Bézard B, Coustenis A, McKay C P
Département de Recherche Spatiale, Observatoire de Paris (Section de Meudon), France.
Icarus. 1995 Feb;113(2):267-76. doi: 10.1006/icar.1995.1023.
During the 1981 Voyager encounter, Titan's stratosphere exhibited a large thermal asymmetry, with high northern latitudes being colder than comparable southern latitudes. Given the short radiative time constant, this asymmetry would not be expected at the season of the Voyager observations (spring equinox), if the infrared and solar opacity sources were distributed symmetrically. We have investigated the radiative budget of Titan's stratosphere, using two selections of Voyager IRIS spectra recorded at symmetric northern and southern latitudes. In the region 0.1-1 mbar, temperatures are 7 K colder at 50 degrees N than at 53 degrees S and the difference reaches approximately 13 K at 5 mbar. On the other hand, the northern region is strongly enriched in nitriles and hydrocarbons, and the haze optical depth derived from the continuum emission between 8 and 15 micrometers is twice as large as in the south. Cooling rate profiles have been computed at the two locations, using the gas and haze abundances derived from the IRIS measurements. We find that, despite lower temperatures, the cooling rate profiles in the pressure range 0.15-5 mbar are 20 to 40% larger in the north than in the south, because of the enhanced concentrations of infrared radiators. Because the northern hemisphere appears darker than the southern one in the Voyager images, enhanced solar heating is also expected to take place at 50 degrees N. Solar heating rate profiles have been calculated, with two different assumptions on the origin of the hemispheric asymmetry. In the most likely case where it results from a variation in the absorbance of the haze material, the heating rates are found to be 12-15% larger at the northern location than at the southern one, a smaller increase than that in the cooling rates. If the lower albedo in the north results from an increase in the particle number density, a 55 to 75% difference is found for the pressure range 0.15-5 mbar, thus larger than that calculated for the cooling rates. Considering the uncertainties in the haze model, dynamical heat transport may significantly contribute to the meridional temperature gradients observed in the stratosphere. On the other hand, the latitudinal variation in gas and haze composition may be sufficient to explain the entire temperature asymmetry observed, without invoking a lag in the thermal response of the atmosphere due to dynamical inertia.
在1981年“旅行者号”飞掠期间，土卫六的平流层呈现出巨大的热不对称性，高纬度的北半球比纬度相当的南半球更冷。鉴于辐射时间常数较短，如果红外和太阳不透明度源分布对称，在“旅行者号”观测的季节（春分）就不会出现这种不对称性。我们利用在对称的南北纬度记录的两组“旅行者号”红外干涉光谱仪（IRIS）光谱，研究了土卫六平流层的辐射收支。在0.1 - 1毫巴区域，北纬50度的温度比南纬53度低7K，在5毫巴时差异约达13K。另一方面，北半球富含腈类和碳氢化合物，从8至15微米的连续发射得出的霾光学厚度是南半球的两倍。利用从IRIS测量得出的气体和霾的丰度，计算了这两个位置的冷却率剖面。我们发现，尽管温度较低，但由于红外辐射体浓度增加，在0.15 - 5毫巴压力范围内，北半球的冷却率剖面比南半球大20%至40%。由于在“旅行者号”图像中北半球看起来比南半球暗，预计在北纬50度也会有增强的太阳加热。基于对半球不对称性起源的两种不同假设，计算了太阳加热率剖面。在最有可能的情况下，即半球不对称性是由霾物质吸收率变化导致的，发现北半球的加热率比南半球大12% - 15%，增幅小于冷却率的增幅。如果北半球较低的反照率是由粒子数密度增加导致的，在0.15 - 5毫巴压力范围内会发现55%至75%的差异，因此比冷却率计算得出的差异更大。考虑到霾模型的不确定性，动力热传输可能对平流层中观测到的经向温度梯度有显著贡献。另一方面，气体和霾成分的纬度变化可能足以解释观测到的整个温度不对称性，而无需因动力惯性而援引大气热响应的滞后。
