A hexagon in Saturn's northern stratosphere surrounding the emerging summertime polar vortex.

• Published in: Nature Communications
• Main Authors: Fletcher, L. N., Orton, G. S., Sinclair, J. A., Guerlet, S., Read, P. L., Antuñano, A., Achterberg, R. K., Flasar, F. M., Irwin, P. G. J., Bjoraker, G. L., Hurley, J., Hesman, B. E., Segura, M., Gorius, N., Mamoutkine, A., Calcutt, S. B.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Nature Publishing Group 2018
• Subjects: South Pole; Moses; Leicester; South pole
• Online Access: https://www.nature.com/articles/s41467-018-06017-3; http://hdl.handle.net/2381/42858; https://doi.org/10.1038/s41467-018-06017-3

All data can be obtained from the primary author (L.N.F., email: leigh.fletcher@leicester. ac.uk) upon request or can be accessed from the following GitHub repository doi:10.5281/zenodo.1286856, which contains the temporally and latitudinally averaged spectra used in this study. Raw and calibrated Cassini Composite Infrared Spectrometer observations are available from NASA’s Planetary Data System (PDS). The entire CIRS database was used in this study, but we provide unique data identifiers where data subsets were used in our figures. The NEMESIS spectral retrieval tool is available upon reasonable request from P.G.J.I. ([e-mail]). The reconstructed temperature and hydrocarbon fields are also available at the DOI listed above. Saturn's polar stratosphere exhibits the seasonal growth and dissipation of broad, warm vortices poleward of ~75° latitude, which are strongest in the summer and absent in winter. The longevity of the exploration of the Saturn system by Cassini allows the use of infrared spectroscopy to trace the formation of the North Polar Stratospheric Vortex (NPSV), a region of enhanced temperatures and elevated hydrocarbon abundances at millibar pressures. We constrain the timescales of stratospheric vortex formation and dissipation in both hemispheres. Although the NPSV formed during late northern spring, by the end of Cassini's reconnaissance (shortly after northern summer solstice), it still did not display the contrasts in temperature and composition that were evident at the south pole during southern summer. The newly formed NPSV was bounded by a strengthening stratospheric thermal gradient near 78°N. The emergent boundary was hexagonal, suggesting that the Rossby wave responsible for Saturn's long-lived polar hexagon-which was previously expected to be trapped in the troposphere-can influence the stratospheric temperatures some 300 km above Saturn's clouds. This work is based on data acquired by the Cassini Composite Infrared Spectrometer and would not have been possible without the tireless efforts of the instrument design, operations, and calibration team over more than two decades. L.N.F. was supported by a Royal Society Research Fellowship and European Research Council Consolidator Grant (under the European Union’s Horizon 2020 research and innovation programme, grant agreement No 723890) at the University of Leicester. The UK authors acknowledge the support of the Science and Technology Facilities Council (STFC). A portion of this work was performed by GSO at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. J.A.S. was supported by the NASA Postdoctoral and Caltech programs as well as by a contract with NASA. S.G. was supported by the Centre National d’Etudes Spatiales (CNES). This research used the ALICE High Performance Computing Facility at the University of Leicester. We are extremely grateful to S. Guerlet, V. Hue. A.J. Friedson, T. Greathouse and J.I. Moses for sharing the outputs of their Saturn simulations. Peer-reviewed Publisher Version