土星上阴影和反阴影的解释和反对南极眼墙的证据
Interpretation of shadows and antishadows on Saturn and the evidence against south polar eyewalls
摘要：
2006年卡西尼号探测器对土星的观测揭示了南极云的阴影，对此的普遍解释是由Dyudina等人（2008，Science 319，1801）提出的，他们认为这些阴影是由同心云墙投射的，类似于飓风的物理和光学厚眼墙。在这里，我们使用Sromovsky等人的辐射转移结果。（2019，Icarus，doi.org/10.1016/j.j.jumus.2019.113398），结合蒙特卡罗计算和物理模型，表明这种解释几乎肯定是错误的，因为（1）光学厚眼壁应该在极方向上产生非常明亮的特征，而这些特征是看不到的，而所看到的适度明亮的特征出现在相反的方向，（2）眼壁阴影应该非常暗，但所观察到的阴影仅产生5- 10%的I/F变化，（三）在这一地区，对云的辐射传输模拟没有探测到光学厚壁云，模型云的压力也没有明显的变化层，和（4）有一个替代的解释，这是更符合观察。最合理的情况是，87.9度和88.9度附近的阴影都是由光学深度逐步减少所产生的边缘上的半透明气溶胶层所投射的，第一个在50毫巴水平的平流层中，第二个在350毫巴附近的假定双膦层中，在752 nm处，每个台阶的向极侧的光学深度分别减小0.15和0.12。这些步骤足以产生大致正确大小和形状的阴影，主要落在900毫巴附近的氨冰层上，并产生我们称之为反阴影的明亮特征。
英文摘要：
Cassini spacecraft observations of Saturn in 2006 revealed south polar cloud shadows, the common interpretation of which was initiated by Dyudina et al. (2008, Science 319, 1801) who suggested they were being cast by concentric cloud walls, analogous to the physically and optically thick eyewalls of a hurricane. Here we use radiative transfer results of Sromovsky et al. (2019, Icarus, doi.org/10.1016/j.icarus.2019.113398), in conjunction with Monte Carlo calculations and physical models, to show that this interpretation is almost certainly wrong because (1) optically thick eyewalls should produce very bright features in the poleward direction that are not seen, while the moderately brighter features that are seen appear in the opposite direction, (2) eyewall shadows should be very dark, but the observed shadows create only 5-10\% I/F variations, (3) radiation transfer modeling of clouds in this region have detected no optically thick wall clouds and no significant variation in pressures of the model cloud layers, and (4) there is an alternative explanation that is much more consistent with observations. The most plausible scenario is that the shadows near 87.9 deg S and 88.9 deg S are both cast by overlying translucent aerosol layers from edges created by step decreases in their optical depths, the first in the stratospheric layer at the 50 mbar level and the second in a putative diphosphine layer near 350 mbar, with optical depths reduced at the poleward side of each step by 0.15 and 0.12 respectively at 752 nm. These steps are sufficient to create shadows of roughly the correct size and shape, falling mainly on the underlying ammonia ice layer near 900 mbar, and to create the bright features we call antishadows.
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