Bézard Bruno
LESIA, Observatoire de Paris, CNRS, 92195 Meudon, France.
Philos Trans A Math Phys Eng Sci. 2009 Feb 28;367(1889):683-95. doi: 10.1098/rsta.2008.0186.
Our present knowledge of the composition and chemistry of Titan's stratosphere is reviewed. Thermal measurements by the Cassini spacecraft show that the mixing ratios of all photochemical species, except ethylene, increase with altitude at equatorial and southern latitudes, reflecting transport from a high-altitude source to a condensation sink in the lower stratosphere. Most compounds are enriched at latitudes northward of 45 degrees N, a consequence of subsidence in the winter polar vortex. This enrichment is much stronger for nitriles and complex hydrocarbons than for ethane and acetylene. Titan's chemistry originates from breakdown of methane due to photodissociation in the upper atmosphere and catalytical reactions in the stratosphere, and from destruction of nitrogen both by UV photons and electrons. Photochemistry also produces haze particles made of complex refractory material, albeit at a lower rate than ethane, the most abundant gas product. Haze characteristics (vertical distribution, physical and spectral properties) inferred by several instruments aboard Cassini/Huygens are discussed here.
本文综述了我们目前对土卫六平流层成分和化学性质的了解。卡西尼号航天器的热测量结果表明，除乙烯外，所有光化学物质的混合比在赤道和南半球纬度随高度增加，这反映了从高层源向平流层下部冷凝汇的传输。大多数化合物在北纬45度以北的纬度富集，这是冬季极涡下沉的结果。腈类和复杂碳氢化合物的这种富集比乙烷和乙炔更强。土卫六的化学过程源于高层大气中光解离导致的甲烷分解以及平流层中的催化反应，以及紫外线光子和电子对氮的破坏。光化学还会产生由复杂难熔物质构成的霾粒子，尽管其产生速率低于最丰富的气体产物乙烷。本文讨论了卡西尼号/惠更斯号上的几种仪器推断出的霾特征（垂直分布、物理和光谱特性）。
