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Secondary Features near Neptune’s Dark Spot NDS-2018

Presentation #212.02 in the session “Giant Planets, Exoplanets and Systems”. Cross-listed as presentation #115.05.

Published onOct 03, 2021
Secondary Features near Neptune’s Dark Spot NDS-2018

Observations: The newest dark vortex on Neptune, NDS-2018, was discovered in Hubble Space Telescope (HST) annual imaging data from the OPAL program (Simon et al. 2019, GRL, DOI 10.1029/2019GL081961). We observed NDS-2018 seven times between Sept. 2018 and Dec. 2020. Secondary features: We observed a new phenomenon on Neptune: transient secondary dark features that may be related to NDS-2018 and its interactions with the zonal flow. At each epoch from Sept. 2019 to Aug. 2020, a single secondary dark feature was seen at latitudes of 4 to 7 deg N. The shape of the secondary features varied among the epochs, and their sizes ranged from about 4000 to 9000 km in east-west width, and 3000 to 4000 km in north-south width. We suspect they are not coherent anticyclonic vortices, because they were located well within the equatorial region where vortices should not remain stable against disruption (LeBeau and Dowling 1998, Icarus, DOI 10.1006/icar.1998.5918). In global maps of Neptune, we detected these secondary features offset in longitude from NDS-2018 itself by about 40 to 110 deg. Contrast: Neptune’s dark spots reach highest contrast at wavelengths near 450 nm. NDS-2018 reached a maximum contrast of -18.7% ± 0.8% in our June 2020 observations in the HST/WFC3 468-nm filter. This contrast was greater than the contrast of the Great Dark Spot seen by Voyager, -12% in the blue filter (Smith et al. 1989, Science, DOI 10.1126/science.246.4936.1422). The secondary dark features had much lower contrast, in the -2% to -4% range. We achieved the highest signal-to-noise ratio in our Dec. 2020 observations at 468 nm, but the transient dark features had completely vanished.

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