Evolution of a dark vortex on Neptune with transient secondary features

Dark spots on Neptune observed by Voyager and the Hubble Space Telescope are thought to be anticyclones with lifetimes of a few years, in contrast with very long-lived anticyclones in Jupiter and Saturn. The full life cycle of any Neptune dark spot has not been captured due to limited temporal coverage, but our Hubble observations of a recent feature, NDS-2018, provide the most complete long-term observational history of any dark vortex on Neptune. Past observations suggest some dark spots meet their demise by fading and dissipating without migrating meridionally. On the other hand, simulations predict a second pathway with equatorward migration and disruption. Our HST observations suggest NDS-2018 is following the second pathway. Some of the HST observations reveal transient dark features with widths of about 4000 to 9000 km, at latitudes between NDS-2018 and the equator. The secondary dark features appeared before changes in the meridional migration of NDS-2018 were seen. These features have somewhat smaller size and much smaller contrast compared to the main dark spot. Discrete secondary dark features of this scale have never been seen near previous dark spots, but global-scale dark bands are associated with several previous dark spots in addition to NDS-2018. The absolute photometric contrast of NDS-2018 (as large as 19%) is greater than previous dark spots, including the Great Dark Spot seen by Voyager. New simulations suggest that vortex internal circulation is weak relative to the background vorticity, presenting a clearly different case from stronger anticyclones observed on Jupiter and Saturn