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A pole-to-pole pressure–temperature map of Saturn’s thermosphere from Cassini Grand Finale data

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Abstract

Temperatures of the outer planet thermospheres exceed those predicted by solar heating alone by several hundred degrees. Enough energy is deposited at auroral regions to heat the entire thermosphere, but models predict that equatorward distribution is inhibited by strong Coriolis forces and ion drag1,2. A better understanding of auroral energy deposition and circulation are critical to solving this so-called energy crisis. Stellar occultations observed by the Ultraviolet Imaging Spectrograph instrument during the Cassini Grand Finale were designed to map the thermosphere from pole to pole. We analyse these observations, together with earlier observations from 2016 and 2017, to create a two-dimensional map of densities and temperatures in Saturn’s thermosphere as a function of latitude and depth. The observed temperatures at auroral latitudes are cooler and peak at higher altitudes and lower latitudes than predicted by models, leading to a shallower meridional pressure gradient. Under modified geostrophy3, we infer slower westward zonal winds that extend to lower latitudes than predicted, supporting equatorward flow from approximately 70° to 30° latitude in both hemispheres. We also show evidence of atmospheric waves in the data that can contribute to equatorward redistribution of energy through zonal drag.

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Fig. 1: Distribution of exospheric temperatures with latitude.
Fig. 2: Meridional profile of observed and model temperatures.
Fig. 3: Estimated winds.
Fig. 4: Temperature profile.

Data availability

The data that support the plots within this paper and other findings of this study are available from the NASA Planetary Data System (https://pds-imaging.jpl.nasa.gov/portal/cassini_mission.html) or from the corresponding author upon reasonable request.

Code availability

The equations required to produce our results are available in Methods. However, the horizontal wind code is available from the corresponding author upon request.

Change history

  • 09 April 2020

    In the legend of Fig. 1b, ‘UVIS solar (2010)’ has been changed to ‘UVIS solar (2007–2010)’ and ‘Voyager stellar (1983)’ has been changed to ‘Voyager solar and stellar (1980/1981)’.

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Acknowledgements

Z.B. and T.K. acknowledge support from the NASA Cassini Data Analysis Program (80NSSC19K0902). R.W.’s contributions were made at the Jet Propulsion Laboratory, under a contract with NASA (80NM0018D0004). L.E. was supported by the Cassini Project.

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Authors

Contributions

Z.B. performed the analysis, developed analytical tools and wrote the manuscript. T.K. developed analytical tools, conceived of the strategy to infer horizontal winds and provided feedback on the manuscript. I.M.-W. provided guidance on the interpretation of the results, STIM outputs, and conductivity and electric field data. R.W., A.J. and T.K designed the observing campaign and commented on the manuscript. L.E. is the principal investigator of the UVIS instrument, and contributed to UVIS observation campaigns and commented on the manuscript.

Corresponding author

Correspondence to Z. Brown.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Henrik Melin and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Information

Supplementary Table 1 and Figs. 1–4.

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Brown, Z., Koskinen, T., Müller-Wodarg, I. et al. A pole-to-pole pressure–temperature map of Saturn’s thermosphere from Cassini Grand Finale data. Nat Astron 4, 872–879 (2020). https://doi.org/10.1038/s41550-020-1060-0

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