Retrieval of H2O abundance in Titan's stratosphere: A (re)analysis of CIRS/Cassini and PACS/Herschel observations
Retrieval of H2O abundance in Titan's stratosphere: A (re)analysis of CIRS/Cassini and PACS/Herschel observations
复制标题
泰坦平流层中 H2O 丰度的反演：CIRS/卡西尼号和 PACS/赫歇尔观测的（重新）分析
DOI：
10.1016/j.icarus.2018.04.003
复制
发表时间：
2018
期刊：
影响因子：
3.2
通讯作者：
Bauduin S
中科院分区：
文献类型：
--
作者：
Bauduin S
自从20年前红外空间天文台(ISO)首次测量土卫六以来，由于现有测量结果之间的巨大差异，土卫六平流层中的水(H2O)摩尔分数仍然不确定。更具体地说，赫歇尔天文台(PACS和HIFI)最近的测量估计，在12.1百万巴时，H2O摩尔分数为0.023 ppb。然而，从Cassini/CIRS的最低空间分辨率观测中恢复了10.7mbar处0.14ppb的混合比。在相同的气压水平(10.7毫巴)，这使得PACS和CIRS的测量结果相差5.5倍，这明显阻止了当前的模型完全限制流入土卫六大气的氧通量。在这项工作中，我们试图理解从Herschel/PACS和Cassini/CIRS观测到的H2O摩尔分数之间的差异。这样做的策略是：1)分析CIRS最近的圆盘平均观测，以调查观测几何结构是否能解释先前观测到的差异；2)(重新)用相同的检索方案分析三种类型的观测，以评估以前检索代码/方法上的差异是否可能是造成先前差异的原因。通过这种分析，我们表明，使用相同的检索方法可以更好地协调这些仪器之前的测量结果。然而，增加盘平均CIRS观测值，而不是确认不同数据集之间的一致性，揭示了其中一组CIRS盘平均观测值与PACS测量之间的差异。这就提出了新的问题，即水的纬度变化的可能性，这可能是由经向环流的季节变化引发的。正如已经对氰类和碳氢化合物所显示的那样，这种环流可能会通过富含H2O的空气的下沉或上升，潜在地影响H2O的纬度分布。文中还讨论了土卫六大气中OH/H2O输入通量时空变化的可能影响。在今后的工作中，需要对更多的观测结果进行分析，以解决这项工作中出现的问题，并提高对土卫六大气中H2O来源的理解。
Since its first measurement 20 years ago by the Infrared Space Observatory (ISO), the water (H2O) mole fraction in Titan’s stratosphere remains uncertain due to large differences between the determinations from available measurements. More particularly, the recent measurements made from the Herschel observatory (PACS and HIFI) estimated the H2O mole fraction to be 0.023 ppb at 12.1 mbar. A mixing ratio of 0.14 ppb at 10.7 mbar was, however, retrieved from nadir spatially-resolved observations of Cassini/CIRS. At the same pressure level (10.7 mbar), this makes a difference of a factor of 5.5 between PACS and CIRS measurements, and this has notably prevented current models from fully constraining the oxygen flux flowing into Titan’s atmosphere. In this work, we try to understand the differences between the H2O mole fractions estimated from Herschel/PACS and Cassini/CIRS observations. The strategy for this is to 1) analyse recent disc-averaged observations of CIRS to investigate if the observation geometry could explain the previous observed differences, and 2) (re)analyse the three types of observation with the same retrieval scheme to assess if previous differences in retrieval codes/methodology could be responsible for the previous discrepancies. With this analysis, we show that using the same retrieval method better reconcile the previous measurements of these instruments. However, the addition of the disc-averaged CIRS observations, instead of confirming the consistency between the different datasets, reveals discrepancies between one of the CIRS disc-averaged set of observations and PACS measurements. This raises new questions regarding the possibility of latitudinal variations of H2O, which could be triggered by seasonal changes of the meridional circulation. As it has already been shown for nitriles and hydrocarbons, this circulation could potentially impact the latitudinal distribution of H2O through the subsidence or upwelling of air rich in H2O. The possible influence of spatial/time variations of the OH/H2O input flux in Titan’s atmosphere is also discussed. The analysis of more observations will be needed in future work to address the questions arising from this work and to improve the understanding of the sources of H2O in Titan’s atmosphere.