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

摘要

在2013年5月，搭载在“卡西尼”飞船上的“成像科学子系统”(Imaging Science Subsystem)在土星最大卫星“土卫六”的深色南极上空识别出像云一样的一个很大结构。自那时以来，这块云在“土卫六”大气层非常高的地方(300 km)和高南纬地区都已被看到，那里的温度太高，不允许“土卫六”上已知的任何痕量气体冷凝。现在，这块神秘的云已利用来自“卡西尼”的“可见和红外绘图光谱仪”的近红外光谱被定性。该数据显示了一种由微米大小的氰化氢(HCN)冰颗粒组成的云，与2012年初冬季极地涡旋内的“土卫六”大气的急剧冷却相一致。这种冷却（冷到比环流模型预测的温度还要低100度的温度）与以前在极地涡旋内观测到的高空变暖形成对比。%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 observed 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 vortex, and temperatures are a hundred degrees colder than predicted by circulation models. These results show that post-equinox cooling at the winter pole of Titan is much more efficient than previously thought.