We update the record of cloud opacity observations conducted by the Mars Science Laboratory (MSL) Curiosity rover to cover the first five Mars Years (MYs) of the mission (L _s = 160° of MY 31 to L _s = 160° of MY 36). Over the three MY period that we add to the previously analyzed two MY record, we achieve good diurnal coverage between 07:00 and 17:00 with nearly 1200 new observations. We derive a new scattering phase function for the clouds of the Aphelion Cloud Belt (ACB) using results from the Zenith and Suprahorizon movie data sets. Our phase function is generally smooth and featureless, which is consistent with the overall lack of atmospheric optical phenomena on Mars aside from a single instance of an observed halo. Applying our new phase function to the data, we find that there is very minimal variability in the ACB's opacity, either diurnally, intraseasonally, or interannually, noting that our observations are only sensitive to ice clouds and cannot detect any ice hazes that may be present over Gale. This contrasts with previous results, which observed a 57% difference in the opacity of morning and afternoon clouds in MY 33. The MY 33 results now appear to be an outlier, not replicated at any point during the MSL mission. We conclude that the higher morning opacities in MY 33 were a consequence of an incomplete understanding of the nature of the scattering phase function close to the Sun.

中文翻译：

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盖尔陨石坑的五年火星云观测：不透明度、变化性和冰晶习惯

我们更新了火星科学实验室 (MSL) 好奇号漫游车进行的云不透明度观测记录，以涵盖任务的前五年 (MY) (L MY 31 的_{s = 160° 至}L _s = MY 36 的 160°）。在我们添加到之前分析的两个 MY 记录中的三个 MY 期间，我们在 07:00 到 17:00 之间实现了良好的日间覆盖，并有近 1200 个新观测值。我们利用天顶和超地平线电影数据集的结果导出了远日点云带 (ACB) 云层的新散射相位函数。我们的相位函数通常是平滑且无特征的，这与火星上除了观察到的光晕的单个实例之外总体上缺乏大气光学现象是一致的。将我们的新相位函数应用于数据，我们发现 ACB 的不透明度的变化非常小，无论是日间、季节内还是年际，注意到我们的观测仅对冰云敏感，无法检测到可能存在的任何冰霾盖尔之上。这与之前的结果形成鲜明对比，之前的结果观察到 MY 33 中上午和下午云的不透明度有 57% 的差异。MY 33 结果现在看来是一个异常值，在 MSL 任务期间的任何时候都没有复制。我们得出的结论是，MY 33 早晨较高的不透明度是对靠近太阳的散射相位函数的性质不完全理解的结果。