Teanby N A, Sylvestre M, Sharkey J, Nixon C A, Vinatier S, Irwin P G J
School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, UK.
Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
Geophys Res Lett. 2019 Mar 28;46(6):3079-3089. doi: 10.1029/2018GL081401. Epub 2019 Feb 25.
Titan's stratosphere exhibits significant seasonal changes, including break-up and formation of polar vortices. Here we present the first analysis of mid-infrared mapping observations from Cassini's Composite InfraRed Spectrometer (CIRS) to cover the entire mission (L=293-93°, 2004-2017) - mid-northern winter to northern summer solstice. The north-polar winter vortex persisted well after equinox, starting break-up around L∼60°, and fully dissipating by L∼90°. Absence of enriched polar air spreading to lower latitudes suggests large-scale circulation changes and photochemistry control chemical evolution during vortex break-up. South-polar vortex formation commenced soon after equinox and by L∼60° was more enriched in trace gases than the northern mid-winter vortex and had temperatures ∼20 K colder. This suggests early-winter and mid-winter vortices are dominated by different processes - radiative cooling and subsidence-induced adiabatic heating respectively. By the end of the mission (L=93°) south-polar conditions were approaching those observed in the north at L=293°, implying seasonal symmetry in Titan's vortices.
土卫六的平流层呈现出显著的季节性变化，包括极地涡旋的瓦解和形成。在此，我们首次分析了卡西尼号综合红外光谱仪（CIRS）在整个任务期间（L = 293 - 93°，2004 - 2017年）——从北极冬季到北半球夏至——的中红外测绘观测结果。北极冬季涡旋在春分之后仍持续存在，大约在L ∼ 60°时开始瓦解，到L ∼ 90°时完全消散。缺乏富集的极地空气扩散到低纬度地区，这表明在涡旋瓦解期间，大规模环流变化和光化学作用控制着化学演化。南极涡旋在春分后不久开始形成，到L ∼ 60°时，其痕量气体比北极冬季中期涡旋更富集，温度约低20 K。这表明初冬和冬季中期的涡旋分别受不同过程主导——辐射冷却和下沉诱导的绝热加热。到任务结束时（L = 93°），南极的状况接近在L = 293°时在北极观测到的状况，这意味着土卫六涡旋存在季节性对称性。
