Jupiter's third largest and longest-lived oval: Color changes and dynamics

Abstract

The transition region between the North Equatorial Band (NEBn) and North Tropical Zone (NTrZ) in Jupiter is home to convective storms, systems of cyclones and anticyclones and atmospheric waves. Zonal winds are weak but have a strong latitudinal shear allowing the formation of cyclones (typically dark) and anticyclones (typically white) that remain close in latitude. A large anticyclone formed in the year 2006 at planetographic latitude 19°N and persists since then after a complex dynamic history, being possibly the third longest-lived oval in the planet after Jupiter's Great Red Spot and oval BA. This anticyclone has experienced close interactions with other ovals, merging with another oval in February 2013; it has also experienced color changes, from white to red (September 2013) and back to white with an external red ring (2015–2016). The oval survived the effects of the closely located North Temperate Belt Disturbance, which occurred in October 2016 and fully covered the oval, rendering it unobservable for a short time. When it became visible again at its expected longitude from its previous longitudinal track, it reappeared as a white large oval keeping this color and the same morphology since 2017 at least until the onset of the new convective disturbance in Jupiter's North Temperate Belt in August 2020. Here we describe the historic evolution of the properties of this oval. We use JunoCam and Hubble Space Telescope (HST) images to measure its size obtaining a mean value of (10,500 ± 1000) x (5,800 ± 600) km2 and its internal rotation finding a value of -(2 ± 1)·10−5 s−1 for its mean relative vorticity. We also used HST and PlanetCam-UPV/EHU multi-wavelength observations to characterize its color changes and Junocam images to unveil its detailed structure. The color and the altitude-opacity indices show that the oval is higher and has redder clouds than its environment but has lower cloud tops than other large ovals like the GRS, and it is less red than the GRS and oval BA. We show that in spite of the dramatic environmental changes suffered by the oval during all these years, its main characteristics are stable in time and therefore must be related with the atmospheric dynamics below the observable cloud decks.