier to explain than others, but a major challenge remains: the link between dynamical and microphysical properties of dark vortices is not known.