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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Teanby, N. A. et al. Active upper-atmosphere chemistry and dynamics from polar circulation reversal on Titan. Nature 491, 732–735 (2012)
Vinatier, S. et al. Seasonal variations in Titan’s middle atmosphere during the northern spring derived from Cassini/CIRS observation. Icarus (submitted)
West, R. A. et al. Post-equinox evolution of Titan’s detached haze and south polar vortex cloud. Abstr. 305.03 (AAS/Division for Planetary Sciences Meeting Abstracts, Vol. 45, 2013)
Rannou, P., Lebonnois, S., Hourdin, F. & Luz, D. Titan atmosphere database. Adv. Space Res. 36, 2194–2198 (2005)
Griffith, C. A., Owen, T., Miller, G. A. & Geballe, T. Transient clouds in Titan’s lower atmosphere. Nature 395, 575–578 (1998)
Samuelson, R. E., Mayo, L. A., Knuckles, M. A. & Khanna, R. J. C4N2 ice in Titan’s north polar stratosphere. Planet. Space Sci. 45, 941–948 (1997)
Anderson, C. M., Samuelson, R. E., Bjoraker, G. L. & Achterberg, R. K. Particle size and abundance of HC3N ice in Titan’s lower stratosphere at high northern latitudes. Icarus 207, 914–922 (2010)
Griffith, C. A. et al. Evidence for a polar ethane cloud on Titan. Science 313, 1620–1622 (2006)
Achterberg, R. K., Gierasch, P. J., Conrath, B. J., Michael Flasar, F. & Nixon, C. A. Temporal variations of Titan’s middle-atmospheric temperatures from 2004 to 2009 observed by Cassini/CIRS. Icarus 211, 686–698 (2011)
dello Russo, N. & Khanna, R. K. Laboratory infrared spectroscopic studies of crystalline nitriles with relevance to outer planetary systems. Icarus 123, 366–395 (1996)
Moore, M. H., Ferrante, R. F., Moore, W. J. & Hudson, R. Infrared spectra and optical constants of nitrile ices relevant to Titan’s atmosphere. Astrophys. J. 191 (suppl.) 96–112 (2010)
Samuelson, R. E., Smith, M. D., Achterberg, R. K. & Pearl, J. C. Cassini CIRS update on stratospheric ices at Titan’s winter pole. Icarus 189, 63–71 (2007)
Lavvas, P., Griffith, C. A. & Yelle, R. V. Condensation in Titan’s atmosphere at the Huygens landing site. Icarus 215, 732–750 (2011)
Bellucci, A. et al. Titan solar occultation observed by Cassini/VIMS: gas absorption and constraints on aerosol composition. Icarus 201, 198–216 (2009)
Maltagliati, L. et al. Titan’s atmosphere as observed by VIMS/Cassini solar occultations: gaseous components. Icarus (submitted); preprint at http://arXiv.org/abs/1405.6324 (2014)
de Kok, R., Irwin, P. G. J. & Teanby, N. A. Condensation in Titan’s stratosphere during polar winter. Icarus 197, 572–578 (2008)
Jennings, D. E. et al. First observation in the south of Titan’s far-infrared 220 cm−1 cloud. Astrophys. J. 761, L15 (2012)
Coustenis, A., Schmitt, B., Khanna, R. K. & Trotta, F. Plausible condensates in Titan’s stratosphere from Voyager infrared spectra. Planet. Space Sci. 47, 1305–1329 (1999)
de Kok, R. et al. Characteristics of Titan’s stratospheric aerosols and condensate clouds from Cassini CIRS far-infrared spectra. Icarus 191, 223–235 (2007)
Anderson, C. M. & Samuelson, R. E. Titan’s aerosol and stratospheric ice opacities between 18 and 500 µm: vertical and spectral characteristics from Cassini CIRS. Icarus 212, 762–778 (2011)
Irwin, P. G. J. et al. The NEMESIS planetary atmosphere radiative transfer and retrieval tool. J. Quant. Spectrosc. Radiat. Transf. 109, 1136–1150 (2008)
Teanby, N. A., Irwin, P. G. J., de Kok, R. & Nixon, C. A. Seasonal changes in Titan’s polar trace gas abundance observed by Cassini. Astrophys. J. 724, L84–L89 (2010)
Tomasko, M. G. et al. Heat balance in Titan’s atmosphere. Planet. Space Sci. 56, 648–659 (2008)
Lebonnois, S., Burgalat, J., Rannou, P. & Charnay, B. Titan global climate model: a new 3-dimensional version of the IPSL Titan GCM. Icarus 218, 707–722 (2012)
Warren, S. G. Optical constants of carbon dioxide ice. Appl. Opt. 25, 2650–2674 (1986)
Warren, S. G. & Brandt, R. E. Optical constants of ice from the ultraviolet to the microwave: a revised compilation. J. Geophys. Res. D 113, 14220 (2008)
Vinatier, S. et al. Optical constants of Titan’s stratospheric aerosols in the 70–1500 cm−1 spectral range constrained by Cassini/CIRS observations. Icarus 219, 5–12 (2012)
Clark, R. N. et al. Detection and mapping of hydrocarbon deposits on Titan. J. Geophys. Res. 115, E10005 (2010)
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.
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) doi:10.1038/nature13789
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