Observations of water ice clouds and aerosols on Mars can provide important insights into the complexity of the water cycle. Recent observations have indicated an important link between dust activity and the water cycle, as intense dust activity can significantly raise the hygropause, and subsequently increase the escape of water after dissociation in the upper atmosphere. Here present observations from Nadir and Occultation for MArs Discovery/Trace Gas Orbiter that investigate the variation of water ice clouds in the perihelion season of Mars year 34 (April 2018–2019), their diurnal and seasonal behavior, and the vertical structure and microphysical properties of water ice and dust. These observations reveal the recurrent presence of a layer of mesospheric water ice clouds subsequent to the 2018 global dust storm. We show that this layer rose from 45 to 80 km in altitude on a time scale of days from heating in the lower atmosphere due to the storm. In addition, we demonstrate that there is a strong dawn‐dusk asymmetry in water ice abundance, related to nighttime nucleation and subsequent daytime sublimation. Water ice particle sizes are retrieved consistently and exhibit sharp vertical gradients (from 0.1 to 4.0 μm), as well as mesospheric differences between the global dust storm (<0.5 μm) and the 2019 regional dust storm (1.0 μm), which suggests differing water ice nucleation efficiencies. These results form the basis to advance our understanding of mesospheric water ice clouds on Mars, and further constrain the interactions between water ice and dust in the middle atmosphere.

中文翻译：

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ExoMars痕量气体轨道器/ NOMAD揭示沙尘暴期间火星水冰云的强烈变化

对火星上水冰云和气溶胶的观测可以提供有关水循环复杂性的重要见解。最近的观察表明，粉尘活动与水循环之间存在重要的联系，因为强烈的粉尘活动会显着提高湿润性，并在上层大气解离后增加水的逸出量。本文介绍了天底星和掩星对MArs发现/跟踪气体轨道器的观测，这些观测研究了火星34年（2018年4月至2018年）近日点季节水冰云的变化，其昼夜和季节行为以及垂直结构和微物理性质水冰和灰尘。这些观察结果表明，在2018年全球沙尘暴之后，中层水冰云层经常出现。我们显示，由于暴风雨，在较低的大气层加热之后的几天内，这一层的高度从45 km上升到80 km。此外，我们证明了水冰丰度存在强烈的黎明-黄昏不对称性，与夜间成核和随后的白天升华有关。水冰的粒径大小一致，并显示出陡峭的垂直梯度（从0.1到4.0μm），以及全球沙尘暴（<0.5μm）和2019年区域性沙尘暴（1.0μm）之间的中层差异，这表明水的差异冰成核效率。这些结果为进一步了解火星上的中层水冰云奠定了基础，并进一步限制了中层大气中水冰与尘埃之间的相互作用。