16454

A dark vortex (called NDS-2018) has been discovered on Neptune comma and tracked over three years. This very dark feature has a higher contrast than any other previous vortex (including the Great Dark Spot seen by Voyager). After its first sighting in 2018 comma it has also lacked obvious d_commacompanion cloudsd_comma that are common to most other dark spots. But most critically comma 2019 data showed that the vortex was drifting equatorward. This was the first instance when a dark vortex tracked by HST actually followed the patterns of equatorward drift seen in numerical models. Models predict that vortices disrupt after crossing a critical latitude of virgul15 deg from the equator. But 2020 HST data showed that instead of drifting and subsequently disrupting comma NDS-2018 had arrested its motion just short of the predicted critical latitude. We propose imaging observations prior to Jan 2021 (solar avoidance) to investigate vortex evolution and potential global-scale changes in Neptune.s atmosphere. We will search for oscillations in position and shape (as seen by Voyager for previous vortices) comma and measure drift in both latitude and longitude. Drift rates and oscillations are key parameters used by numerical models to constrain atmospheric properties such as the thermal structure comma methane abundance comma and deep winds---all of which remain open questions slated for eventual investigation by atmospheric entry probes in the coming decades. We will also follow up on a small transient dark spot in January 2020 data that may represent a stage of the disruption process comma characterize a dramatic drop in northern CH4-cloud activity comma and track south polar features unrelated to the vortex.