School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, UK.
LESIA-Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris-Diderot, 92195, Meudon, France.
Nat Commun. 2017 Nov 21;8(1):1586. doi: 10.1038/s41467-017-01839-z.
Saturn's largest moon Titan has a substantial nitrogen-methane atmosphere, with strong seasonal effects, including formation of winter polar vortices. Following Titan's 2009 northern spring equinox, peak solar heating moved to the northern hemisphere, initiating south-polar subsidence and winter polar vortex formation. Throughout 2010-2011, strengthening subsidence produced a mesospheric hot-spot and caused extreme enrichment of photochemically produced trace gases. However, in 2012 unexpected and rapid mesospheric cooling was observed. Here we show extreme trace gas enrichment within the polar vortex dramatically increases mesospheric long-wave radiative cooling efficiency, causing unusually cold temperatures 2-6 years post-equinox. The long time-frame to reach a stable vortex configuration results from the high infrared opacity of Titan's trace gases and the relatively long atmospheric radiative time constant. Winter polar hot-spots have been observed on other planets, but detection of post-equinox cooling is so far unique to Titan.
土星最大的卫星土卫六拥有浓厚的氮-甲烷大气层，且具有强烈的季节性变化，包括冬季极地涡旋的形成。在 2009 年土卫六的北部春季春分之后，太阳辐射的峰值转移到了北半球，引发了南极下沉和冬季极地涡旋的形成。在 2010 年至 2011 年期间，下沉的加强产生了中层热斑，并导致光化学产生的痕量气体的极度富集。然而，在 2012 年，观测到了出乎意料的中层迅速冷却。在这里，我们展示了极地涡旋内痕量气体的极度富集极大地提高了中层长波辐射冷却效率，导致春分后 2-6 年出现异常寒冷的温度。达到稳定涡旋结构的长时间框架是由于泰坦的痕量气体具有较高的红外不透明度和相对较长的大气辐射时间常数。在其他行星上已经观察到冬季极地热斑，但到目前为止，只有土卫六上观测到了春分后的冷却现象。
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