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

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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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Figure 1: Rapid south polar atmospheric change observed using infrared spectra.
Figure 2: South polar seasonal temperature changes.
Figure 3: Seasonal changes in south polar trace-gas abundances.
Figure 4: Profiles of temperature and composition taken close to the equinox.

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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.

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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.

Corresponding author

Correspondence to Nicholas A. Teanby.

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The authors declare no competing financial interests.

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This file contains Supplementary Text and Data, Supplementary Tables 1-4, Supplementary Figures 1-2 and additional references. (PDF 234 kb)

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Teanby, N., Irwin, P., Nixon, C. et al. Active upper-atmosphere chemistry and dynamics from polar circulation reversal on Titan. Nature 491, 732–735 (2012). https://doi.org/10.1038/nature11611

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