Letter

Active upper-atmosphere chemistry and dynamics from polar circulation reversal on Titan

Received:
Accepted:
Published online:

Abstract

Saturn’s moon Titan has a nitrogen atmosphere comparable to Earth’s, with a surface pressure of 1.4 bar. Numerical models reproduce the tropospheric conditions very well but have trouble explaining the observed middle-atmosphere temperatures, composition and winds1,2. The top of the middle-atmosphere circulation has been thought to lie at an altitude of 450 to 500 kilometres, where there is a layer of haze that appears to be separated from the main haze deck3. This ‘detached’ haze was previously explained as being due to the co-location of peak haze production and the limit of dynamical transport by the circulation’s upper branch4. Here we report a build-up of trace gases over the south pole approximately two years after observing the 2009 post-equinox circulation reversal, from which we conclude that middle-atmosphere circulation must extend to an altitude of at least 600 kilometres. The primary drivers of this circulation are summer-hemisphere heating of haze by absorption of solar radiation and winter-hemisphere cooling due to infrared emission by haze and trace gases5; our results therefore imply that these effects are important well into the thermosphere (altitudes higher than 500 kilometres). This requires both active upper-atmosphere chemistry, consistent with the detection of high-complexity molecules and ions at altitudes greater than 950 kilometres6,7, and an alternative explanation for the detached haze, such as a transition in haze particle growth from monomers to fractal structures8.

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Acknowledgements

This work was funded by the UK Science and Technology Facilities Council, the Leverhulme Trust and the NASA Cassini mission.

Author information

Affiliations

  1. School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK

    • Nicholas A. Teanby
    •  & Elliot Sefton-Nash
  2. Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK

    • Patrick G. J. Irwin
    •  & Simon B. Calcutt
  3. Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

    • Conor A. Nixon
    •  & F. Michael Flasar
  4. SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

    • Remco de Kok
  5. LESIA Observatoire de Paris, CNRS, UPMC Université Paris 06, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon Cedex, France

    • Sandrine Vinatier
    •  & Athena Coustenis
  6. Department of Earth and Space Sciences, University of California Los Angeles, Los Angeles, California 90095-1567, USA

    • Elliot Sefton-Nash

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Contributions

N.A.T. designed the study, performed the radiative transfer analysis and wrote the initial manuscript. P.G.J.I., N.A.T., C.A.N., R.d.K. and S.B.C. developed and maintained the radiative transfer code used for the analysis. S.V. performed independent tests on the results. A.C. performed further checks on the inversion method. All authors contributed to the interpretation of the results, in addition to editing and improving the final manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Nicholas A. Teanby.

Supplementary information

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  1. 1.

    Supplementary Information

    This file contains Supplementary Text and Data, Supplementary Tables 1-4, Supplementary Figures 1-2 and additional references.

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