Cassini UVIS Detection of Saturn's North Polar Hexagon in the Grand Finale Orbits

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Abstract

Cassini's final orbits in 2016 and 2017 provided unprecedented spatial resolution of Saturn's polar regions from near-polar spacecraft viewing geometries. Long-wavelength channels of Cassini's Ultraviolet Imaging Spectrograph instrument detected Saturn's UV-dark north polar hexagon near 180 nm at planetocentric latitudes near 75°N. The dark polar hexagon is surrounded by a larger, less UV-dark collar poleward of planetocentric latitude 65°N associated with the dark north polar region seen in ground-based images. The hexagon is closely surrounded by the main arc of Saturn's UV aurora. The UV-dark material was locally darkest on one occasion (23 January 2017) at the boundary of the hexagon; in most Ultraviolet Imaging Spectrograph images the dark material more uniformly fills the hexagon. The observed UV-dark stratospheric material may be a hydrocarbon haze produced by auroral ion-neutral chemistry at submicrobar pressure levels. Ultraviolet Imaging Spectrograph polar observations are sensitive to UV-absorbing haze particles at pressures lower than about 10–20 mbar.

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Pryor, W. R., Esposito, L. W., Jouchoux, A., West, R. A., Grodent, D., Gérard, J. C., … Koskinen, T. (2019). Cassini UVIS Detection of Saturn’s North Polar Hexagon in the Grand Finale Orbits. Journal of Geophysical Research: Planets, 124(7), 1979–1988. https://doi.org/10.1029/2019JE005922

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