HCN ice in Titan’s high-altitude southern polar cloud


Titan’s middle atmosphere is currently experiencing a rapid change of season after northern spring arrived in 2009 (refs 1, 2). A large cloud was observed3 for the first time above Titan’s southern pole in May 2012, at an altitude of 300 kilometres. A temperature maximum was previously observed there, and condensation was not expected for any of Titan’s atmospheric gases. Here we report that this cloud is composed of micrometre-sized particles of frozen hydrogen cyanide (HCN ice). The presence of HCN particles at this altitude, together with temperature determinations from mid-infrared observations, indicate a dramatic cooling of Titan’s atmosphere inside the winter polar vortex in early 2012. Such cooling is in contrast to previously measured high-altitude warming in the polar vortex1, and temperatures are a hundred degrees colder than predicted by circulation models4. These results show that post-equinox cooling at the winter pole of Titan is much more efficient than previously thought.

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Figure 1: Identification of HCN ice in VIMS observations.
Figure 2: Cloud observed at Titan’s limb.
Figure 3: South-polar temperatures from models and retrievals.


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R.J.d.K. thanks the PEPSci programme of the Netherlands Organisation for Scientific Research (NWO) for support. N.A.T. and P.G.J.I. were supported by the UK Science and Technology Facilities Council. L.M. thanks the Agence Nationale de la Recherche for support (ANR Project “APOSTIC” no. 11BS56002, 968 France). We thank B. Bézard, T. M. Ansty, C. Nixon and M. López-Puertas for discussions; we also thank the VIMS and CIRS operation and calibration teams.

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R.J.d.K. conceived the study. R.J.d.K., L.M., N.A.T. and P.G.J.I. performed the VIMS analysis. N.A.T. and S.V. performed the CIRS analysis. All authors contributed to the interpretation, in addition to editing and improving the final manuscript.

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Correspondence to Remco J. de Kok.

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The authors declare no competing financial interests.

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de Kok, R., Teanby, N., Maltagliati, L. et al. HCN ice in Titan’s high-altitude southern polar cloud. Nature 514, 65–67 (2014). https://doi.org/10.1038/nature13789

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